HP-AN1287-7_Improving Network Analyzer Measurements of Frequency-TranslatingDevices

8/14/2019 HP-AN1287-7_Improving Network Analyzer Measurements of Frequency-TranslatingDevices

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Improving Ne tw ork AnalyzerMeasu remen ts of Freque ncy-Translat ing Dev ices

Applica t ion Note 1287-7

LO-RF

RF

LO

LO+RF

IFIF

RF

LO

IF

LO-RF

RF

LO

LO+RF

IFIF

RF

LO

IF

LO-RF

RF

LO

LO+RF

IFIF

RF

LO

IF

LO-RF

RF

LO

LO+RF

IFIF

RF

LO

IF


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Introduction Fr equency-tr an slation devices (FTDs) such as mixers, converter s,an d tun ers ar e critical components in m ost RF and m icrowavecommun icat ion systems. As commu nication systems adopt m ore

advan ced types of modulat ion, F TD designs ar e increasinglycomplex, tests a re more st ringent with t ighter specificat ions, an dthe n eed to reduce costs is more importa nt th an ever.

The m easur ement tra de-offs for frequency-tran slating devices varywidely am ong different indust ries. Measur ement accura cy, speed,cost an d ease of setup a re a mong th e considerat ions fordeterm ining the best t est equipment . This applicat ion n ote explorescurrent test equipment solutions and techniques that can be usedto accur at ely cha ra cterize an d test frequen cy-tr an slating devices.Frequency-translating devices present unique measurementcha llenges since their inp ut an d outpu t frequencies differ. Theserequire different m easur ement t echn iques than th ose used for alinear device such as a filter. This note covers linear frequency-

tran slation measurements, such a s ma gnitude, relative phase,reflection and isolation. Corresponding accuracy issues are alsodiscussed.

To get t he m ost from th is note, you sh ould have a ba sic un der-standing of frequency translation terminology, such as RF port,IF port an d LO port. Und ersta nding of fun dam enta l RF a ndnetwork a na lyzer ter ms su ch as S-para meters , VSWR, group delay,mat ch, port, full two-port calibrat ion, a nd test set is also expected.For a bett er un dersta nding of such term s, a list of referencematerial appear in the Appendix section.

Network a na lyzers u sed for t esting frequency-tran slation devices

include scalar network a na lyzers, vector net work an alyzers withfrequency offset capability, and vector network analyzers using anupconversion/downconversion configuration. Each solution has itsown a dvant ages an d disadvant ages. This section provides asynopsis of the three configurations so you can quickly evaluatewhich is th e best fit for your mea sur ement needs. Detailedinform ation a bout each solution is discussed in lat er sections.

The most economical instr um ent for F TD tests is a scalar networkan alyzer. A scalar network a na lyzer u ses diode detectors t ha t candetect a very wide band of frequencies. This capability enables ascalar network a na lyzer to detect signals when the r eceiverfrequen cy is different from the s ource frequ ency. Magnit ude-only

measu remen ts su ch a s conversion loss, absolute outpu t power,retu rn loss an d isolation can be made, as well as nonlinearmagn itude measu remen ts such as gain compression. Group delayinform ation is available in some scalar net work ana lyzers using a nAM-delay technique, which employs a mplitude modulation.AM-delay measur ement s are less accur at e tha n group delay

Netw ork AnalyzerMixer Measuremen tConfigurations

ScalarNetwork AnalyzerConfigurat ion


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measu remen ts obtained with a vector network a na lyzer. AM delaytypically has an un cert aint y of around 10 to 20 ns, whereas group

delay with a vector network a na lyzer ha s an u ncertaint y as good as150 ps. Advant ages of th e scalar s olut ion in clude low cost an d goodmagn itude a ccur acy. As shown in F igure 1, fully integrat ed scalarnetwork a na lyzers such a s th e HP 8711C or H P 8713C provideeconomical RF measu remen ts u p to 3 GHz, an d include AM delaycapa bility. The HP 8757D scalar network an alyzer, shown inFigure 2, m easur es up to 110 GHz, an d pr ovides very good absolut epower measurement s, particularly when installed with a n intern alpower calibra tor an d used with precision det ectors. In certa incases, such as measu ring FTDs with an int erna l filter, theHP 8757D with intern al power calibrator a nd pr ecision detectorcan typically make more accura te ma gnitude measurements tha na vector n etwork a na lyzer.

Figure 2.HP 8757DScalar NetworkAnalyzerConfiguration

External LO source

Directional bridge

Precision detector

SweeperHP 8757D

scalar network analyzer

Lowpass filter

Figure 1.HP 8711C Sca larNetwork AnalyzerConfiguration

HP 8711CRF network analyzer

External LO source Lowpass filter

10 dB

10 dB


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A more versatile solution for FTD t est is a vector n etwork an alyzer.A vector network a na lyzer uses a tu ned-receiver na rrowbanddetector, which a llows mea sur ement s of both ma gnitude a nd

relat ive phas e. The vector n etwork a na lyzers frequen cy offsetmode offsets t he a na lyzers r eceiver from its sour ce by a given LOfrequen cy, and ma kes frequency-tra nslat ion mea sur ement spossible.

There are two common vector network analyzer configurations forFTD measu rement s. The simplest configura tion is shown in F igure 3,an d is pra ctical for t esting upconverter s an d downconverters. Thisconfigurat ion a llows ma gnitude-only measu remen ts with a limiteddynamic range. For example, if you a re interest ed in th e magnitu deresponse of th e FTDs pa ssband, t he H P 8753E vector n etworkan alyzer h as 35 dB of dynam ic ran ge in the R chan nel and providesa qu ick a nd ea sy solution.

Vector Network Analyzerin Freque ncy OffsetMode Configurat ion

Figure 3.Vector Netw orkAnalyzer inFrequencyOffset Mode Vector Network Analyzer

Lowpassfilter

10 dB10 dB

RF in

Start: 900 MHzStop: 650 MHz

Start: 100 MHzStop: 350 MHz

Fixed LO: 1 GHzLO power: 13 dBm

FREQ OFFSON off

LOMENU

DOWNCONVERTER

UPCONVERTER

RF > LO

RF < LO

VIEWMEASURE

RETURN


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A vector net work an alyzer in norma l operat ing mode can also beconfigured for frequency-translation measurements. This config-urat ion h as two main advanta ges. First , the instrument can beused to measur e a FTDs magnitu de and relat ive pha se responsewithout the need for frequency offset. As shown in Figure 5, twomixers are u sed to upconvert a nd downconvert t he signals,

ensur ing the sa me frequencies at t he net work a na lyzers sourceand receiver ports. Second, this configuration provides a potentiallymore accura te m eth od for meas ur ing absolut e group delay. You cansimply measure t wo mixers an d ha lve th e response, accepting th eresulting u ncertaint y. You can a lso use a m ore elaborat e techniquetha t involves cha ra cterizing the am plitude and ph ase of acalibration mixer a nd th en a pplying extern al err or correction forthe most accuracy. Alth ough this t echn ique is more accurat e, it is

To also measur e th e F TDs r elative phase an d out-of-band response,Figure 4 illustra tes a high-dyna mic range configura tion. An a lter-na tive high-dyna mic ran ge configur at ion can be achieved by

splitting the a na lyzers RF output power between t he device un dertest (DUT) and th e reference mixer (This configura tion is similar tothe one shown in F igure 24). In both configurat ions the vectornetwork a na lyzer ha s ar ound 100 dB of dyna mic range. A signa linto the r eference R cha nn el is always necessar y for proper pha se-locking of the vector network analyzer. In addition, theR cha nn el provides a reference for r atioed measur ement s such asrelative phase or magnitu de and ph ase tr acking. Vector networkana lyzers such as th e HP 8720D series and the H P 8753E havefrequen cy offset capability t o 40 GHz an d 6 GH z, respectively.

Vector Network Analyzer

Lowpass filter Reference mixer

C H1 C O NV M EA S l og M AG 1 0 d B/ R EF 1 0 d B

START 640.000 000 MHz STOP 660.000 000 MHz

RF out

RF in

10 dB10 dB

Powersplitter

External LO source

Figure 4.Vector Netw orkAnalyzer, HighDynamic RangeConfiguration

The Upconvers ion/ Downconvers ionTechnique


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Conversion loss, as shown in Figure 6, measures how efficiently amixer converts energy from one frequency to another. It is definedas th e rat io of th e out put p ower to the input power at a given LO(local oscillator) power. A specified LO power is necessa ry becausewhile the conversion loss of a mixer is usu ally very flat within t hefrequen cy span of its int ended operat ion, t he a verage loss will vary

Figure 5.Upconversion/ DownconversionConfiguration

ConversionLoss

Defini tion and Importanceof Conve rsion Loss

External LO source

Vector network analyzer

Bandpassfilter

BandpassfilterDUT Mixer

6 dB 6 dB6 dB6 dB6 dB

Powersplitter

RF

LO

IFIF RF

LO

Figure 6.ConversionLoss

Frequency

Powerlevel

Conversion loss

Conversion loss =

20*log [ ]mag(f )

mag(f )

RF

LO

IF

IF

RF

also more complicat ed, requiring a n exter na l controller a nd anoperat or who is fam iliar with net work-ana lyzer dat a tr an sfer an derror-term manipulation. Different levels of error correction can be

applied to achieve the desired accuracy. This technique will bediscussed in more deta il in the section titled,Absolute Group Delay A M ore Accurat e, Lower Ripple Techn ique.


