MIXERSweb.rfoe.net:8000/ZILIAOXIAZAI/FYG/MITEQ/Compon/mixers/c... · 2010. 10. 17. · Application Guidelines 2 QUICK REFERENCE Single-, Double- and Triple-Balanced Mixers 3 Biasable,

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MIXERSMICROWAVE AND MILLIMETER WAVE

• Single-, Double-, and Triple-BalancedMixers for Double-Sideband Up/DownConverting and Demodulation

• MESFET for High IP3• Mixer/Amplifiers for Single or

Multichannel Applications• Mixer Subsystems

SPECIAL MIXER PRODUCTS

ISO 9001REGISTERED COMPANY

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http://www.miteq.com

CONTENTS PAGE

INTRODUCTIONApplication Guidelines 2

QUICK REFERENCESingle-, Double- and Triple-Balanced Mixers 3Biasable, Harmonic and MESFET Mixers 4

DETAILED DATA SHEETSDouble Balanced – Ultra-Broadband 5Double Balanced – High Isolation 27Triple Balanced – Microwave IF 53Biasable Mixers – Low LO Power 67MESFET – High Dynamic Range 69Even Harmonic – 1/2 LO 87Waveguide Mixers 91Multichannel Assemblies 97

GENERAL INFORMATIONMixer Terminology 103Additional Literature 104Double-Sideband Mixer Circuits 105Double-Sideband Mixer Subsystems 106Application Data Needed to Specify Downconverters and Demodulators 107Fax-Back Form 108

QUESTIONS AND ANSWERSBalanced Schottky Diode Mixers 109MESFET Mixers 114

TECHNICAL ARTICLEFundamental, Harmonic and Sampling MESFET Mixer Circuits 118

MIXER DESIGN REFERENCES 124

CROSS-REFERENCESAvantek 125Watkins-Johnson 125RHG 125

PRODUCT INDEX 126

TABLE OF CONTENTS



2

SCHOTTKY/MESFET PRODUCTS

This detailed, double-sideband mixer catalog summarizes the important input, output and transfer characteristicsof these devices. A short-form catalog is also available upon request, that describes three other special mixerproduct groups: image rejection products, single-sideband modulator products, and millimeter-wave products. Theshort-form catalog is also published on our web site: http://www.miteq.com. We look forward to helping you choosethe best mixer from our increasing core of state-of-the-art products, so that your system will be more competitivein today’s demanding marketplace. Most importantly, we are committed to satisfying not only the written technicalspecifications of any new product, but to ensure that the product satisfies its intended application requirements.

Don Neuf, Department HeadSteve Spohrer, Product Line ManagerMary Becker, Sales Manager

CRITICAL SPECIFICATIONS BEST MODELS CIRCUIT DESCRIPTIONLow cost DB0218, TB0218 2 to 18 GHz double/triple balancedLimited LO power, < 0 dBm, good RF VSWR SBB0218 Biasable 2 to 18 GHz bridge mixer

-10 to +10 dBm LO

+5 dBm 1 dB input RF compression DB/DM, “L” option diode Double balanced +10 dBm typical LO Schottky diodes

+15 dBm 1 dB input RF compression TB...H option diode Triple balanced +20 dBm typical LO Schottky diodes

+23 dBm 1 dB input RF compression SBF Double balanced +23 dBm typical LO MESFETS

Highest IP3 termination insensitive, +36 dBm input DBF Double balanced MESFET, LO = +26 dBm typical

Even harmonic (1/2 LO) SBE Back-to-back ring quads

Low DC output offset for demodulator applications DB, DM Double balanced tapered or tuned balun

High single-tone m x n rejection TB, DM “H” diode Triple balanced, Schottky diode

High LO AM noise rejection, low conversion loss DM octave units Double balanced, 40 dB typical LO-to-RF isolation

System mismatch immunity TIM Quadrature coupled Schottky

Phase or amplitude matched multichannel DA4, 4 channels Double-balanced with LO splitter,DSS, 5 channels IF amp, BIT

DOUBLE-SIDEBAND MIXER APPLICATION GUIDELINES

+10 +15 +20 +25

+25

+20

+15

+10

+5

0

LO POWER (dBm)

COM

PRES

SION

(dBm

)

MIXER COMPRESSION (INPUT)

L

M

H

MESFETSCHOTTKY DIODES



MIXER TERMINOLOGY

The subject of mixers is often confused by the variety of different technical terms that often describe thesame piece of hardware. For example, the common double-balanced mixer is useful as a downconverter,demodulator, upconverter or modulator. Other adjectives are also used to further subdivide each categorysuch as linear, saturated, double sideband etc. Ultimately, it is the relationship between the two input anddesired output frequency bands and powers that uniquely specify each device classification. During our dis-cussion, we will refer to the two input signal bands of any mixer as f1 and f2 (in increasing frequency) withrespective powers P1 and P2. In this manner, any confusion defining the IF, RF, LO for up- and downcon-version is avoided. The two output bands are f3 = (f1 - f2) or difference frequency and f4 = (f1 + f2) or sumfrequency. In general, downconverters and demodulators are separated in classification from upconvert-ers and modulators by the obvious fact that the output frequency (f3, f4) of the latter group is alwaysgreater than f1, whereas f3 is less than f2 and f1 for downconverters/demodulators. These two groupsare further subdivided into either single- or double-sideband responses. An example of a single-sidebanddownconverter would be the image rejection mixer. A single-sideband upconverter rejects either outputupper or lower sideband (i.e., f2 + 1 or f2 - f1). The figure and table below show how all of our mixer prod-ucts are defined in the available catalogs (see cover reproduction next page).

103

f1 f3

f2

f2 > f1

(Note 1)

|P1 or P2| > 5 dB|P1 or P2| > +10 dBm min.

f3 = f2 - f1

f4 = f2 + f1

INPUTS OUTPUTS

f4

1.Double-Sideband Mixers ...................... ❏ No image or sideband rejection

Upconverter ............................... ❏ f2/f1 > 2 using f3, or f4 = output

Downconverter ........................... ❏ f3 min. > 0 and f2/f1 < 2

Demodulator............................... ❏ f3 min. = DC (i.e., f2 = f1)

2. Single-Sideband Downconverters........ ❏ Image rejection required

Image Rejection .......................... ❏ f3 min. > 0 and f2/f1< 2

I/Q Demodulator......................... ❏ f3 min. = DC (i.e., f2 = f1)

3. Single-Sideband Upconverters............ ❏ f2/f1 > 2

I/Q Modulator ............................ ❏ f3 and f4 required and f1 = 0

Modulation Driven........................ ❏ P2 < P1Carrier Driven ............................. ❏ P2 > P1SSB Upconverter......................... ❏ f3 or f4 required and f1 min. is not = 0

4. Low-Noise / Millimeter Subsystems ... ❏ f1 or f2 or f3 or f4 > 30 GHz

Low Noise .................................. ❏ SSB noise figure < 5 dB

Note 1. When f2 or f1 is each a range or frequencies, use their midband values in the table formulas above.

MIXER MODEL SELECTION GUIDELINE



DOUBLE-SIDEBAND MIXER CIRCUITS

105

F

SIGNALINPUT

LO INPUT

SIGNALOUTPUT

SIGNALINPUT

LO INPUT

SIGNALOUTPUT

SIGNALINPUT

SIGNALOUTPUT

SIGNALOUTPUT

SIGNALINPUT

LO INPUT

SIGNALOUTPUT

LO INPUT

SIGNALINPUT

LO

RF

IF

LOINPUT

X XZ Z Y W

W WY Y Z X

RFIN

IF

LO INPUT

X XZ Z Y W

W WY Y Z X

LOINPUT

SIGNALOUTPUT

RF

LO

RF IF

DM

LO

IF

RF

OUT

DM

SBW SERIESWaveguide RF, SMA LO/IF

SBB SERIESDC Biasable, Low LO Power

DB, DM SERIESGeneral Purpose

SBE SERIESEven Harmonic(1/2 LO)

TB, TBR SERIESBest Spurs,Overlap RF/IF

TIM SERIESLow VSWR,Load Insensitive

SF SERIESGeneral PurposeHigh IP3/LO Ratio

SBF SERIES+30 dBm, IP3

SRD SERIESSampling,0.5 TO 1.5 GHz LO

DBF SERIES+36 dBm, IP3



DOUBLE-SIDEBAND MIXER SUBSYSTEMS

106

DA4 SERIES

DSS SERIES

J3

J15J6BAND 2

CHANNEL D

BAND 3IF OUTCHANNEL D

DIPLEXER PHASE ADJ GAIN ADJ

J5

J17J10BAND 2

CHANNEL C

BAND 3IF OUTCHANNEL C


J2

J14J7BAND 2

CHANNEL B

BAND 3IF OUTCHANNEL B


J1 SW/LIM.

J13

PHASE AND GAIN BOARD

J6BAND 2

CHANNEL A

BAND 3IF OUTCHANNEL A


INPUT

LOINPUT

BITINPUT

EXTERNAL ADJUST (TYPICAL 5 PLACES)

EXTERNAL ADJUST (TYPICAL 5 PLACES)

J4

J16J9BAND 2

CHANNEL V

BAND 3IF OUTCHANNEL V


MIXER ASSEMBLY

Channel 1 Output

Channel 2 Output

IF LNADIPLEXERMIXERDIODE LIMITERDOUBLE-BALANCED

Channel 3 Output

Channel 4 Output

Channel 1 Input

Channel 2 Input

Channel 3 Input

Channel 4 Input

LO Input LO Power Divider

BASIC FOUR-CHANNEL DIRECTION-FINDING FRONT END

EXTENDED FEATURE FIVE-CHANNEL DIRECTION-FINDING FRONT END WITH BACK-LOBE COVERAGE



107

APPLICATION DATA NEEDED TO SPECIFYDOWNCONVERTERS AND DEMODULATORS

STEP 1: ❏ P1, lower band (f1 = ___ GHz to ___ GHz)Enter power and frequency of two RF inputs. ❏ P2, upper band (f2 = ___ GHz to ___ GHz)

STEP 2: ❏ f3 “difference” f2 - f1 rangesEnter desired output frequency ranges. ❏ f4 “sum” of f2 + f1 ranges

❏ “Both” f3 and f4 range limits

STEP 3: ❏ P1 dB ❏ IP3 ❏ IP2

Enter dynamic range parameters for application. ❏ Conversion loss❏ SFDR (dB)

STEP 4:Is the LO frequency source specified or are the merits of ❏ Fundamental LOharmonic or sampling mixers a consideration? ❏ 1/2 or 1/3 LO

❏ Sampling mixer

STEP 5: ❏ Block ❏ LO > RF ❏ LO < RFIs this requirement for a tracking LO/RF or a fixed LO ❏ Tracking ❏ LO > RF ❏ LO < RFblock downconverter? ❏ Fixed-tuned

RF, LO, IF

STEP 6:For this “tracking” LO application, is the usage for a ❏ Downconverterdownconverter or demodulator? ❏ Demodulator

STEP 7:

Identify all single-tone spurious products that can yield ❏ 1 ❏ 2 ❏ 3 ❏ M (LO)false outputs. ❏ 1 ❏ 2 ❏ 3 ❏ N (RF)

STEP 8:Maximum input RF and minimum LO power needed for ❏ M ❏ N (MLO ± NRF)desired SFDR. ❏ RF dBm ❏ LO dBm

STEP 9: ❏ Image signal noise rejection or reflection problemsPotential system application? ❏ IP3 versus IF VSWR

❏ LO AM noise rejection

STEP 10: ❏ Cost-driven applicationChoosing the circuit and semiconductor? ❏ Performance driven

❏ Mixed



108

INPUTSRF frequency _______ GHz to _______ GHzRF input 1 dB compression point _______ dBm, min.RF power

Maximum pulse _______ dBm _______ µsMaximum CW _______ dBm

RF VSWR (50 ohm ref.) _______ RatioLO frequency _______ GHz to _______ GHzLO power _______ dBm to _______ dBmLO VSWR (50 ohm ref.) _______ RatioDC voltage (current) _______ Volts _______ mA

OUTPUTRF frequency _______ GHz to _______ GHzRF VSWR (50 ohm ref.) _______ RatioSideband rejection (when required) ❏ Lower (difference)

❏ Upper (sum)DC offset, max. (demodulator) _______ mV

TRANSFER CHARACTERISTICSRF to IF gain (conversion loss) _______ dBSSB noise figure _______ dBImage rejection (when required) _______ dBLO to RF isolation _______ dBLO to IF isolation _______ dBInput two-tone IP3 _______ dBmSingle-tone intermod at -10 dBm input _______ dBc (MLO±NRF) _______ M _______ N

ENVIRONMENTAL SPECIFICATIONSTemperature, operating _______ to _______ degrees CHumidity _______ with condensationVibration _______ to g’s

QUALITY ASSURANCETest data supplied at 25° C _______ Gain/loss _______ Noise figure

_______ Isolation _______ VSWR_____________________________ Other

SOFTWARE REQUIRED ___ ATP ___ MTBF ___ QTP

APPLICATION❏ Downconverter

❏ Double sideband❏ Image rejection❏ With LNA❏ With IF amplifier

❏ Demodulator❏ Biphase❏ I/Q

❏ Upconverter❏ Double sideband❏ Single sideband

❏ Modulator❏ BPSK (biphase)❏ QPSK (I/Q phase)❏ QAM (linear I/Q)❏ Vector (phase shifter)

❏ Analog control❏ Digital control

❏ Integrated subsystem❏ 2 channels❏ 4 channels❏ With LNA❏ With LO source❏ Phase matched❏ Gain matched❏ BIT (built-in-test)❏ IF amplifiers, filters

MIXER CIRCUIT❏ Single balanced❏ Double balanced❏ Triple balanced❏ DC biasable❏ Schottky diodes❏ MESFETs❏ Fundamental LO❏ Harmonic LO❏ Sampling LO

MIXER SPECIFICATION GUIDELINES

TEL.: (631) 439-9423FAX: (631) 436-7430E-MAIL: [email protected] DATE

SPECIAL MIXER PRODUCTS

COMPANY

CONTACT

TEL.

FAX

ADDRESS



109

BALANCED SCHOTTKY DIODE MIXERS

Questions and Answers about...

SINGLE-, DOUBLE- AND TRIPLE-BALANCED SCHOTTKY DIODEMIXERS

Q1: What are the differences between single- and double-balanced mixers?

A1: Before explaining this difference we should mention that a one-diode or unbalanced mixer is oftenused in economical receiver front ends, where tunable or fixed bandpass filters can easily separate the LO,RF and IF energy coupled to and from the diode. Early wideband receivers utilized two diodes in a single-bal-anced mixer circuit with a 90° hybrid to couple RF and LO power to a pair of diodes. This technique allowedoverlapping LO and RF bandwidths without filters, but the isolation was dependent on how well the diodeswere impedance matched. Broadband 180° hybrid balanced mixers eliminated this problem. The figurebelow shows the equivalent circuit and the single-tone intermodulation table of the MITEQ modelSBB0618LA1 biasable single-balanced mixer with 0 dBm LO applied to the in-phase port of the 180° hybridand -10 dBm RF at the delta port. In this mode of operation only the RF energy is balanced or applied outof phase to each diode, with a subsequent reduction or cancellation of even harmonic mixing products (i.e.,LO ± 2RF, LO ± 4RF).