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Figure 8 illust rat es th e importa nce of a flat conversion-lossresponse. The DUT is a st an dar d television-cha nn el convert er. Theinput signal consists of a visual carrier, audio carrier an d a colorsubcarrier. Since th e frequency response of the converter ha s a

notch in th e passban d, the color subcarr ier is suppr essed and th eresulting output signal no longer carries a valid color-informationsignal.

with the level of the LO, as the diode impedance changes. As shownin Figure 7, conversion loss is usually measured versus frequency,either the IF frequency (with a fixed LO) or t he RF frequency (with

a fixed IF ). The configur at ion for a fixed IF measu remen t isdifferent from th ose described up to th is point . (See the Fixed IFMeasurementsection.)

Figure 7.Two Types o fConversion LossMeasurements

RF IF

LO

RF IF

LO

Loss

IF freq

Loss

RF freq

Conv loss vs IF freq(fixed LO freq)

Conv loss vs RF freq(fixed IF freq)

0 0

Figure 8.TV Tun erConversionLoss Example

Converter response

Input Signal DUT Output signal

Audiocarrier

Visualcarrier Color

sub-carrierColor sub-carrier

attenuated

LO


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Conversion-loss measu remen ts can be mad e with either a scalarnetwork a na lyzer or a vector net work a na lyzer, using theconfigura tions shown in Figures 1 thr ough 5. The measu remen t

un cert aint ies are d ifferent for each type of analyzer. For both t ypesof an alyzers, the two main systema tic errors ar e port m ismat ch an dfrequen cy response. The scalar network a na lyzer appr oach r equiresadditional care to minimize errors due t o the an alyzer s broadban ddetector. For some vector n etwork an alyzers, a n intern al pr ocess,called sam pling, an d ph ase-lock r equirement s can a lso createerrors. Next we will examine ea ch of these error t erms an d exploretechniques to minimize their effects.

Mismatch errors resu lt when t here is a conn ection between twoports t ha t h ave differen t imp edan ces. Comm only, a devicesbehavior is chara cterized within a Z0 environm ent, typically havingan impedance of 50 or 75 ohms. Alth ough th e test ports of a

network a na lyzer a re designed to be perfect Z0 impedances, theyare not. The imperfect source an d r eceiver ports of th e net workan alyzer creat e errors in the calibration sta ge. Therefore, evenbefore a device under test (DUT) is connected, some errors havealready been creat ed in th e calibrat ion st age (see Figur e 9). Oncethe DUT is connected, the tota l measur ement u ncertaint y is equalto the sum of th e calibration error plus the measu remen t error.

Once the DUT is connected, int eraction between t he DUTs portsan d th e network a na lyzer s ports cause misma tch err ors. As shownin Figure 9, mismatch effects gener ate t hr ee first-order errorsignals. The first is int eraction between the network a na lyzerssource port an d th e DUTs input port. The second is between thenetwork a na lyzers r eceiver port an d th e DUTs outpu t port .

MeasurementConsiderat ions

Mismatch Errors

Figure 9.Mismatch Effects

Source

Receiver

Measurement:

Calibration:

Source

Receiver

Total Uncertainty = Calibration Error + Measurement Error

( )

( )

( )

RF IF

LO

source

receiver

DUT input

DUT output

source receiver

Calibration plane

source receiverDUT


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The th ird is between the n etwork an alyzer s source port an dreceiver port. For an F TD measur ement , this third inter action isusually negligible because the conversion loss and isolation of the

FTD will atten ua te th e reflected signals. As frequen cy tra nslat ionprecludes conventional t wo-port er ror corr ection, att enua tors canbe used to improve port ma tch.

By adding a h igh-quality at tenu at or to a port, th e effective portmat ch is improved by up t o approximat ely twice the value of th eatt enua tion. A high-quality att enua tor has a round 32 dB of portmatch. The effective match is a function of the quality of theattenu ator as well as i ts attenu ation, as sh own in F igure 10.

Figure 10.Effective Matchas a function of Attenuato rsMatch.

Attenuator

( )

( )( ) 2

Source

( ) += ( 2

source E source match

attenuator

source attenuation

attenuator source attenuation)( )

ff

E source matchff

As shown in Figure 11 and Figur e 12, a well-mat ched atten ua torcan significantly improve the effective port match. For example,a 10-dB att enua tor, with a port m at ch of 32 dB, can tra nsform a norigina l port m at ch of 10 dB int o an effective mat ch of 25 dB.

0 5 10 15 20 25 30 3510

15

20

25

30

35

Original match (dB)

Effectivematch(dB) 32dB attenuator match

26dB attenuator match



Region when attenuator

no longer results in improved match

Figure 11.Effective Matchas a Function of Attenuato rsMatch (Fixed10 dB Attenuator).


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However, as th e match of the a tten ua tor approaches the ma tch ofthe original source, the impr ovement diminishes. As shown inFigure 12, the larger th e att enuation, the more nearly the resulting

mat ch approaches th at of th e at tenu at or. However, excessiveatt enua tion is not desired since this will decrease t he dyna micran ge of th e measur ement system . The port m atch of an F TD canbe poor, typically around 14 dB. Therefore, it is recommended thatatt enua tors be placed at th e FTDs input an d outp ut ports.

Figure 12.Effective Matchas a Function of Attenuation(AttenuatorMatch = 32 dB)

0 5 10 15 20 25 30 350

510

15

20

25

30

35

Original match (dB)

Effec

tivematch(dB) 20 dB attenuation

10 dB attenuation

6 dB attenuation

3 dB attenuation

Region when attenuatorno longer results in improved match

Scalar network a na lyzers use different detection met hods tha nvector network an alyzers th at s hould be considered when t estingFTDs. Scalar net work a na lyzers use broadban d diode detectors.Alth ough capa ble of both na rr owband an d broadband detection, th eHP 8711 series, which includes th e HP 8712C and H P 8714C vector

network an alyzers, uses broadband detection for FTD m easur ements.Therefore, if you u se an H P 8712 or 8714, use th e same F TD testconsidera tions as you would for a scalar n etwork an alyzer.

Importance of Proper Filtering

A scalar n etwork a na lyzers broadba nd diode detector will detectan y signal tha t falls within its passban d. Although a broadban ddiode detector is an economical way to measure FTDs, it also canallow certain detection errors. The diode detector will detect thedesired IF signal, as well as other mixing products or spu rioussignals. To minimize the detection of undesired signals, a filtershould be placed at th e detector port to pass t he desired IF signa lbut reject a ll other signa ls. Figure 13 shows an example of the

incorrect measurements tha t m ight result when improper IFfiltering is used in a scalar net work an alyzer configur ation.

ConsiderationsUnique tothe ScalarNetwork Analyzer


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Figure 13.Conversion LossResponse withand without an

IF Filter

In F igure 13, the conversion loss measur ement without t he IF filterappear s to be better th an it really is. The lack of an IF filtergenerat es erroneous results. The broadban d diode detector cann otdiscriminate the frequency of the received signal(s) it measuresth e composite res ponse. If th e source is set at 1 GHz, it is assu medtha t t his is the frequency of the detected signa l. Any signal tha tfalls within t he pa ssband of the diode detector will be detected. Ifthe output of a DU T is composed of th e desired IF signal plus th eimage frequency, LO and RF feedthr ough and other spu rioussignals, th e diode detector will detect th e composite of all th e signalswithin its passband. This composite signal will be incorrectlydisplayed as a r esponse th at occurs a t 1 GHz.

Frequency Response Error

Without performing any sort of calibration on a scalar or vectornetwork a na lyzer, the frequency response of the test syst em cann otbe separa ted from th e FTDs r esponse. One wa y to correct t heseerrors is to perform a frequency-response n ormalization orcalibration, using a through connection in place of the DUT.

For scalar network a na lyzers such as t he HP 8757D, which veryaccur at ely measur es absolute power, th e norma lization calibrat ioncan be performed in t wo steps. See Figure 21. First, th e absolut eRF power is measu red a nd st ored in memory. Second, the DUT isinserted a nd t he a bsolute IF power is measu red. Conversion loss isdisplayed u sing th e Dat a/Memory forma t. Th e conversion loss value

is very accur at e since the mea sur ements of th e two absolut e powerlevels, RF an d IF, are very a ccura te. Rat ioing two very accur ateabsolute power levels removes the frequency response error. Insome cases, a scalar H P 8757D with an intern al power calibratoran d pr ecision detector can m ake more accurat e conversion lossmeasurements than a vector network a nalyzer. In th e AccuracyComp arison of the HP 8757D an d a Vector N etwork An alyzer

Start 900.000 MHz Stop 1 000.000 MHz

2:Conv Loss /M Log Mag 1.0 dB/ Ref 0.00 dBSwept Conversion Loss

1

2

1:Conv Loss /M Log Mag 1.0 dB/ Ref 0.00 dB

-9

-8

-7

-6

-5

-4

-3

-2

-1

Abs

dB

IF filter

No IF filter

2

1

Ch1:Mkr1 1000.000 MHz-6.38 dB

Ch2:Mkr1 1000.000 MHz-4.84 dB


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section, err or ter ms ar e used to illust ra te how a scalar an alyzerwith inter na l power calibrat or can be more accurat e tha n a vectornetwork an alyzer.

For an alyzers t ha t do not precisely measu re absolute power,corr ections for th e frequency response error are less accur ate.The input an d output of the DUT a re at different frequencies, butthe norma lization can only be performed over one frequency ran ge.The result is th at pa rt of th e test system is chara cterized over adifferent frequency range than that which is used during the actualmeasurement.

There a re t wo choices for t he frequency ran ge used for thenorma lization: either th e DUTs inpu t (RF source) range, or th eDUTs out put (receiver) ran ge. The n orma lization should be doneto corr ect t he portion of the test system tha t cont ributes t he lar gestun cert aint y; for exam ple, this would be th e portion with th e most

loss or frequen cy roll-off. System s an d componen ts t end t o havepoorer performa nce at t he h igher frequencies, therefore t hecalibrat ion sh ould normally be performed at th e higher frequencies.In gen era l, high-quality, low-loss cables a nd connectors sh ould beused to minimize frequency-response errors.