Alternately, in any single-balanced mixer one could choose to apply the LO to the 180° port andobserve suppression of the even harmonic LO products instead (2LO ± RF, 4LO ± RF etc.). The circuit andresulting products are shown below:

Both single-balanced mixer configurations, however, suppress any RF or noise energy that may be pre-sent with the LO (common mode or noise rejection). In addition, single-balanced mixer circuits are particu-larly easy to bias and monitor the diode currents.

Alternately, one could also make an easily biasable single-balanced mixer with multioctave bandwidthcoverage using a diode bridge (shown below). This appears very similar to the ring double-balanced mixer(also shown), but the key difference is that all even order products are canceled in the output of the double-balanced, whereas only even products of the RF are canceled in the single-balanced circuit. The MITEQ modelSBB0218LR5 uses this circuitry for RF coverage from 2 to 18 GHz and 2 to 26 GHz, however the IF out-put cannot overlap the RF coverage.

0/180°

∑

SINGLE-BALANCED RF/LO PORT

RF, LO IF 3 45 46 412 38 42 411 0 17 15

1 2 3

∆ = RF (-10 dBm)∑ = LO (0 dBm)

RF

HA

RM

ON

IC

LO HARMONIC

3 49 53 402 28 44 281 0 30 15

1 2 3

∆ = LO (0 dBm)∑ = RF (-10 dBm)

RF

HA

RM

ON

IC

LO HARMONIC

none L ± ne R

none L ± no R

RF INPUT

LO INPUT

SINGLE BALANCED (Bridge)

IF OUTPUT

RF INPUT

no L ± ne RLO INPUT

DOUBLE BALANCED (Ring)

ne L

± n

o R

IF OUTPUT

no L

± n

o R

ne L ± ne R



110


The double-balanced mixer circuit provides mutual isolation of LO, RF and IF energy, without filters,because of the combined properties of the ring diode circuit and wideband baluns. This results in suppres-sion of all even-order harmonic mixing products of both the LO and RF (i.e., 2LO ± RF, LO ± 2RF, 2LO ± 2RF,etc.). The double-balanced mixer, however, requires 3 dB more LO power than the two-diode single-balancedcircuit assuming, of course, that the same barrier voltage diode is used in each case.

Q2: What are the major differences between triple- (or double-double) and double-balanced mixers?

A2: The triple-balanced mixer employs two diode quads (eight junctions in total) fed by two power splittersat the RF and LO microwave baluns. The architecture allows both quads to be coupled together with mutu-al LO-to-RF isolation. The most significant advantage of this circuit is that the output IF signal is available attwo separate balanced and isolated terminals with large bandwidth (typical 0.5 to 10 GHz). The IF signal andreturn path are isolated from both the RF and LO ports, thus allowing for overlapping frequencies at all threeports. A slight disadvantage of this circuit is that it will not yield a DC IF. In contrast, the standard microwavedouble-balanced mixer often uses diplexing techniques to separate the IF signal from the LO band. As aresult, a microwave double-balanced mixer cannot support widely overlapping RF and IF frequencies whilemaintaining a DC response at the IF port. The theoretical single-tone spur product port cancellation relationsare the same for each mixer circuit, however, in practice the triple-balanced mixer and only certain designsof double-balanced mixers with high port isolation yield the best spur suppression (MITEQ DM series).

Q3: For what applications are triple-balanced mixers best suited?

A3: They are especially valuable for translating large bandwidth segments from one frequency range toanother with low intermodulation distortion. The high IF-to-LO and IF-to-RF isolation of this class of mixersmakes the conversion loss flatness much less dependent on IF frequency mismatches that almost alwaysexist at the RF and LO ports. Recently MITEQ perfected a triple-balanced 4 to 40 GHz RF/LO mixer with a0.5 to 20 GHz IF (model TB0440LW1). Many customers are using this mixer with several fixed LOs to down-convert the 26 to 40 GHz portion of the millimeter band into existing receivers in the 0.5 to 18 GHz range.This mixer is also useful for upconverting the 0.5 to 18 GHz band into a fixed Ku-band second converter, thuseliminating the image response without tunable preselectors.

RF

IF

DOUBLE-BALANCED MIXER

LO

RF

IF

TRIPLE-BALANCED MIXER

LO



111


Q4: For what applications are microwave double-balanced mixers best suited?

A4: Double-balanced mixers are most utilized in lower cost applications where there is no requirement foroverlapping RF and IF frequencies and moderate LO power is available. In addition, the DC-coupled output ofthe double-balanced design makes it a prime candidate as a building block for phase detectors, I/Q modula-tors and demodulators that operate over narrow or extremely wide bandwidths. Lower frequency torroidbalun type mixers below 2 GHz often have excellent LO-to-RF balance or isolation (40 to 50 dB) and, there-fore, function well as low offset phase demodulators or high carrier rejection I/Q modulators. Conventionalmicrowave double-balanced mixers with tapered line baluns seldom exceed 20 dB LO-to-RF isolation. TheMITEQ DM series of double-balanced mixers uses a unique balun (patent pending) that yields 30 dB minimumLO-to-RF isolation over multioctave bandwidths and 40 dB typical over communication bands (modelsDM0208LW2, DM0416LW2). In addition, the 4 to 16 GHz version has a DC to 4 GHz IF range with 30dB minimum isolation to the RF and LO ports.

Q5: How much LO power is required for double- and triple-balanced mixers?

A5: Nonbiasable double-balanced mixers with so-called “zero bias” silicon Schottky diode quads will oper-ate with +3 to +6 dBm LO power. Schottky diodes made with other junction metals and base semiconduc-tor material, such as gallium arsenide (GaAs), can operate up to +23 dBm of LO power. The required LOpower is usually determined by the desired input 1 dB compression point of the mixer and is typically spec-ified at 5 dB above this level. Triple-balanced mixers typically require 3 dB more LO power than single-quadmixers since there are twice as many diode junctions.

Q6: What is meant by single- and two-tone intermodulation products?

A6: Using amplifier terminology, a single-tone input at a frequency (f1) can produce outputs at the har-monic frequencies (2f1, 3f1, 4f1...mf1). Each harmonic has an input-to-output power slope equal to the orderof the product (m). For example, if we double the input power (3 dB increase), we expect to see the 2ndharmonic frequency increase in power by 6 dB, the 3rd by 9 dB, etc.

In the case when two nonharmonically related tones are simultaneously fed into an amplifier, the out-put spectrum becomes more complex. The two tones can mix with each other due to the nonlinear trans-fer in the amplifier, and produce new additional signals (two-tone intermodulation products) of the order m ±n. Certain products are of particular interest because no amount of input filtering can eliminate them, suchas the two-tone third order (i.e., 2f1 ± f2 and f1 ± 2f2). In this case, we recognize this as third order becausem + n = 3.

The former discussion is applicable to mixers with the additional complexity that the power supply fora mixer is not DC, but a time-varying voltage classified as the LO signal. The LO does not switch the mixerin a sinusoidal fashion, but rather as a square wave and, therefore, an additional set of harmonics are pre-sent at the output of the device. Single-tone spurs are not only harmonically related to the frequency of theRF input signal (fRF), but are also related to the harmonics of the LO input signal (fLO). The output spurioussignals are typically classified by their order (i.e., mfRF x nfLO) and represented in a spur table or m x n matrixchart.



112


The two-tone third-order outputs of a mixer are defined the same way as for an amplifier, but are usu-ally referred to the input. The LO shifts the third-order product into the IF range by the relation:

(m1 fRF1, ± m2 fRF2) ± n LO

The rules for determining the RF input to IF output power slope of each RF intermodulation productremain the same for all LO harmonics.