For higher accuracy, combine a normalization calibration withexterna l error-term correction. Dur ing th e norma lizat ion, only onesection of the test configuration should be connected, either theDUTs input ra nge or the DUTs outpu t r an ge. For h ighestaccuracy, the removed section can be characterized separately.An extern al compu ter is u sed to extra ct the r emoved sectionsS-param eters from th e network ana lyzer. This dat a is then u sedto modify th e net work an alyzer s err or term s to accoun t for th e

effects of the rem oved section.

Now that we have covered the important measur ement considerationsof th e scalar n etwork a na lyzer, lets cont inu e with a d iscussion ofthe vector net work an alyzer. The importa nt considerat ions include:the n eed for proper filtering, an accur ate a nd st able LO, and powermeter calibration for the most accurate measurements.

Importance of Proper Filtering

A vector network a na lyzer ha s a n ar rowband tu ned r eceiver. Sincethe r eceived signa l is heavily filtered by an inter na l nar rowband IFfilter, broadband detection issues encountered by the scalar networkan alyzer a re n ot present . However, proper filtering is st ill very

importan t for vector network a na lyzers with s ampler-basedreceivers, such as t he HP 8753E a nd th e HP 8720D.

Sampl ing Archi tecture and Issues

A samp ler-based r eceiver consists of a volta ge-tu na ble oscillat or(VTO), a pulse generator, and a sampler (switch). The VTO drivesthe pu lse genera tor, which in tu rn dr ives the sa mpler. As a result,with pr oper t un ing of the VTO, this combinat ion r eplicat es a down-converted inpu t signal at th e correct int ermediat e frequency (IF)

Considerat ions Unique tothe Vector Netw orkAnalyzer


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for furth er pr ocessing. This combinat ion is similar to a h ar monicmixer in which t he h arm onics of th e LO are genera ted in t he mixer,an d the inpu t signal can mix with an y harm onic. With proper

tuning of the LO, one of the LO harmonics is offset from the inputsignal to produce the corr ect IF signal.

Since th ere ar e man y LO harm onics, any signal (desired or not)tha t is one IF away from an y of the LO ha rmonics will bedownconverted t o the net work an alyzer s IF an d detected.To illustr at e th is sampler effect, lets use t he H P 8753E a s anexample.

The IF of the HP 8753E vector network a na lyzer is 1 MHz. Errorsmight r esult because th e incoming signal is not filtered un til afterit is downconvert ed to th e IF. If th ere is only one signal a t t hereceiver, this signal will mix with one LO harmonic and is properlydownconvert to 1 MHz. However, if there are multiple signals th at

are 1 MHz awa y from an y of the LO h arm onics, these signals willbe downconverted to 1 MHz, which creates erroneous responses.

Figure 14 illust rat es an example of this sampler effect wher e thedesired IF outpu t signal of the mixer is 110 MHz. In order t ocorrectly detect th is signal, the H P 8753E will use a VTO of54.5 MHz, wher e its second h ar monic (109 MHz) will properlydownconvert 110 MHz to th e desired 1 MHz IF signal. In th eillustr at ion, we sh ow two mixer pr oducts (6 LO-2RF a nd 9 LO-RF)tha t would also produce IFs at 1 MHz. Notice th at t hese two spursoccur on eith er side of th e LO ha rm onics (18 VTO an d 42 VTO,respectively), but as long as they are 1 MHz away, they will bedownconver ted t o 1 MHz. Aside from t he signa ls which

(RF LO) 1

Spur: 18 VTO (6LO 2RF)981 980 = 1 MHz

Spur: 42 VTO (9LO RF)2289 2290 = 1 MHz

MixerIFoutput

LOharmonics

RF LO6LO 2RF 9LO RF

110 980 2290

2VTO 18VTO 42VTO

109Frequency

Frequency

1 MHz 1 MHz 1 MHz

IF 1 MHz

LO harmonics

2

Desired signal: 2 VTO (RF LO) =109 (410 300) = 1 MHz

Given RF = 410 MHz IF = RF LO = 110 MHzLO = 300 MHz

VTO = = 54.5 MHz

981 2289

Figure 14.Diagram of SpuriousMeasurementResponses


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R-Channe l Phase-Locking Conside rations

For F TD tests, u sing frequency offset m ode with a vector n etworkan alyzer, man y times you ar e required to remove the R-cha nn el

jumper an d input a signal into the R Ref-In port. Once an externa lsignal is introduced into the R chan nel, proper filtering a t t he Ref-In port is essential. Certain r equirement s must be met in order forthe R chann el to phaselock properly. First, t he signal into th eR chann el must be within a specified power level thr oughout theentire IF r an ge. For example, in th e HP 8753E vector networkan alyzer, the R-cha nn el signal mu st be between 35 dBm an d 0 dBmthr oughout the selected IF ra nge, oth erwise proper pha se lock willnot occur. If the R-channel filter does not cover the entire IF range,modifying th e selection of th e IF r an ge accordingly will elimin at eth e pha se-lock error. For example, if th e IF r an ge is set from 10 MHzto 300 MHz, but th e R-cha nn el filter is a lready rolling off at 300 MHz,then change th e IF st op frequen cy to around 250 MHz, or u se adifferent filter. The second r equirement is tha t t he R-cha nn el

samp ler needs to phaselock with t he corr ect signal. If th ere ar espur ious signals enter ing the R-cha nn el sampler, it might tr y toestablish ph ase lock with an improper frequency. This situa tion canbe avoided with proper R-channel filtering. (If you are using theHP 8712C or H P 8714C vector net work an alyzers, R-chann elconsidera tions are irrelevant becau se broadband det ectors ar e used.)

LO Accuracy an d Stabil i ty

For th e vector network a na lyzer, the u se of an accur ate, sta ble LOis required for accurate ma gnitude and relat ive phase measu rement s.As shown in Figure 3, a network a na lyzer in t he frequency offsetmode will au tomat ically set its int erna l source to sweep over t hecorresponding RF ran ge once you have ent ered th e necessaryinformation (i.e., IF range, LO information, upconversion ordownconver sion, RF LO). Therefore, if th e LO sour ce is notaccur at e, a network a na lyzer will not receive the an ticipat edIF signal. As a result, th e IF signal can fall on t he skirt s of th e IFfilter or worse, it can fall entirely outside of the filter passband. Inthe first case, this inaccur acy is tran sferred to the mea sur ementresults, and in both cases, phase lock can possibly not even occur.


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This LO accur acy and sta bility requirement can be explained withthe help of Figure 16, which illustra tes t he block diagra m of theHP 8753E in frequency offset m ode. At the st art of th e sweep, the

RF source is pretu ned t o the IF frequency plus th e LO offset, th enth e ma in ph ase-lock loop (PLL) is locked up . The r eceiver willsweep over the IF ran ge, an d th e source will tra ck it with th e fixedoffset. As shown, t he m ixer's IF signa l is downconverted in th eR-chann el sampler to provide th e 1 MHz ph ase-lock signa l.Therefore, the LO source mu st be stable and a ccura te in order toprovide a n LO signal t ha t will properly convert th e mixers RFinput to its desired IF out put . This mixers IF signal, oncedownconverted by t he sa mpler, is compar ed to an intern al 1-MHzreference signal wher e a resulting volta ge, proportiona l to theirpha se differen ce, is used t o fine-tu ne t he RF s ource. If th e mixersIF s ignal is t oo far from t he expected r eceiver s frequen cy, th en itmight lie outside th e acquisition ra nge of th e PLL. The pha se-lockalgorith m will not work a nd a pha se-lock er ror is displayed. For

example, the H P 8753E r equires a LO frequen cy accur acy of with ina few kHz of the nominal frequency.

Figu re 16. HP8753E BlockDiagram of

Frequency OffsetMode

1 MHzBPF

1 MHzBPF

IFdetectors

Pulsegenerator

PretuneDAC

RF source0.03 3000 MHz

RFout

DUT

ExternalLO

source A / B

R

FLO

15 60 MHz

Reference1 MHz

Main PLL

RF IF

LO


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Pow er Meter Calibration

Some vector network an alyzers, such as th e HP 8753E an d theHP 8720D, ar e capa ble of using power meter s to enha nce their

source and receiver accuracy. This power-meter calibration cansignificant ly minimize frequen cy resp onse err or, as well as correctfor some m isma tch effects. Th e following discussion r elat es to adownconverter m easur ement , although the sam e concepts alsoapply for u pconverter s. It is a t hree-step process:

1. Perform power-meter calibrat ion over th e IF r an ge.2. Perform response calibrat ion over th e IF r an ge.3. Perform power-meter calibrat ion over th e RF r an ge.

The power-meter calibration procedure needs to be done correctlyor it can lead to un expected err ors. Lets look a t ea ch step forperforming a power-meter calibration.

Step 1. Perform powe r-mete r cal ibration over the IF range.The pu rpose of this step is to calibrat e th e network a na lyzerssource for a very accur at e power level over th e IF ran ge. After th is,a r esponse calibra tion complet ely corrects t he r eceiver s frequen cyresponse over the IF ra nge. With t he power-meter calibrat ion, th eaccur acy of the power m eter is tr an sferred t o the network a na lyzer.

First , the power met er is preset an d zeroed. The network ana lyzeris set to sweep over th e IF ra nge.

Before connecting th e equipment as sh own in Figure 17, you n eedto ensur e th at the R chann el will phase lock pr operly. For networkana lyzers, such a s the HP 8753E, that do not h ave anintern al/externa l switch for the R cha nn el, you mu st tu rn on th efrequen cy offset before disconnecting t he R-chan nel jum per. Thisstep is importa nt s ince it prevents th e network an alyzer fromatt empting t o do a pretun e calibrat ion when ther e is no validR-channel signal. If the network analyzer attempts a pretunecalibration without a valid R-chann el signal, the pret un e constan tswould be invalid. Once the pretu ne consta nt s ar e erroneous, properphase lock will not occur even with a valid R-channel signal. If thisshould happen , reconnect the R-cha nn el jum per an d allow theau tomatic pretu ne calibration to fix itself. This step is notnecessary for a n H P 8720D with Opt ion 089 which provides anintern al/externa l R-chann el switch.

Figure 17.Configuration

for Power MeterCalibration overthe IF Range


Lowpassfilter

Power meter

HP-IB

Powersplitter 10 dBPower sensor

Point A Point B


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Step 2. Perform respon se cal ibration ove r IF range.

With a very accur ate p ower level at point B, now calibrate thenet work an alyzer s receiver for accur at e absolute power

measurements over th e IF ran ge.

The equipment is still conn ected a s shown in Figure 17. As men-tioned ear lier, th e power into the R chann el must be within t hecorrect p ower r an ge (35 to 0 dBm, in the case of an H P 8753E),thr oughout the selected IF ra nge, or pr oper ph ase lock will notoccur.

Exam ine the R-cha nn el response. The values displayed for theR cha nn el can appear surp rising. Notice th at t he values displayedare positively offset by approximately 16 dB. For example, with asignal of 0 dBm, th e R cha nn el will display appr oximat ely +16 dBm.Normal operat ion u sua lly displays ra tioed mea sur ement s (A/R orB/R). A mat hem at ical offset is us ed to accoun t for th e differen ces

between the R-cha nn el path an d the A-or B-cha nn el path. Thenetwork a na lyzer cannot discern th at the signa l is directly appliedto the R sam pler, so the signal level is displayed appr oximately16 dB too high. This offset a nd the frequen cy-response err or ar eaccounted for after a response calibrat ion is performed on th eR cha nn el.

Go to the calibration menu an d perform a r esponse calibra tion.The th rough sta nda rd in Figur e 17 is composed of th e att enua tor,filter, an d cabling. After th e response calibrat ion, t he R chann elwill indicate 0 dBm. At t his point, th e accur acy of the power met erha s been tra nsferred to the net work a na lyzer's receiver at point B.In a ddition, th e an alyzer h as a ccoun ted for t he frequency responseof the t hr ough stan dar d. Point B is now calibrated for a very

accur at e 0 dBm absolute power rea ding.

Step 3. Perform powe r-mete r cal ibration for RF range.

This last step, an other power-meter calibration over the RF r an ge,provides a very accur at e power level at the RF port of the mixer.

An altern at ive is to do one power-meter calibrat ion for both th eRF a nd IF ra nges. As men tioned earlier, power-meter calibrat ionint erpolat ion is a useful, tim e-saving tool. To save time, th e firstpower-meter calibration can be performed over a frequen cy ra ngethat encompasses both t he RF an d IF ra nge. Then tu rn oninterpolation to preserve the calibration in the separa te RF an d IFranges. This does compromise accuracy to some extent.

Connect th e equipment as sh own in F igure 18. Once theappropriate informat ion such as the LO informat ion,upconversion/downconversion, and RF LO have been set,the n etwork an alyzer will automat ically set t he RF r an ge. Afterselecting th e desired calibrat ion power at port B, perform th epower-meter calibrat ion.


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Upon completion of the power-meter calibration, a very accurate

reading of the DUTs absolute outpu t p ower is displayed on th eR cha nn el, which has been calibrated t o 0 dBm. Notice tha t t hereading is not necessarily the conversion loss of the DUT; it is theabsolut e power read ing of th e DUTs outpu t power. For exam ple, ifthe DU T ha s a 6 dB conversion loss when stimu lated with 10 dBmof RF input power, the R channel will display 16 dBm. If you wantthe R chann el to display t he conversion loss directly, th en in Step 1select the p ower at point B t o be the RF drive power for t he DU TsRF port. Th e disadvan ta ge here is some loss in accur acy; powermeter calibrat ion is m ost accur ate a t 0 dBm power level.

Pow er-Meter Calibration for a High-Dynamic-RangeMeasurementFor a high-dyna mic-range configur at ion, F igure 19 is an alogous to

Figure 17 a bove. Notice that th e R-cha nn el jum per is conn ected,since a valid signa l is required int o the R chann el for pha se locking.In t his configura tion, the R chan nel is used only for t he pu rpose ofpha se locking. It is not included as a direct pat h in th e actualmeasu remen t. This is th e high-dyna mic range configur at ion youshould use for per forming Steps 1 a nd 2.

Figure 18.Configurationfor Pow er-MeterCalibration over

the RF Range.


Lowpassfilter

External LO source

Power meter

HP-IB

Powersplitter

10 dB

6 dB

Powersensor

Point APoint B

DUT

Figure 19.Powe r MeterCalibration overthe IF Range,High-Dynamic-RangeConfiguration.


Power meter

HP-IB

Powersplitter 10 dB

Powersensor

Point A Point B


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Figure 20 is a na logous to Figure 18. This h igh-dynam ic-ra ngeconfigura tion is us eful for devices th at includes a filter, as sh own. Afilter is especially importan t in t he R chann el to suppress spur ious

signals tha t can cause false phase locking. A filter into the Bcha nn el might not be n ecessary. This is t he configurat ion you woulduse for performing Step 3.

Figure 20.Powe r MeterCalibrationover the RFRange, High-Dynamic-RangeConfiguration


External LO Source

Power meter

HP-IB

Powersplitter 10 dB

Powersensor

RFin

RFout

Reference mixer

Point APoint B

DUT

Lowpass filter

Accuracy Comparison

of the HP 8757D and aVector Network Analyzer

In some cases, an H P 8757D with an intern al power calibrator an d

precision detectors will make more accurate conversion lossmeasurements than a vector network ana lyzer with power metercalibrat ion. To illustr at e th e differen ce in a ccur acy, lets look a t t hethe err or terms involved in a downconverter m easur ement in eithercase.

In F igure 21a, the source is set to the RF r an ge. The precisiondetector very a ccura tely detects the absolut e power a t its port overthe RF range. The resulting errors are m 1, the mismat ch err or, andr detector RF,the u ncertaint y in th e detector s calibrat ion over th e RFran ge. The rdetector RF error is very sma ll, appr oximat ely 0.18 dB.


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Next, th e RF response is stored int o memory. The pu rpose of thisstep is to correct for t he frequency response of th e at tenu at or andthe source, r source RF.

In F igure 21b, the DU T is connected. The err ors consist of misma tch

errors, m2 an d m3 , an d th e uncerta inty of th e detectorscalibration over th e IF r an ge, rdetector IF. The latter er ror is verysmall. Notice tha t a n a ttenua tor is not u sed at t he output of theDUT since the precision det ector measu res most a ccur at ely at itsport. If additiona l devices, such as an att enua tor or filter, areadded, th eir effects will add a dditiona l calibrat ion steps a ndtherefore added uncertainty. This configuration is especiallyadvan tageous for FTDs with intern al filters.

Figure 21.(a) AbsoluteP o w e rMeasurement

over theRF Range(b) AbsoluteP o w e rMeasurementover theIF Range.

Sweeper

HP 8757Dwith internal power calibratorSweeper

HP 8757D

with internal power calibrator

Precisiondetector

Precisiondetector

m1

m3m2

LO

(a)

(b )

B

B

Systemtest ports


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In th is HP 8757D calibration and m easurement, th e totalun cert aint y is equal to the sum of th ree un corrected misma tcherror terms, m1, m 2, and m 3. The detectors err ors are very sma ll

an d a re n egligible.

With t he vector n etwork an alyzer with power-meter calibrat ion,five mismatch er rors r esult. Lets t ake a closer look a t t he er rorterm s involved in a vector n etwork an alyzer with power m etercalibration measurement.

In F igure 22a, the source is set to the IF ran ge. A power met ercalibration is per formed. Th e err ors consist of th e misma tch err or,m1, and t he source frequency response error over th e IF r an ge,r source IF. The latter er ror is small.

In F igure 22b, a norma lization calibrat ion is performed. Err ors =m2 + r receiver IF, where m 2= m 1 + r detector IF and r receiver IF is the

receiver frequency response error over the IF r an ge.

In Figur e 22c, the source is set to the RF r an ge and a power metercalibration is performed. E rr ors = m3 + r source RF , where r source RF isthe source frequen cy response error over th e RF ra nge.

In F igure 22d, the DU T is connected. Mismatch er rors, m4 and m5,are genera ted. As illustr ated, th e total uncerta inty is due to fiveun corr ected misma tch error term s, m1, m 2, m 3, m 4, m 5. The othererrors ar e relatively small.


Vector network analyzerVector network analyzer

Power meter


m1

m3

m2

m5

m4

Power meter

Detector

Detector

Systemtest ports

LO

RF IF

(a)

(d)

(b)

(c)

Systemtest ports

Systemtest ports

Figure 22.(a) Power MeterCalibration overthe IF Range.

(b) NormalizationCalibration overthe IF Range.(c) Power MeterCalibration overthe RF Range.(d) DUT isMeasured.


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In sum mar y, the HP 8757D with its interna l power calibrator an dprecision det ectors can be m ore a ccur ate tha n a vector net workan alyzer with power met er calibra tion since ther e are less

conn ections, resulting in less misma tch effects an d t herefore lessmeasu remen t u ncertain ty. However, the vector network a na lyzer iscapable of furt her error corr ection using err or-term man ipulationswith a n extern al compu ter. With a dditiona l error corr ection, thevector network analyzer might be the more accurate instrument.In t he section, Absolute Group Delay A M ore Accurat e, LowerRipple Techn ique , procedures for external err or-term m anipulat ionsare discussed.