Q7: What determines the level of undesired single- and multitone intermodulation products in a mixer?

A7: This is a rather complex question that requires knowledge of the mixer circuit used, power ratiobetween the LO and applied RF, the order of the product, the degree of mixer circuit balance and the termi-nating impedances at each port, including out-of-band responses.

In general, mixer intermodulation products at multiples of the RF frequency are produced when theRF power level is sufficient to affect the conducting state of the diode or semiconductor used for the mixerswitching action. Intermodulation products at multiples or harmonics of the LO frequency are caused by thenonsinusoidal resistance variation of the diodes due to the exponential forward voltage/current characteris-tic. Typically, RF harmonics can be reduced by increasing the LO power and mixer circuit complexity (i.e., sin-gle, double or triple balanced). Basically, when the incoming RF is subdivided between many diodes and theindividual output IFs are recombined, each diode will generate disproportionally less intermodulation.However, each time we double the amount of diodes, both the LO power and the RF dependent intercept pow-ers will double (+3 dB).

More recently, MESFETs (metal epitaxial semiconductor field effect transistors) have been utilized forpassive mixing by applying the LO signal to the gate source junction and RF/IF to the drain source junction.The principal advantage of these mixers is much lower levels of the single-and two-tone third-order productsfor a given amount of LO power. For example, a typical Schottky diode mixer has a 3 dB greater input IP3

power level than the LO power, but the MESFET version is 10 dB higher. The MITEQ model SBF0812HI3(8 to 12 GHz) has an input IP3 level of +33 dBm when using +23 dBm LO (see catalog section 2, MESFETmixers).

Intermodulation levels in most mixers are influenced by external and internal terminating impedancesat the RF, IF and LO ports. Internally terminated and load insensitive mixers are also available, including anew MITEQ design that redirects reflected IF, RF and sum energy to separate ports (patent pending).

In general, a good practice is to:

1. Use a mixer requiring a high or medium drive level.

2. Use a mixer with the high interport isolation (i.e., good balance).

3. Have broadband resistive terminations at all ports (beyond the desired pass bands). If this is notpossible, use a broadband termination at the IF or RF port.

4. Compare each mixer design by measuring data in the system reflection environment actually encountered.



113


Q8: What are the differences between the DB and DM series of double-balanced mixers?

A8: The DB series of mixers utilize the more conventional tapered ground microstrip balun (invented in1972 at RHG by present MITEQ personnel). This balun is ideally suited for extremely broadband microwaveapplications (2 to 18 and 1 to 30 GHz), requiring modest LO-to-RF isolation (20 dB typical). The major lim-itations of this design relate to the high and unsymmetric balun leg impedances, making it difficult to achievehigh IF frequency coverage with DC capability.

More recently at MITEQ, we have perfected a new more symmetric balun which yields typical LO-to-RFisolation of 35 dB over 4 to 1 bandwidth ratios. This design is synthesized from double- and triple-tunedmicrowave filter theory and, therefore, has much higher out-of-band rejection than conventional double-bal-anced mixers. In addition, the IF capability is greatly extended. For example, the model DM0520LW1 hasan IF coverage of DC to 8 GHz with simultaneous RF and LO coverage of 5 to 20 GHz.

Q9: What advantage does the new DM and FDM mixer baluns offer for narrow RF bandwidthapplications?

A9: In general, the new balun design exhibits best performance at band center and, therefore, the nar-rower band units yield progressively better LO-to-RF isolation (45 dB typical for 10 percent bandwidth units).In addition, the spurious mixing products of these microwave units are similar to that expected fromVHS/UHF double-balanced mixers having similar isolation. The 10 percent RF bandwidth units typically havethe same RF skirt selectivity as a two-pole filter, thus reducing the system input preselection requirements(see model FDM0325HA1).

Another advantage of the FDM design is that the LO and IF coverage are relatively broadband andone can choose an IF frequency that causes the RF image response to fall on the skirt of the balun, thusyieding image rejection without the usual more expensive matched mixers and hybrid circuit topology.

Finally, special versions of the FDM design can be optimized for simultaneous image rejection andimage recovery in selected communication bands requiring relatively high IF frequencies. The typical con-version loss in this mode is 3.5 dB.

Q10: What is the principle advantage of even harmonic mixing?

A10: Aside from requiring an LO at half the normal frequency, one can achieve ultra-high (-55 to -60 dB)rejection of the LO leakage out the RF port relative to the input power. This means an input isolator can oftenbe eliminated, but more important, for linear upconverter or modulation requirements, the carrier rejectioncan be maintained at high levels. Some customers employ pairs of I/Q even harmonic up- and downcon-verter mixers for lower cost data links. The principle disadvantages of the even harmonic mixer are slightlyhigher (2 dB) conversion loss, more LO power sensitivity and, of course, doubling of the LO phase noise.



114

MESFET MIXERS

Questions and Answers about...

MESFET MIXERS

Q1: What does MESFET mean?

A1: Metal Epitaxial Semiconductor Field Effect Transistor (i.e., the gate electrode is a metal to semicon-ductor junction similar to a Schottky diode).

Q2: Why use a MESFET for mixing instead of a Schottky diode?

A2: The principal advantage of a FET mixer is a reduction in the third-order distortion, thus yieldingimproved single-tone (i.e., LO ± 3RF) and two-tone (2RF, - RF2 - LO) intermodulation products relative to aSchottky diode mixer that operates at the same LO power. The figure below illustrates the source of mixingdistortion (E/I characteristic) of a Schottky diode and a typical MESFET.

The dotted sine wave represents an applied RF signal across each semiconductor junction at theinstant that the LO voltage is zero (in the case of the MESFET curve a fixed negative bias on the gate resultsin the E/I VD curve shown). The most significant difference in the two curves is how they each compare toan ideal fixed 50 ohm resistor, shown by the dotted straight line. The resistor, of course, would yield no dis-tortion in the resulting current sine wave. We notice that the deviation from a straight line for the Schottkydiode is considerably greater, thus yielding a poor IP3 at this bias point. The measured IP3 of both mixers isthe average of the instantaneous IP3 distortion at each LO operating voltage. The input IP3 of a MESFETmixer is typically 10 dB or greater than the LO power. A general rule for Schottky diode mixers is 3 dBgreater than the LO power (the intercept powers of mixers are usually specified relative to the maximum sig-nal power at the input). The third-order intercept point of amplifiers is, therefore, relative to the output port.

-1V

100

50

0

-50

-100-0.5 0

Vo

VRF

Ioo

+0.5 +1

25ΩRESISTOR

V

SCHOTTKY DIODE

-1V

100

50

0

-50

-100-0.5 0

VDS

VRF

IDS

IDSmA

IomA

DS

+0.5 +1

25ΩRESISTOR

VgsV

-2V

MESFET

25ΩRESISTOR

25ΩRESISTOR



115

MESFET MIXERS

In addition, the linear mixing region of a Schottky diode is approximately 5 dB below the applied LO powersince both the RF signal and LO signal exist at the same terminal. However, a FET mixer, configured in thepassive mode, has the LO applied to the gate and controls the drain to source channel resistance with lowpower. RF and IF signals that are present at the drain cannot easily modulate the channel resistance and,therefore, produce an RF 1 dB compression point approximately equal to the LO power. At the lower switch-ing rates (UHF and VHF frequencies) the power difference is more dramatic (e.g., a FET switch controls +25dBm RF with microwatts of gate power).