When a commu nication system receives a signal u sing asuperh eterodyne receiver, th e informa tion is downconvert ed toan appr opriat e frequency called th e interm ediate frequency. Thecomponents in the IF section of the receiver usually provide most

of the gain and frequency selectivity of the communication system.Using a fixed IF allows a designer to optimize th e IF filtering an damplification which yield the highest performance. Typical IFfrequen cies for F M ra dios a re 10.7 an d 21.4 MHz, rada r r eceiversare 30 an d 70 MHz and sat ellite receivers ar e 70 and 140 MHz.

The other way in wh ich conversion loss or gain flat ness can bemeasu red is by keeping th e IF a t a constan t frequency. This isknown as a fixed-IF m easur ement . In order to accomplish t his, theLO must sweep in conjun ction with th e RF inpu t signal, keeping aconst an t frequency offset equa l to tha t of the IF. In man y cases,this m easur ement more closely ma tches the operat ion of the DU Tin the a ctu al applicat ion.

The most common way to perform t his measu remen t is to use thevector n etwork an alyzer in a tun ed-receiver mode (with out

frequen cy offset). Figur e 23 illust ra tes this configur at ion. As

shown, two extern al sources are u sed. One externa l sour ce provides

the RF signal an d the other pr ovides the LO signal. Both sources

sweep in a stepped-frequen cy mode un der t he direction of a

measu remen t controller via the HP-IB bus. This measur ement is

an ideal candidate for au tomat ion u sing the built-in test -

sequencing or IBASIC capability of many modern network

an alyzers. Also, notice th at an externa l reference must be

conn ected between th e VNA and t he sources (i.e. a 10 MHz

reference). When testing a tu ner with a fixed-IF sweep, the

cont roller must be able to tun e the intern al LO. If an a na log

voltage is used, t hen a programm able power su pply or digital-to-an alog conver ter is needed. If th e LO is tun ed digita lly, the proper

interface must be used.

Testing a mixer under fixed IF conditions can also be accomplishedwith one external source and th e network a nalyzer un der thecontrol of an external computer. The configuration is very similar toFigure 23, except t here is only one extern al LO source and acompu ter int erface for both the n etwork ana lyzer an d the extern alLO source. The configuration is shown in Appendix C.

Fixed IF Measurements


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Two cases should be considered for a fixed IF measurement. Inboth, t he LO an d RF frequencies are stepped over t heir respectiveran ges but in each case th e network a na lyzer is configur ed slightly

differently.Figure 23.Fixed IFConfiguration


External RF Source External LO Source

HP-IB

10 MHzbandpass

filter

Externalreference

Externalreference

10 dB

3 dB6 dBRF LO

IF

DUT

HP-IB

The first considers t he F TD as a downconverter, where t he RFoutput frequen cy of th e ana lyzer is stepped over the RF r an ge andthe a na lyzer measu res a fixed IF frequen cy at th e R-chann el inpu t.The second u ses the FTD as an upconverter an d th e outputfrequency of the analyzer (test port 1) is fixed at the IF frequency.The ana lyzer m ust m easure th e stepped RF frequency at theR-cha nn el inpu t. Both m easur ements a re accomplished using thefrequency offset mode capability available on many modernnetwork an alyzers. This tun ed receiver mode allows measu remen tto be ma de at frequencies other th an t ha t of th e ana lyzer s intern alsource.

Th e Appendix provides a pr ogram for a fixed IF measu remen t on amixer th at operat es as a d ownconverter. This HP Basic for Windowsprogram uses the H P 8753E as t he RF source and receiver an d th eLO signal is provided by a HP ESG-D3000A signal genera tor. TheLO source comman ds ar e written in SCPI, so any compliant signalgenerat or will work. Th e program set s an absolut e power level tothe m ixer s RF input port an d measu res th e absolute power level atthe IF p ort. The a na lyzers r eceiver is first calibrated a t t he fixed IFfrequen cy, then th e an alyzer s RF sour ce level is set . The differen cebetween th e two signals is th e conversion loss of the mixer u ndertest. The program a ssumes t he RF dr ive level from the t est portrema ins const an t over t he st epped frequen cy ran ge and a lso doesnot include insertion loss from the test port cable. The accuracy ofthe m easur ement can be enhan ced by including a power-metercalibration a t t he IF and RF ran ges as detailed in the H P 8753EOperating Manual. Care must be taken to ensure adequate signallevel to the R-cha nn el inpu t or t he system will not pha se lock to theIF signal. The R-cha nn el inpu t r equires a signa l level between0 dBm a nd 35 dBm. Adequat e filtering is also required to preventspur ious response from en tering t he r eceiver.


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Measuring mixer relative phase param eters such as tracking,relative phase linear ity and group delay requires a vector networkan alyzer. Relative phase between two mixers can be measu red, but

the a bsolute pha se response of a single mixer can not. In order tomak e a pha se measu remen t of a F TD, a second mixer is needed toprovide a reference phase signal. This r eference mixer is neededbecause when the analyzer is in frequency-offset mode, the sourcean d receiver function a t different frequencies an d pha se is notdefined between two different frequencies. This reference mixerneeds to be driven by the sam e RF an d LO signals th at are u sed todrive the mixer under test. The IF outpu t from th e reference mixeris applied t o the r eference pha se-lock R cha nn el of the vectornetwork an alyzer.

The capa bility to measu re th e amplitude an d relative phase mat chbetween frequency-tra nslat ion devices su ch as mixers is increasing

in importan ce as t he nu mber of multichan nel signal processingsystems increases. These multichan nel systems, such as direction-finding radars, require that the signal transmission through eachcha nn el be am plitude- an d pha se-ma tched. To achieve the requiredmatch between channels, each channel usually is manufacturedwith m at ched set s of components.

The ma tch between FTDs is defined as th e difference in amplitudeand relative phase response over a specified frequency range. Also,the tra cking between F TDs is measu red by how well the devicesare matched over a specified interval after the removal of any fixedoffset. For exam ple, this inter val can be a frequen cy int erval or atemper atu re inter val, or a combinat ion of both .

The configur at ion sh own in F igure 24 can be used to mak emagnitude and relative phase ma tching measurement s. First , oneDUT is measur ed and its r esponse is stored in memory. Then th esecond DUT is measur ed an d th e an alyzer s dat a/memory featu re isused t o compare t hem. The a na lyzer a ccoun ts for t he r esponse ofthe t est system since the two DUTs ar e measu red with exactly thesame conditions.

Relat ive Ph aseMeasurements

Relative Phase andMagnitud e Tracking

Figure 24. MixerMatching Magnitude an dGroup Delay


Powersplitter 10 dB

DUT 1

Powersplitter

DUT 2External LO Source

RF in

LO

RF

IF

LO

RF

IF10 dB

Low pass filter10 dB

Step 1: Measure DUT 1, store data in memoryStep 2: Measure DUT 2, display data/memory

10 dB


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Figure 25.TrackingResponses

CH1 S21/M log MAG .02 dB/ REF 0 dB

CH1 START 250.000 000

MHz STOP 400.000 000 MHz

PC

Cor

Ofs

CH2 S21/M phase REF 0200 m /

CH2 START 250.000 000

MHz STOP 400.000 000 MHz

PCCor

HldOfs

Figure 25 shows the magnitude and relative phase match betweenthe two DUTs.

If both DUTs must be measu red simulta neously (for tun ing), thenyou can place one DUT in t he B chan nel and one in the R chan nel.However, in this configurat ion you ar e limited by the d ynam icran ge of the R chann el. Therefore, the DU T in th e R-chann el pathshould n ot ha ve a filter. If it does, you can only view th e DUTspassban d ma tch. For two DUTs with int erna l filters, you cann otsimulta neously view the r esponse u sing th e configur ation inFigure 24.

Figure 26 illustra tes a configura tion th at t akes ad vant age of thehigh dynam ic ran ge of the A an d B chan nels. The ana lyzer compar eschann el A and B an d can be used t o actively display A/B responses,or for making high-dyna mic-ra nge measur ement s such as tra ckingof out-of-band r esponses. In th is case, t he R cha nn el is used only for

phase locking.Figure 26.SimultaneousTuning an d High-Dynamic RangeConfiguration

Vector network analyzerwith direct receiver access

External LO source

LPFDUT 1

DUT 2

RF

LO

IF

RF

R A B

Powersplitter

Powersplitter


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Group delay is a m easur e of a signals tr an sition t ime th rough adevice. Relative group-delay measur ement s can be mad e with th esame setup as that shown for relative phase measur ements.

The resu lt of a group-delay response depends on man ymeasurement factors, including aperture, averaging and IFbandwidth . When u sing a network a na lyzer to obtain a group-delayresponse, we calculate the slope of the phase over a frequencyincrement. As illustra ted in F igure 27, if we wan t to obta in th egroup delay at a par ticular frequen cy, we do not calculat e th etan gent at th at frequency point. Rath er, we find the values of th efrequen cy points on either s ide of the frequency of int erest a ndcalculat e th e corr esponding ph ase slope. As shown, th e twometh ods yield different results. Th e frequency points on eith er sideare determined by the aperture t hat is used. The default a pertureis equal to the sma llest increment between th e frequency points.

Apertu re = frequency span / (nu mber of points 1)

Group De lay

Important Parameterswhe n Speci fyingGroup Delay

Figure 27.Group Delay

Phase

Frequency()

Tangent

Average

delay

Frequency

Group delayripple

t o

Group delay (t )g =

d

d =

1

360d

d f*

fo

G

roupdelay

The a pertu re, which is used to calculat e group delay, can beadjusted. Th is adjustm ent cont rol is called th e smooth ingapert ur e. With th e smoothin g apertu re cont rol, you can ad just th enetwork a na lyzer to use a wider frequen cy incremen t t o calculatethe phase slope, although you are limited to 20% of your frequencyspan.


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Figure 28 illustra tes t he t ra de-off between resolution a nd n oise asa fun ction of apert ur e. Any effects in the pha se response directlyaffect the group-delay response. For example, noise in t he ph ase

response directly tran slates int o noise in a group-delay response.It is the apertu re tha t determines the informa tion t hat istra nsferred from one response to th e other. For example, if you seta na rr ow apert ur e, the noise in the ph ase response becomes moresignifican t, m aking the group-delay response noisier. At the sametime, a na rr ow apert ur e allows you to see var iations in the group-delay res ponse, which you would miss oth erwise. But , if youincrease the smooth ing apert ure too much you begin t o loseresolution since th e peaks sta rt t o disappear. Figure 28 illust ra teshow dram at ically different th e group delay responses can be,depending on th e smooth ing apert ur e settings. Always specify th esmoothing apertu re when m aking a group delay measurement .

Figure 28.Group Delay

as a Functionof Aperture

20% Aperture

0

0.2

0.4

0.6

0.8

1

1.2

Frequency

Delay(ns)

Minimum Aperture

0

0.2

0.4

0.6

0.8

1

1.2

Frequency

Delay(ns)

2% Aperture

0

0.2

0.4

0.6

0.8

1

1.2

Frequency

Delay(ns)

10% Aperture

0

0.2

0.4

0.6

0.8

1

1.2

Frequency

Delay(ns)

If you do not want to trade off resolution and noise, you can achievea balan ce by tra ding off some measur ement speed. Using averagingor n arrowing the IF ban dwidth results in slower measurement

speeds, without degrading r esolution or n oise.


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If you a re interest ed in absolut e group-delay measu remen ts acalibrat ion m ixer is r equired. When mea surin g the group delay of alinear device, sta nda rd vector er ror corr ection calls for a th rough

conn ection (delay=0) to be used as a calibration sta nda rd.The solution to the problem of measur ing th e delay of a nonlineardevice like a m ixer is t o use a calibrat ion mixer with very smallgroup delay as the calibration sta nda rd. Two mixers h ave beenchara cterized by Hewlett-Pa ckard Compan y for this pur pose:

Mixer Frequency Ran ge Group De lay

ANZAC MDC-123 30 MHz to 3,000 MHz 0.5 ns

MCL ZFM-4 dc to 1,250 MHz 0.6 ns

The group delay values were obtained by measur ing three sa mplesof each m ixer, usin g th e up conver sion/downconver sionconfigura tion shown in Figure 5. The mea sur ed group delay wasvery consistent, with flat frequen cy response. F or example, a pa ir

of MDC-123 mixers was measured, resulting in 1 ns of group delay.A single mixer, th en, must ha ve a group delay between 0 a nd 1 n s.Therefore, one can stat e th at th e group delay of the MDC-123 is0.5 ns, with a n a ccur acy of 0.5 ns. This is a very conservat iveun cert aint y window. Therefore, th e un certa inties of both mixers ar eequal t o their r espective group delay, alth ough th ese ar e worse-caseestimat es. After t he th rough calibrat ion a nd you are rea dy to testthe frequen cy-tran slating DUT, enter a value of electr ical delay tocompen sat e for t he gr oup delay of th e calibrat ion mixer (0.5 ns forthe MDC-123 or 0.6 ns for the ZFM-4). The resulting accuracyderived from th is technique is typically th e group-delay un certa intyof th e calibrat ion m ixer.

An import an t considera tion t o remember when selecting a

calibrat ion m ixer is t ha t t he delay of th e device should be as sma llas p ossible. The conven tion is to select a mixer wit h very widebandwidth compar ed to the mixer to be tested the wider thebandwidth , the sma ller th e resulting delay. For fur ther informa tionregarding th e desirable char acteristics of a calibra tion mixer, seethe Appendix section, Calibration Mixer Attributes.

Anoth er technique for m easur ing absolute group delay uses a nupconversion/downconversion configuration that yields moreaccur at e results. Th e technique is discussed in the section,Absolute Group Delay A M ore Accurat e, Lower Ripple Techn ique.

Measur ing the group delay of converters an d tu ners with

inaccessible internal LOs is very difficult using the methoddescribed above becau se we lack t he necessary ph ase-coherentLO for phase locking. Fortun ately, an a lterna te met hod exists t ha tis based on modulation delay. This technique can m easur e groupdelay on devices without needing a reference mixer. Using afrequen cy-response norma lization, absolute an d r elative groupdelay can be measu red.

Absolute Group De lay

AM Delay


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As shown in Figure 29, th e modulation-delay meth od for measu ringgroup delay is a ccomplished by am plitu de modulat ing or frequencymodulating an RF carr ier with a low-frequency sinusoid. Then t he

carrier is swept th rough th e frequency-tra nslat ing device, the IFoutput is demodulated, and fina lly, the ph ase of th e demodulatedsignal is compared with the origina l baseband m odulation t one(which can be derived by demodulating th e carr ier at t he input tothe DU T). As th e car rier is swept th rough th e DUT, the ph ase ofthe m odulation var ies proportionat ely to the ph ase r esponse of th edevice itself. Group d elay is calculat ed a s follows:

where e is the ph ase difference between the dem odulated inputan d output carriers in degrees, and fmod is the modulationfrequency. The apert ur e of this measu remen t is equal to twice th emodulation frequency. The effects of apertu re a re t he sa me a s th epha se-derivative meth od of measur ing group delay pr eviously

discussed. As the a pertu re gets larger, th e measu remen t becomesless noisy, but resolution degrades.

Gd= e (360 * f ) mod

Figure 29.ModulationDelay Method forMeasuring GroupDelay

AMModulator

RF

Sweep fmod

DUT

Phase detector

Gd=e

(360 * f ) mod

LO

Measure phasebetween twodemodulatedsignals

AM-delay measur ement s ar e easily performed with the H P 8711Cseries of network analyzers (see Figure 30). A responsenorma lization calibrat ion is performed without t he DU T conn ected.Then th e DUT is conn ected for relat ive or absolut e group-delaymeasu rement . The apertur e of this measu rement is fixed at 27.8 kHz,with t ypical accur acy around 10 to 20 ns. The accuracy of thistechnique is limited by th e diode detectors, which h as less

sensitivity tha n th e nar rowband detectors used in th e HP 8753E orHP 8720D. Group-delay accura cy with th e HP 8753E or H P 8720Dis more accur at e, typically around t he ps r an ge, depending on thesmooth ing apertu re settings. As shown in Figure 30, the ph ase-derived method (top response) is less noisy th an the AM delaymeth od (bottom response). H owever, vector n etwork a na lyzers u sea ph ase-derived meth od of group delay measu remen t, ma king itdifficult to measure group delay of FTDs with internal inaccessibleLOs.


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Now that tr an smission m easur ements ha ve been covered, letsmove on t o reflection measu remen ts. Reflection measu remen ts a relinear, even when testing frequency-tr an slating devices since thereflected signal does not undergo a frequency shift. Therefore, thesemeasurements are essentially the same as for filters andamp lifiers, with a few minor variat ions. The system at ic errors inthe test system an d the calibrat ion techniques used to remove themare t he sa me. Nar rowband detection sh ould be used if available.

Mixers a re t hr ee-port devices, an d t he r eflection from an y one portdepends on th e conditions of the other two ports. When measu ringreflection on a thr ee-port device, it is importa nt to term inat e th eports th at a re not being tested with an impedance typical of actua loperat ion. While this is often th e chara cteristic impedance Z0(usua lly 50 or 75 ohms), it could also be more complex. Forexam ple, if th e IF port of th e mixer is directly conn ected to a filter,then th is filter should be used wh en t esting RF- or LO-portreflection. In th is case, it m ight be n ecessary t o disconnect the IFport of the m ixer from the t est set a nd conn ect t he ap propriate loadimpedance.

When test ing the RF or IF ports, th e LO signal should be applied atthe power level that will be used in a ctu al operation. Since th e bias

point a nd im peda nce of th e mixer diodes is a fun ction of LO level,th e reflection at th e other ports will also be a function of LO level.

Figure 30.AM DelayMeasurementsusing the

HP 8711 Series

ReflectionMeasurements

Center 175.000 MHz Span 100.000 MHz

5101520253035

Abs

2

1:Memory Delay 5 ns/ Ref 0 sns

35

Center 175.000 MHz Span 100.000 MHz

51015202530

Abs

ns2:AM Delay /M 55.6 kHz 5 ns/ Ref 0 s

1

Ch2: Mkr1 25.370 MHz-16.45 ns

M1

1

Ch1: Mkr1 25.480 MHz-16.31 ns

Ext detX input

Ext detY input

DUT

Detectors

HP 871XCscalar orvectornetworkanalyzer

Normalize without DUT first,then measure with DUT

Powersplitter


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Figure 31 sh ows the different RF-port VSWR resu lts obtained witha broadband load versus th e IF t ermination t hat would be used inactua l operat ion. The lower tr ace was measur ed with th e IF port

terminated with a 50-ohm load. The upper trace was term inatedwith a filter an d th en a 50-ohm load. The effect of mismatchbetween th e mixer and filter is readily apparen t. Usu ally, themismat ch is even worse in th e stopband where th e mat ch istypically quite poor. This is a very common operating environmentfor a mixer. This measu rement was done using narr owbanddetection.

Figure 31.ReflectionMeasurement

The measu remen ts of VSWR on t he LO and IF ports ar e verysimilar. For IF-port SWR, the RF port sh ould be term inat ed witha ma tched impedance and th e LO signal should be applied at itsnorma l operat ing level. For the LO port VSWR, th e RF an d IF portsshould both be term inat ed in conditions similar to what will bepresent during normal operation.

Testing converter s an d tu ners is easier since ther e are only twoports an d th e LO will already be a t t he corr ect power level. Thesame r equirement for proper ter minat ion of either th e RF or IFport also applies.