Another difference between the MESFET and the Schottky diode is that the latter is a two-terminal deviceand, therefore, requires filters or multiple diodes and balanced circuits to separate the LO, RF and IF circuits(this is essential when signal LO and RF bandwidth overlap). The MESFET is a three-terminal device and allowsdecoupling between the LO (gate to source) and RF/IF circuitry (drain to source). Single- and double-balancedFET mixer circuits also exist.

Q3: What are the disadvantages of a MESFET mixer relative to the Schottky device?

A3: There are two, cost and LO VSWR, particularly for broad bandwidth applications. At the presenttime, the fabrication process for making 4 silicon diodes in a quad configuration is considerably less costlythan that of 4 GaAs MESFETS, therefore, if the P1 dB or IP3 requirements are moderate (up to +10 and+20 dBm respectively), a Schottky diode device is adequate. For P1 dB and IP3 of greater than +17 and+27 dBm the MESFET cost may be justified in view of the extra cost of an LO amplifier needed for the Schottkydevice. The Schottky device will typically require LO powers of +24 dBm to achieve IP3 of +27 dBm, where-as the MESFET mixer requires only +17 dBm LO power.

Another difficulty of designing octave and multioctave bandwidth MESFET mixers is impedance match-ing the FET gate circuit to a 50 ohm source impedance. Unlike the Schottky mixer, the FET gate circuit isnot driven into full conduction during LO operation, but rather swings from pinch-off to zero bias and thusalways has a high reflection coefficient. For narrow bandwidth applications, one can impedance match to thelow series resistance of the gate and achieve large voltage swings with little LO power (a desirable condition).More recently at MITEQ, we have achieved octave bandwidth operation with 15 dB or more gate return lossby employing balanced circuitry. This technique has been employed to make a series of octave high level (P1dB = +23 dBm input) MESFET mixers from 6 to 18 GHz that are suitable for a second-stage image rejectionmixer following a high-gain low-noise RF preamplifier.

Q4: What are the differences between active and passive FET mixing?

A4: Active FET mixers are typically DC biased like an amplifier and employ a dual gate or two series FETs.The LO and RF signals are applied to separate gates and the IF signal (or sum frequency) is coupled from thedrain. This circuit yields low IP3 and moderate gain with high shot noise at low IF frequencies.

Passive FET mixers have conversion loss and noise similar to Schottky diode mixers. The RF signalis applied across the drain source channel of the MESFET without any DC drain voltage. The LO signal is fedto the gate, effectively modulating the channel resistance. This produces a mixing action with the sum anddifference appearing across the drain source. External or self gate biasing is used to prevent forward gateconduction from the LO signal, however, since no average current is drawn, the main noise source in thismixing is thermal.



116

MESFET MIXERS

Q5: Are there preferred frequency ranges for MESFET mixers?

A5: No, since the advantage of their high IP3 with moderate LO power has been proven at UHF throughmillimeter bands.

Q6: Where should MESFET mixers be used?

A6: In any application requiring high dynamic range. For example, in receiver front end downconverterswhere one or more high level RF signals result in intermodulation distortion spurs (such as in EW, radar orcommunication front ends). MESFET mixers are also well suited to second-stage mixing following a low-noise,high-gain RF preamplifier. In the latter usage a filter or imageless mixer must be used to reject the addednoise. A typical communication example is in any wireless cable TV link where up to 60 tones will be fre-quency multiplexed onto a single carrier. In this case, the Schottky diode mixer is no match for the spur han-dling capability of the MESFET mixer.

Q7: What about the relative cost of Schottky diode versus MESFET mixers?

A7: Broadband double- and triple- (double-double) balanced Schottky mixers are a mature technology andare available from many suppliers. Therefore, diode mixers are more likely to be the winner in any moder-ate quantity cost contest where LO power is easily available. In addition, the unbiased Schottky diode doesnot require a separate DC power supply. However, when the issue is maximum RF power handling with lowLO power, the comparison is not always obvious. Particularly, when a separate LO amplifier may be requiredto supply the extra 6 dB needed to make the Schottky diode mixer perform at the same signal powers as theFET mixer. Typical cost ratios put the MESFET mixer 2 to 4 times higher in unit price to that of the Schottkymixer. This can often compensate the cost of an LO amplifier or the enhancement in overall system perfor-mance.

Q8: Is the MESFET mixer more susceptible to burnout from a high power RF pulse or CW signal when compared to a conventional Schottky ring?

A8: Quite the contrary, since the RF is applied across the channel (drain source) of the FET, the FET powerdissipation is more like the limits for the DC supply power in a FET amplifier. The CW RF power limit of a typ-ical 10 GHz balanced MESFET mixer is approximately 1 watt (the CW power limit of a typical Schottky ringmixer is about 300 mW). Thus, in some system applications, an RF limiter is not required.

Q9: What about the noise level of FET versus Schottky diode mixers?

A9: When using FETs in the passive mode (no average drain current), the 1/f and thermal noise is verysimilar to GaAs Schottky diode mixers, i.e., corner frequency (defined as the point where the 1/f noise equalsthe thermal noise) is about 100 kHz.

Q10: Are MESFET mixers more temperature sensitive than Schottky diode mixers?

A10: No, particularly if one employs zener voltage regulating diodes in the MESFET gate bias circuit. Eachtype mixer will then commonly have a conversion loss variation of +0.25 dB for a temperature variation of+50°C when using a constant LO power.



117

MESFET MIXERS

Q11: Are there passive modes of operation for the MESFET mixer other than LO on gates andRF/IF on the drain source?

A11: It is possible to get very low conversion loss (-3 to 0 dB) by applying LO between the drain and sourceand RF to the gate with IF output at the drain. In this mode of operation, the LO periodically powers the FETinto the active amplifier region and one obtains normal amplifier gain less the Fourier LO switching coefficient(approximately -6 dB). The input IP3, burnout and noise figure for this mode of operation are all considerablylower than drain source mixing. The lower limit of noise figure for this mode of operation is 3 dB becauseof the image response.

Q12: What are the performance characteristics of a typical MITEQ narrow bandwidth MESFETmixer?

A12: The curves below show averaged measured data on four L-band units:

0

10

20

30

40

5

4

3

2

1

> 100

> 100

82

68

REF

1

-

> 100

95

70

50

2

-

-

> 100

80

70

4

-

> 100

> 100

80

47

3

CONVERSION LOSSRELATIVE IF RESPONSE

(LO = +23 dBm)0

2

4

6

8

0

2

4

6

8LO HARMONIC

RF H

ARM

ONIC

FREQUENCY (GHz)

CONV

ERSI

ON L

OSS

(dB)

VSW

R (R

ATIO

)

ISOL

ATIO

N (d

B)

+50

+40

+30

+20

+10

IP3 POW

ER (dBm)

IF RESPONSE (dB)

1.7 1.74 1.78 1.82 1.84 1.9FREQUENCY (GHz)

1.7 1.74 1.78 1.82 1.84 1.9

1.7DC

RF FREQUENCY (GHz)IF FREQUENCY (GHz)

1.92

5:1

4:1

3:1

2:1

1:1

RF AND LO VSWR(LO = +23 dBm)

RF VSWR

LO VSWR

LO = +26 dBm

LO = +20 dBm

IF RESPONSE

CONVERSION LOSS

LO-TO-RF ISOLATION AND IP3

SINGLE-TONE (m) RF x (n) LO SPUR LEVELRELATIVE (dBc) TO REF (RF = -10 dBm, LO = +26 dBm)

-

-

> 100

85

58

5

INPUT IP3

LO TO IF ISOLATIONLO TO RF ISOLATION

TYPICAL TEST DATAMODEL DBF1800W3



118

tortion products, such as 3RF ± LO and 2RF1 ± RF2.In this passive mode, the MESFET channel acts as anLO voltage time-dependent linear resistor. In contrast,the active MESFET mixer has an RF input gate sourceresistance and intermodulation similar to the Schottkydiode mixer.