RF Port SWR

1:Reflection &MSWR 0.1 / Ref 1.00 C

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

AbsCh1

1

M1

IF filter

50-ohmload


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IsolationMeasurements

Isolat ion is a measu re of the leaka ge or feedthr ough from one portto an other. The more isolation a mixer provides, the lower t hefeedthr ough will be. Isolat ion is a t ra nsm ission measu remen t since

the st imulus is applied at one port a nd th e response is measu red atan other. However, you ar e measu ring a signal at t he sam efrequen cy as t he stimu lus, and n ot the frequency-shifted signal.As is the case for reflection measurements, isolation/feedthrough isdependent a lso on the ter minat ion a nd LO power on the ports n otbeing tested.

Three m ain isolat ion t erms ar e of interest : RF-to-IF feedthr ough,LO-to-IF feedthrough a nd LO-to-RF feedthrough. The lat ter term isoften importan t if the m ixer is n ear th e front end of the receiver ortun er tha t is connected to a cable or an tenn a. In th is case,significant LO leakage could cause interference in other frequencybands.

Figure 32.Isolation

LO to IFIsolation

RFFeedthrough

V = 0RF(1)

VLO(2)

LO(3)V

LO toRFIsolation V = 0IF(3)

VLO(2)

LO(1)V

V = 0LO(2)

VRF(1)

RF(3)V

VRF

VIF

VLO

1

VLO

VLO

VRF

2

3

Both th e LO isolation term s a re sm all for single- and double-balanced mixers. RF t o IF feedthrough is a lso low in double-balanced mixers. RF feedth rough is usu ally less of a pr oblem tha nLO to IF feedthr ough becau se in m ost cases, the LO p ower level issignifican tly higher t ha n t he RF power level.

RF-to-IF feedthrough is measured with the sa me instrum ents a ndsetu p us ed to mea sur e conver sion loss (except frequ ency-offsetmode is not needed when using a vector n etwork an alyzer su ch a sthe H P 8753E). Because th e source an d receiver frequencies are th esame, set up th e network ana lyzer to use narr owband det ection tomake the measurement. The only difference in the hardware

configurat ion is t ha t t he IF filter needs t o be removed so the RFfeedthr ough will not be filtered out. Depending on which n etworkanalyzer is used, a frequency-response calibration or full two-portcalibration sh ould be perform ed to impr ove mea sur ement accura cy.


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FeedthroughMeasuremen t of Converters and Tune rs

36

The RF feedthr ough level is very depen dent on th e level of appliedLO signal. For this reason, the measu remen t should be ma de withthe LO signal present a t its norma l operat ing level.

LO feedthrough measu rements ar e made the sa me way asdescribed above. Remember to termina te th e unu sed port with t heproper impedan ce. The LO power level used for t he m easur ementshould be the same level th at is used du ring actua l operat ion of th emixer.

Figure 33.RF Feedthrough


RF Feedthrough

1:Transmission/M Log Mag 2.0 dB/ Ref 0.00 dB

18

16

14

12

10

8

6

4

2

Abs

dB

Ch1

1

Remove IF filter

Apply proper LO

power

The procedur e for m easur ing RF t o IF feedthr ough of a converteror tu ner is identical t o tha t of a mixer. Since an IF filter is oftenincluded in the DUT, th e RF leakage is often very small. Aninstrument with high dynamic ran ge should be used to make themeasurement, an d averaging and a reduced IF bandwidth can alsobe required t o lower t he n oise floor of the test instru ment .

Measur ing LO leaka ge requires a different technique and t estinstr umen t. Since the LO port is t ypically ina ccessible, a n orm alnetwork-analyzer transmission measurement cannot be used. Thebest instru ment t o use for mea surin g LO leakage is a spectruman alyzer. The spectrum an alyzer is conn ected to either t he RF or IFport an d tu ned t o the frequency of the LO signal. Again, th e un usedport sh ould be termina ted. If the LO frequency of the converter ortun er is known to with in a few kH z, th en th e tun ed-receiver modeof a n etwork an alyzer su ch as th e HP 8753E could also be used tomake this measurement.


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Figure 34.Feedthrough of Tuners an dConverters

DUT

ATTEN 10dB CNT26 . 00dBm

RL 0dBm 10dB/ 5. 14983 GHz

MKR5. 149850 GHz

CENTER 5. 150075GHZ SPAN 5 . 000MHz

RBW 30kHz VBW 30 kHZ SWP50. 0ms

26 . 00 dBm

Spectrum analyzer

LO

The plot in F igure 34 sh ows a fairly high level of LO leak agepresent at th e out put of a converter. If a scalar n etwork an alyzerwere used to measu re conversion gain of this device, th is signa l

could cause some measu remen t ina ccura cy becau se th e an alyzer sbroadband d etectors would include the LO a s well as t he desired IFsignal. In cases such as this, additional filtering should be added t oreduce th e level of th e LO feedth rough wh en m easur ing conversiongain.

Up t o this point, t he possible test equipment configura tions h avebeen discussed, along with linear measu rement s such as conversionloss, relative phase, reflection, and isolation. One configuration thatha s not been t horoughly discussed is the u pconversion/downconver sion configura tion. Th is configura tion is u seful fortesting FTDs on vector network an alyzers tha t do not ha ve afrequen cy offset featu re. Fu rt herm ore, this configura tion alongwith external error-term manipulation via an external computercan yield very accur at e, lower ripple magnitu de an d pha semeasurements.


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Alth ough th is section describes a more a ccurat e technique formeasu ring absolute group delay, the sa me pr inciples can be appliedfor ma king more a ccura te, lower r ipple phase-related

measu remen ts such as relat ive pha se and relat ive group delay, aswell as very accur ate conversion loss m easur ement s.

This section provides more advan ced techniques a nd t he operat orshould un derstand network-analyzer dat a tran sfer an d error-termman ipulation. An extern al compu ter is required. Unlike th eprevious technique for a bsolute group d elay (in which t he DUTwas mea sur ed against a single, absolute group-delay valuerepresent at ion of a golden mixer calibrat ion st an dar d), theaccur acy of this t echn ique depends on first fully chara cterizing th ecalibrat ion m ixer st an dar d an d th en u sing vector err or correctionto calibrate th e test system.

This t echn ique uses an upconversion/downconversion configura tion.

It consists of a t wo-step process.

First , three mixer calibration stan dar ds are char acterized. Mixerpairs a re measu red, in which one mixer is used as a n u pconverteran d the other a s a downconverter. This ensur es the sam e frequencyat th e network a na lyzers source and receiver ports. The da ta fileof each individua l mixer is m at hema tically extracted from t hemeasu red mixer-pair data . The individua l mixer da ta file is storedin the compu ter for later use in th e calibration stage.

Second, one of these calibrat ion m ixers is u sed to calibrat e th e testsystem. During t he calibrat ion, th e calibration mixer is the th roughsta nda rd. Since the calibrat ion m ixer h as been fully chara cterized,its effects can be rem oved.

The effects of the t hrough sta nda rd ar e removed by modifyingerror-terms extra cted from th e network a na lyzer. The calibra tionmixer data files along with additional measur ed data a re used tomodify the extr acted error-terms. These m odified error t erms a reimported back into the n etwork ana lyzer where t he calibrat ioncalculat ions ar e perform ed. Therefore, th e net work an alyzerperform all the error-correction calculat ions, but th e externa lcompu ter is u sed to modify th e error ter ms.

Three different levels of error correction can be achieved dependingon how much accuracy is needed. The first level removes frequency-response error, the second level removes frequency response andsource mismatch, an d th e th ird level removes frequency response,

source mismatch, an d receiver misma tch. The complexity an d th estep s requ ired in crease for each level of accura cy. Ea ch will bediscussed in det ail.

Lets begin with a discuss ion of the first st ep.

Absolute GroupDe lay A More

Accu rate, Lowe rRipple Tech niqu e

38


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When making FTD measu rements th e measurement accura cy ofa n etwork an alyzer depends u pon knowing and correcting for thereference (or calibration mixer stan dar d) used during th e

calibration process. The calibration mixer, along with additionalsta nda rds, is used to correct for err ors with in the t est system. Thecalibrat ion mixer also provides the necessary frequency tran slationrequired du ring th e system calibrat ion pr ocess. In order to use th ecalibration m ixer as a sta ndard, i t mu st ha ve certain a ttributes.See Append ix A for a list of at tribut es required in the calibrationmixer.

There is no direct way t o measur e the a bsolute group-delaychara cteristics of a m ixer. Instead, by measu ring th ree similarmixers in pairs, t he simple ma them atical technique for solvingthr ee equa tions with t hr ee unkn owns can be applied. Thisappr oach h as been u sed by Hewlett -Pa ckard for ma ny years forchara cterizing th e pha se n oise of unkn own oscillat ors. This

technique can be u sed to extract t he a bsolute response of eachmixer, thu s providing the r esponse informa tion needed when t hemixer is used as a calibrat ion st an dar d. While the mat h is basic,the a ctual measu remen t process is not. Making vector networkan alyzer m easur ement s often requires correction of errors withinthe m easur ement s ystem. This techn ique is fur th er complicat ed bythe frequen cy-tran slation process th at is occurr ing. With p ropermeasurement techniques and a n u nderstanding of the causes ofmeasu remen t err or, the calibrat ion mixer can be accur atelychara cterized and used as a sta ndard.

The char acterization process consists of measu ring t wo mixers,an u pconverter a nd a downconverter in order to obtain t he sam eRF input a nd outpu t frequency at t he network an alyzer s test

ports. The vector network an alyzer supplies and mea sur es theseRF signals. A common LO sour ce is sha red between the two mixers.A typical measur ement configur at ion of a two-mixer pair a nd a nIF filter is shown in F igur e 35.