FUNDAMENTAL LO MIXER CIRCUITS

The basic mixer design problem arises in situationsthat require LO, RF and IF circuits to be coupled effi-ciently to a common semiconductor element, whilerequiring each port be decoupled or isolated from oneanother. Various multiple diode single-, double- andtriple-balanced circuits have evolved that rely on dif-ferent coupling modes for port separation. Figure 2shows the double-balanced Schottky diode mixer cir-cuit and a MESFET version of the same circuit. TheSchottky circuit advantages are its low cost and itsperformance. It has an IP3/PLO equal to 0 to 5 dB,maximum IP3 of +25 dBm above 2 GHz and aP1dB/PLO equal to -5 dB. The MESFET mixer is easyto bias and has an IP3/PLO equal to 5 to 15 dB, an IP

3

maximum of greater than +35 dBm and a P1dB/PLO of0 dB. The MESFET circuit is usually chosen for receiv-er design because of its increased RF dynamic rangewith the same LO power as normally employed for theSchottky mixer. The cost of the passive MESFETmixer is usually higher, but must be weighed against

FUNDAMENTAL, HARMONIC AND SAMPLING MESFET MIXER CIRCUITS

Schottky diode mixers have generally been used asthe front-end downconverter for commercial and mili-tary receivers. As the density of signals in a givenchannel increases, the input IP3 rather than noise fig-ure of the front end begins to limit the receiver’sdynamic range.1The principles of operation andadvantages of fundamental, harmonic and samplingmixers using MESFETs instead of Schottky diodes, aswell as performance data obtainable with new MES-FET equivalent circuits, are reported.

Don NeufMITEQ Inc.Hauppauge, NY

The input RF compression power of any Schottkymixer is approximately 5 dB below the available LOpower because both signals are simultaneouslyapplied to the diodes. Under normal circumstances, itis not desirable for the RF to control the conductionstate of the diode, which results in RF harmonics.Therefore, higher compression RF powers are achiev-able only with proportional increases in LO power.Greater LO power usually means higher receiver costand volume, and lower battery life. Many designershave extended the original work in which MESFETsare used instead of Schottky diodes for greater mixerRF power handling with less switching or LO power.2

MESFETs can basically be used in either of twomodes for multiplication of the LO and RF signals. Inthe active mode, the LO and RF signals are applied tothe gate (or dual gate) and the IF signal is recoveredfrom the drain. The drain also has a positive DC volt-age, thus providing some gain to the frequency con-version process. In the passive mode, the LO isapplied to the gate of the MESFET, while the RF andIF are both connected between the drain and source.No DC voltage is used on the drain, although a smallnegative voltage is used at the gate.

In the passive mode, the LO at the gate essentiallyswitches the drain/source channel between high andlow resistance states. Unlike the active mode, no gainis achieved but the resulting conversion loss is similarto a Schottky mixer including low phase noise. Thispaper emphasizes the passive MESFET modebecause of its superior third-order distortion.

Distortion is generated in any Schottky diode mixer pri-marily from the exponential shape of the junction volt-age and current, as shown in Figure 1. The small-sig-nal RF resistance of a Schottky mixer is approximate-ly equal to the average value of the time varying slopeof the E/I curve, which at the knee, is quite nonlinear.By contrast, the passive MESFET drain/source resis-tance is almost linear at two different LO bias voltages.The symmetry of the MESFET curves about the origin(VDS = 0) also accounts for the low odd order RF dis-

FIGURE 1

25 Ω RESISTOR Vgs = 0 V Vgs = 1 V

100

50

I D (

mA

)

-1.0 -0.5 +0.5 +1.00VD

VDS-1.0 -0.5 +0.5 +1.00

-50

-100

0

100

50

I DS

(m

A)

-50

-100

0

(a)

(b)

THE E/I CHARACTERISTICS OF (a) A SCHOTTKY DIODE AND (b) A MESFET



119

the extra cost of a higher power LO source needed toget the same dynamic range using Schottky diodes.When operation at low LO power is desired, the dou-ble-balanced MESFET mixer, shown in Figure 3, hasthe additional advantage that the separate LO gate cir-cuit is more easily DC biased than a continuous ring-quad of diodes. The 1/f and uniform thermal phasenoise of the Schottky diode and passive MESFET cir-cuits are similar.

Table 1 lists the typical measured data of a 1.8 GHzMESFET mixer at +30 and +20 dBm LO powers. Ineach case, the input IP3 is approximately 10 dB greaterthan the LO power. The ratio of IP3 to LO power isdependent upon the channel doping profile of theMESFET and the LO port reflection coefficient. Theinput RF 1 dB compression power is approximately

equal to the LO power for this mixer, and it will acceptan RF input power of +30 dBm when the LO is also atthis power. Perhaps the term power mixer is moredescriptive of this device. Thus, each MESFET in thisdouble-balanced quad has a 1 dB RF compression of+24 dBm. Another interesting advantage of the pas-sive MESFET mixer relative to a Schottky diode mixeris the burn-out RF power limit. A general rule used bySchottky diode manufacturers is 75 mW maximumCW power for each diode junction or +300 mW (+25dBm) for a quad. The average high frequency MES-FET will accept an RF power or DC power across thedrain and source of 250 mW (50 mA at 4 V) and 1 Wfor the quad. The described L-band mixer can survive25 W CW. In actual practice, the thermal resistance ofthe microwave copper circuitry and that of theSchottky or MESFET ceramic packages must be con-sidered.

Figure 4 shows the X-band MESFET mixer circuitusing quadrature coupled single-balanced mixers.This four-FET circuit has three unique system advan-tages. The input IP3 is not affected by IF circuit mis-matches (it is termination insensitive). The LO-to-RFisolation is typically 30 dB, and the input LO and RFVSWRs are low and nearly independent of LO power,that is, the circuit behaves as if ferrite isolators wereused at these ports. A 12 to 18 GHz scaled version ofthis mixer circuit was produced with a 2 to 4 GHz IFoutput. Table 2 lists the X-band MESFET mixer’s per-formance. The listed performance was measured withan LO power of +25 dBm. However, when DC bias isused at the gates, operation at +13 dBm LO is possi-ble with 2 dB higher conversion loss.

FIGURE 2

TABLE 1

FIGURE 3

THE 1.8 GHz MESFET MIXER’STYPICAL PERFORMANCE

RF/LO FREQUENCY (GHz) 1.7 to 1.9Input IP3 (dBm)

@ 30 dBm LO +40@ 26 dBm LO +36

Isolation (dB)LO/RF 25LO/IF 30SWRRF 2LO 3IF 2

IF RESPONSE (MHz) 50 to 2000

RF

RF

D

S

D

S

D

S

D

S

LO

LO

IF

IF x

(a)

(b)

FUNDAMENTAL DOUBLE-BALANCED MIXING CIRCUITS; (a) A SCHOTTKY AND (b) A MESFET

A 1.8 GHz DOUBLE-BALANCEDMESFET MIXER

LORF

IF

-3 dB ∆

-3 dB ∑

180°HYBRID




120

HARMONIC LO MIXING CIRCUITS

It is becoming increasingly popular, particularly at mm-wave frequencies, to use Schottky diode mixers thatoperate at one-half or one-third the normal LO fre-quency, that is, second- and third-harmonic mixing.3,4

At these frequencies, there is a considerable savingsin the cost of the LO and a reduction in LO reradiationbecause of the higher inherent 2LO-to-RF isolation ofthese mixers. Figure 5 shows a typical 8 to 18 GHzeven-harmonic balanced, Schottky diode mixer usingan LO frequency at one-half the RF. Its performanceas a downconverter is listed in Table 3. The unusuallyhigh 2LO-to-RF isolation (60 dB) of this circuit alsomakes it useful as an upconverter for digital quadra-ture amplitude modulation radios because linearupconverters or modulators require high suppressionof the LO or carrier in order to maintain accurate RFquadrature phase I/Q states.

The even-harmonic mixer is generally more popularthan third-harmonic mixing because the even harmon-ic has approximately the same conversion loss as fun-damental mixing, whereas third-harmonic mixing istypically 10 dB poorer than fundamental mixing.However, an even-harmonic Schottky mixer generallyhas 6 to 10 dB poorer input RF compression com-pared to fundamental Schottky mixing because the LOpower for optimum conversion loss is more critical andoften lower. Once again, the MESFET has a usefulrole in upgrading the dynamic range of a mixer. Figure6 shows a MESFET even-harmonic mixer. Table 4 listsits performance.

FIGURE 4 FIGURE 5

TABLE 3

TABLE 2

FIGURE 6

PERFORMANCE OF THE X-BAND MESFET MIXER

RF (GHz) 8 8.5 9 9.5 10 10.5 11 11.5 12

LO (GHz) 6 6.5 7 7.5 8 8.5 9 9.5 10

Input IP3 (dBm) 37 37 37 37 36 37 34 37 36

Conversion loss (dB) 8 8 7.9 7.7 7.5 7.9 8.2 8.5 9

Input P1 dB (dBm) 26 27 26 25 25 25 25 25 24

LO/RF isolation (dB) 30 34 36 32 32 40 34 34 32

Return loss (dB)

RF 25 25 23 21 20 17 16 15 15

LO 15 16 20 22 20 19 18 20 23

LO = 26 dBm Bias = -15 V

THE SCHOTTKY DIODE MIXER’STYPICAL PERFORMANCE

RF frequency (GHz) 8 to 18

RF power (dBm) -3

LO frequency (GHz) 4 to 9

LO power (dBm) +7

IF output (GHz) DC to 1

Upconverter carrier rejection (dB) 45

Conversion loss (dB) 10

LO

RF

IF

UP

CO

NV

ER

TE

R

-3 dB

-3 dB

IFD

OW

NC

ON

VE

RT

ER

180°HYBRID

∆

∑

A TERMINATION-INDEPENDENTMESFET MIXER

AN EVEN-HARMONIC BALANCED, SCHOTTKY DIODE MIXER

A MESFET EVEN-HARMONIC MIXER

IF RFLO

IFRFLO




121

The circuit yields approximately 10 dB conversion lossat +13 dBm LO power and exhibits 1 dB RF compres-sion at +10 dBm. The half frequency LO is appliedthrough a 180 degree balun to the gates of the twoidentical MESFETs. The drain-to-source lead pairs areconnected in parallel. Therefore, each FET has thesame RF and IF signal. During one LO cycle each FETconducts during its corresponding positive half cycle,which produces two low impedance states across theRF terminals during each LO cycle, effectively dou-bling the input LO switching rate. The incident RF andreflected IF energy is separated by a diplexer. This cir-cuit is only balanced with respect to the LO/RF and willnot reject RF or IF harmonic spur products. The ther-mal output noise of an even-harmonic mixer is identi-cal to a pad, but any LO phase noise is doubled in themixing process.

The conversion loss penalty is severe for harmonicmixing above n = 2. For example, a third-harmonicmixer made from a ring-type Schottky mixer is typical-ly (1/n)2 or 10 dB poorer than fundamental mixing.Other odd-harmonic products of square wave ringswitched mixers follow the same relation unless reac-tive terminations of unused output frequencies areprovided. Sometimes a step-recovery diode (SRD) isused to generate a comb of output frequencies as anLO source. A conventional Schottky diode mixer willhave progressively higher conversion loss in directproportion to the spectral power output of the SRDpulse harmonic. If only one harmonic of the comb is fil-tered and amplified, low conversion loss is possible,but is considered the same as fundamental mixing.Fortunately, high conversion efficiency can be achievedfrom a mixer using LO harmonic ratios of 10 to 100.

SAMPLING MIXER CIRCUITS

Using sampling mixer circuits the amplitude of anyrepetitive RF signal can be detected by periodicallysampling or connecting a small capacitor with a diodeor MESFET switch and charging it with the unknownvoltage. Figure 7 shows the sampling mixer concept. Ifthe switching action (typically in picoseconds) occursat an exact or submultiple (one, one-half, one-third, ...one/n) of the unknown measured frequency, then the

capacitor charging voltage or sampled RF waveform isidentical during each switching instant. Since theswitching diode is off (high resistance) between sam-ples (typically in nanoseconds), the average capacitorvoltage would not discharge, but rather after many RFcycles would eventually reach the amplitude of the RFsignal. In some cases, such as in a phase-locked sam-pling loop, the capacitor will have zero average volt-age because the samples are timed or in-phase withthe exact zero crossings of the RF signal.5 At thispoint, a small change in sampling frequency phase willyield the positive or negative peak values of theunknown sinusoidal RF signal. Typically, in the phase-locked application, the sampling capacitor voltage isamplified with a high input impedance operational ampand the phase of the much higher frequency-lockedsource is forced to agree with the multiplied phase ofa typically 1 GHz reference or sampling frequency. Inother sampling mixer applications, the multiples of thesampling frequency are chosen to be slightly differentin frequency by the desired IF of the receiver. In gen-eral, the sampling mixer can accommodate slight fre-quency changes or unknown RF signal bandwidths,provided that the reference LO has a frequency that isat least twice that of the RF information bandwidth,that is, the Nyquist criteria. Under-sampling is a com-monly used term to describe bandpass RF signal sam-pling. The receiving system penalty paid for the sav-ings of a microwave LO source is multiple responsesspaced by the fundamental LO frequency, and there-fore, the RF bandwidth is restricted to be less than halfthe LO frequency to prevent response folding. A band-pass filter preceding the sampling mixer would elimi-nate other narrowband harmonic responses.

The sampling mixer is capable of lower and flatter con-version loss than the discussed harmonic mixer, pro-vided that the following circuit conditions are met. Thesampling gate time should be less than a one-halfcycle at the highest RF frequency. The sampling rateis required to be considerably higher than the IF fre-quency. The sampling capacitor and IF load resistancetime constant should be much greater than the periodof the RF being sampled.

TABLE 4

FIGURE 7

THE MESFET EVEN-HARMONIC MIXER’STYPICAL PERFORMANCE

RF frequency (GHz) 5 to 6

RF P1 dB (dBm) +10

LO frequency (GHz) 2.5 to 3

LO power (with bias) (dBm) +13

IF freqeuncy (GHz) DC to 1

Conversion loss (dB) 10

Isolation (2 LO/RF) (dB) 30

THE SAMPLING MIXER CIRCUIT CONCEPT

LO INPUT

RF INPUT DC/IF OUTPUT

RF IF




122

The RF input compression power of the samplingmixer generally is higher than the harmonic mixer, par-ticularly if a MESFET is used as the switch. The RFinput compression point of a harmonic mixer is relatedto the harmonic current of the Schottky diode, and fallsoff as 20 log 1/n.