Figure 35.TypicalConfigurationfor a Mixer Pair

LO input

IF filter

LO input

RF input RF output

IFoutput

IFinput

Two key component s ar e required when ma king upconverter an ddownconverter measurements on the mixer pairs:

IF filter: The fun ction of the IF filter is to separa te t he desiredmixing product from the u ndesired pr oduct.

Characterizing MixerCalibrat ion Stand ards


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Attenuators : These ar e used to insur e well-matched ports on theindividua l mixers an d the t est system. Using sufficient at tenu at ionan d vector err or corr ection dur ing th e chara cterizat ion p rocess will

redu ce these effects. You should a void excessive at ten ua tion sincethis may introduce unwan ted noise in the m easurement system,an d possibly making th e errors larger tha n a system with lessatt enua tion. The value of attenu at ion t o use will depend on yourselected configura tion. To review the effects of at ten ua tion, pleaserefer back to the Magnitude Measurements section. Anoth erappr oach would be to replace the at tenu at ion with an isolatorbetween th e first mixer an d th e filter. The isolat or may be a goodway to redu ce mismatch effects wh ile mainta ining a low insert ion-loss measurement path in the system.

For convenience, each of the three calibration mixers will belabeled A, B, and C. Each m ixer is a mixer assem bly. For examp le,mixer A is an assem bly consisting of a mixer an d thr ee atten ua tors,

one conn ected to each port of th e mixer. The RF port of th e mixerA will be referr ed to as ARF, the IF port as AIF, and the ALO port a sALO. The S-para meter for a r eflected signal from th e RF port willbe referred to as S11ARF. All measu red S-param eters will consistof a complex number with real an d imaginar y values. The measu re-ment group delay of mixer A will be represented by GDA. The groupdelay is specified by GDA with a value in t he un its of time.To simplify the equa tions th roughout t his pa per, refer t oFigure 36 as a guide.

Labeling Conventions

Figure 36.Diagram of theMixer Assem bly

S22A IFS11A RF

S21A

GDA

IF outputRF input =

AMixer A

LO input

AIFRFA

A LO

The symbol F will refer to all the components, connected betweenthe t wo mixers, such as t he isolator, IF filter an d an y adapt ers.Port 1 of F is the in put to the isolator, so a S 11Fmeasurementrefers to th e S-param eter of the r eflected signal a t t he isolat ors

port. Refer t o Figur e 37 for a diagra m of the filter a ssembly andequivalent schema tic.

Figure 37.Diagram of theFilter Assembly

IF filter F

IF outputIF input

Isolator

=S22FS11F

S21F

GDF


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To characterize each mixer-pair, the test configuration is as shownin Figure 38. A good mea surem ent pr actice is to mainta in shortdistan ces between th e component s an d th e VNA. Shorter cable

length s resu lt in less r ipple effects. If possible, connect t estcomponen ts d irectly to port 2 of th e VNA an d us e a cable fromport 1 to complete th e measu remen t pa th (see Figure 38). Thisconfigura tion will minimize ripple in S21 mea surem ents caused bythe receiver misma tch of th e an alyzer.

Figure 38.Diagram of the SuggestedComponentConnections

MeasurementAssumptions


A B

F

CablePort 1 Port 2

RF

LO

IF

RF

LO

IF

Maintain shortdistance here

Pr ior to mixer cha ra cterization, several measu remen ts n eed to beperformed t o verify a few basic assu mpt ions an d un dersta nd th efactors th at could influence the accura cy of th e measu rement s.

Upconvers ion = Downconvers ion

When mea sur ing the m ixer pa irs, one mixer will be used as bothan u pconverter a nd a downconverter. It is assu med th at t his mixerha s th e sam e chara cteristics in both m odes of opera tion or A =A*,where A is the tr an smission response in th e upconverter modeand A* is the r esponse in the downconverter mode. A test can beperformed to verify this a ssum ption as a first order a pproxima tion.Since there is no direct technique for m easur ing absolut e pha se of amixer, th is measu remen t can only be made for the conversion lossofA or | A | . Measur ement s can be ma de on individua l mixers usinga scalar network a na lyzer or a VNA th at is capa ble of performingfrequen cy-offset mea sur ement s, such as the HP 8720D Option 089an d H P 8753E. The conversion loss of th e mixers sh ould bemeasu red in both th e upconverter an d downconverter modes of

operat ion. If th e assum ption that | A | = | A*| is true, then th echara cterization pr ocedure can pr oceed. If the equa lity does nothold, th en th e magnitu de inform ation gather ed here should be usedfor th e ma gnitude pa rt of th e calibrat ion m ixer s char acterizationdata (See Absolute Group Delay and Conversion L oss Calculat ions).


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Proper Filtering

Depending on the level of filtering and isolation provided in themeasurement system, some spurious signals may be measured by

the n etwork an alyzer. It is beneficial to mak e an initial test on th eVNA to verify th e effects of spu rs in th e mea sur ed res ponse.Performing mea sur ements on a pa ir of mixers over the requ iredRF a nd LO frequency ran ges (using th e conn ections shown inFigure 37) is required to determined if additional filtering orisolation is needed in t he test system. This measu remen t will alsobe used to optimize th e VNA settings for IF BW and IF a veraging.

LO Effects

If measurem ents a t different LO frequen cies are requ ired, it isnecessary to verify the frequency response of the systemcomponent s across th e LO range. It is assum ed tha t th e LO drivelevel to the mixer is consta nt as t he LO frequency is steppedthr ough its specified ra nge. If the LO drive level cha ngessufficiently to alter t he mixer perform an ce then adjustm ents m aybe required to minimize these effects. Mismatch effects may alsocha nge th e LO drive to the m ixers, th erefore t he a ddition ofpadding an d isolation might be required to mainta in the a ccura cyof the m easur ement s. The LO sour ce might a lso requiream plificat ion t o sufficient ly drive the t wo mixers.

The following measu remen ts can be performed using t he VNA inman ua l operat ion. Some of th e repetitive measu remen ts ma y beeasily aut omated by a computer program such as H P VEE.In genera l, a compu ter will be required to retrieve measur ed dataan d error t erms from the n etwork an alyzer. The compu ter will alsobe used to archive results to a disk for post-processing.

Measureme nt of the Filter Assem bly

It will be necessary t o cha racterize th e filter a ssembly, F, bymeasu ring all four S-param eters. This informat ion is used laterwhen extr acting th e frequen cy response of th e individua l mixers.To characterize the filter assembly, use a standard full two-portcalibration at th e IF frequencies. This test dat a sh ould also cont ainthe group-delay dat a for t his assembly. Store all measur ed data toa compu ter file for p ost-processing.

Test Proc edure s forCharacterizing theCalibration Mixers


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Measureme nt of Mixer Pairs

Measur ement s are n ow made on th e set of mixer pairs. If th e mixersare well mat ched, we can consider t he t ran smission r esponse, S21, of

each mixer to be independent of th e other, as well as indepen dent ofth e IF filter s resp onse. In th is way, we can describe th e overa ll S21 asthe pr oduct of the t wo mixers a nd t he filter:

If A = S21A of Mixer A, B = S 21B, and C = S21C; F represents t hemeasured S21F of the filter, an d X, Y an d Z represent the m easur edS21 of mixer pa irs, AB, CB, an d CA*, respectively, th en th e r esponseof each can be expressed as

X=A . F . B ; Y = C . F . B ; and Z= C . F . A*

where A* is th e respons e of Mixer A in t he downconvert er position.This assu mes t he LO frequency and filtering is set t o provideupconversion at the first m ixer position at port 1 of the VNA.

The system can also be configured to operate with downconversion inthe first mixer position. Figure 39 shows the configuration formeasurements made on X.

Figure 39.Representationof a Mixer Pair A

FB

X

In order to measu re th e mixer pairs, a sta nda rd full two-portcalibrat ion is performed over t he RF frequency range. Figure 40shows the configuration for the VNA calibration. Since the mixersare configured in a n upconversion an d downconversion pair, theVNA supplies and mea sures t he sam e RF frequency at both ports.An RF ban dpass filter is included in t he calibrat ion a nd is usedlater to filter out an y spur ious signals, such a s LO leakage, whichmight ent er th e measur ement port of th e VNA.

Figure 40.TypicalConfigurationfor Calibratingthe Test System


A B

F

Calibrationplane

Port 1 Port 2


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During the calibration a nd m easurement, i t is best t o use the st epsweep mode. This sweep m ode verifies pha se lock at ea chmeasu remen t point an d yields the h ighest accur acy. If th e mixer

pair is a noninserta ble device, you sh ould calibrate th e VNA withthe swap-equal-ada pter techn ique. Please refer to th e VNA userguide for the details of this calibration procedure.

Required Mixer-Pair Data Files

Once the t est system is calibrated, each set of mixer pairs ismeasu red, as shown in F igure 41, using th e full two-portcalibration. A set of measurements will be recorded for each mixerpair X, Y an d Z. The measu red tr an smission response of each set issaved to a file for dat a processing. As a minimum , the data filesshould contain t he group delay and S21 (both m agnitu de andpha se) for each set of measu remen ts. As discussed ea rlier, car eshould be taken in selecting th e appropriate group-delay apert ur e(please refer to th e Group Delay section for more in form at ion). In

some cases, a wider a pertu re ma y be required in order to smoothout spur ious r esponses caused by improper IF a nd RF filtering inthe t est system. Also, adjusting th e IF BW and IF a veraging canaid in reducing the spu rious signals and n oise present in th emeasured data.

Figure 41.TypicalConfigurationfor Measu ringthe Mixer-Pairs


A B

F

Port 1 Port 2

LO

Optional Mixer-Pair Da ta Files

Saving i

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HP-AN1287-7_Improving Network Analyzer Measurements of Frequency-TranslatingDevices