Figure 8 shows the sampling mixer circuits of theSchottky diode and the MESFET, while Figure 9shows their relative performance. Both units hadapproximately 100 to 400 MHz IF frequency rangesand could accommodate wide bandwidth receiver sig-nals or fast phase-locked loops. The MESFET switchinput RF compression power was approximately +13dBm, whereas the Schottky version was 0 dBm, usingthe same SRD power. Newer I/Q and image rejectionMESFET sampling mixers are currently being devel-oped as preparation for a lower cost, low noise frontend. A low noise input amplifier and 1 GHz LO willallow 500 MHz operating bandwidths up to 26 GHz.CONCLUSION

FIGURE 8

FIGURE 9

0369

12151821

05

101520253035404550

CO

NV

ER

SIO

N L

OS

S (

dB)

SP

UR

IOU

S R

ES

PO

NS

E (dB

c)

2 4 6 8 10RF FREQUENCY (GHz)

12 14 16 18

IF 2ND HARMONIC

IF SPURIOUSRESPONSE

(a)

(b)C

ON

VE

RS

ION

GA

IN (

dB)

HA

RM

ON

IC R

EJE

CT

ION

(dBc)

10

0

-10

-50

-60

-702 5.2 8.4 11.6

RF FREQUENCY (GHz)14.8 18

2 (RF ± n LO) 3 (RF ± n LO)

PERFORMANCE OF (a) SCHOTTKY DIODES AND (b) MESFET MIXERS

THE SAMPLING MIXER CIRCUITS USING(a) SCHOTTKY DIODES AND (b) A MESFET

MICROWAVEINPUT

MICROWAVEINPUT

LO

LO

LO

OUTPUT

OUTPUT

(a)

(b)




123

This paper has demonstrated that almost any existingSchottky diode mixing circuit can benefit in RF powerhandling capacity by substituting MESFETs. Additionaladvantages are increased circuit isolation withoutbaluns and/or bias options by virtue of the three-termi-nal structure of the MESFET. These advantages areparticularly helpful in more complicated mixing circuits,such as image rejection types following a high gaininput low noise amplifier (LNA). In many existing front-end upgrades, the increased sensitivity of the LNA car-ries a trade-off in dynamic range by compression ofthe following imageless mixer due to the increased RFgain. This problem could be avoided with more LOpower, but increased LO power would increase thecost of the system upgrade. As a result, front-enddesigns using broad bandwidth or image rejection mix-ers with MESFETs are growing in popularity.

Figure 10 shows the dynamic range and input noisefigure trade-offs of typical 4 to 8 GHz LNAs with aMESFET second-stage mixer. The corresponding LOpower needed to prevent mixer overload at the inputRF power is also shown. Other harmonic and sam-pling MESFET image rejection mixers are currentlybeing developed.

REFERENCES

FIGURE 10

1. B. Bannon, “Using Wideband Dynamic RangeConverters for Wideband Radios,” May 1995, RFDesign, pp. 50-55.

2. S. Maas, “A GaAs MESFET Balanced Mixer withLow Intermodulation,” 1987 IEEE MTT-S SymposiumDigest, p. 895.

3. M. Cohn, J. Degenford, and B. Newman,“Harmonic Mixing with an Antiparallel Diode Pair,”1974 IEEE MTT-S Digest, pp. 171-172.

4. J. Merenda, D. Neuf, and P. Piro, “4 to 40 GHzEven Harmonic Schottky Mixer,” 1988 IEEE MTT-S

Digest.5. S.R. Gibson, “Gallium Arsenide Lowers Cost and

Improves Performance of Microwave Counters,”Hewlett-Packard Journal, Vol. 37, February 1986,pp. 4-10.

Reprinted with permission of Microwave Journal, December 1995.

6

5

4

3

2

1

0

NO

ISE

FIG

UR

E (

dB)

-20-100P1 dB INPUT (dBm)

+10+20

6

5

4

3

2

1

0

NO

ISE

FIG

UR

E (

dB)

403020GAIN (dB)

100

P1 dB OUTPUT

+15 dBm

+20 dBm

+10 dBm

+20 dBm

+23 dBm PLO MIXER

+15 dBm

(a)

(b)

THE LNA - MESFET MIXER’S (a) INPUT DYNAMIC RANGE AS A FUNCTION OF (b) THE LNA’S PERFORMANCE




124

MIXER DESIGN REFERENCES

Written BySpecial Mixer Products Department Personnel

FEATURES• MMW Block Converters July 1996, Microwave Journal

• 4 to 40 GHz Even Harmonic Mixer May 1988, MTT Symposium

• Designing with Phase Detectors Sept. 1987, M.S.N.

• 2 to 26 GHz Biasable Mixer May 1986, Microwave Journal

• Normalized Intermodulation Curves Feb. 1985, Microwave Journal

• Fundamental Versus Harmonic Mixing Nov. 1984, Microwave Journal

• 2 to 18 GHz Even Harmonic Mixer April 1982, M.S.N.

• Cost Effective Radar Retrofit April 1976, M.S.N.

• Carrier Cancellation Circuit May 1975, MTT Symposium

• Mixer Specifications Nov. 1974, Microwaves & RF

• Mixer Preselector Noise Figure Jan. 1972, Microwave Journal

• 1 to 2 GHz Frequency Converter March 1972, Microwaves & RF

DOUBLE-BALANCED MIXERS/PREAMPLIFIERS• Extended Dynamic Range Mixers Winter 1996, Applied Microwave

• Ultra-Small MIC Mixer Oct. 1985, M.S.N.

• Drop-In Limiter-Mixer-IF Amplifier Sept. 1975, Microwave Journal

• 2 to 18 GHz Biasable Mixer Sept. 1974, Microwave Journal

• Double Balanced Mixer with4-Wire Trans Line May 1974, Microwave Journal

• Multioctave Double Balanced Mixer Jan. 1973, Microwave Journal

IMAGE REJECTION/SSB MODULATORS• New Direct Microwave Modulators Sept. 1995, Wireless Convention, CT

• New Applications of QIFMs Jan. 1983, Microwave Journal

• A Quiet Mixer May 1975, Microwave Journal

MESFET MIXERS• MESFET Mixer Circuits Dec. 1995, Microwave Journal

• Coplanar, Image Rejection, FET Mixer June 1991, MTT Symposium

• 2 to 8 GHz, +30 dBm IP3, MESFET Mixer May 1988, MTT Symposium

Catalog products that stretch the limits of mixer performance...



125

CROSS-REFERENCES

RF/LOREFERENCE MITEQ FREQUENCY IF FREQUENCY PAGE

PART NUMBER PART NUMBER (GHz) (GHz) FORM-FIT NUMBER

DBX–158L/M/H DMX0418L/M/H 8 – 15 DC – 1 YES 47



DBX–184LS/MS/HS DMX0418L/M/H 4 – 18 DC – 1.5 YES 47



DBX–72L/M/H DMY0207L/M/H 2 – 7 DC – 1.5 YES 37

DBY–158L/M/H DMY0418L/M/H 8 – 15 DC – 1 YES 47



DBY–184LS/MS/HS DMY0418L/M/H 4 – 18 DC – 1.5 YES 47



DBY–72L/M/H DMY0207L/M/H 2 – 7 DC – 1.5 YES 37

M67C DM0818LW1 9 – 15 DC – 2.5 YES 51

M74 DM0520LW1 7 – 18 DC – 3 YES 51

M77C DM0812LW2 8 – 12.5 DC – 2.5 YES 51

M79 DM0520LW1 7 – 18 DC – 3 YES 51

M79H DM0520HW1 7 – 18 DC – 3 YES 51

MY88 TB0218LW2 2 – 18 1 – 8 YES 55

MY88C TB0218LW2 2 – 18 1 – 8 YES 55

MY88H TB0218HW2 2 – 18 1 – 8 YES 55

MY88HC TB0218HW2 2 – 18 1 – 8 YES 55

MY89 TB0218LW2 2 – 18 1 – 8 YES 55

MY89C TB0218LW2 2 – 18 1 – 8 YES 55

MZ7407C DM0520LW1 6 – 18 DC – 3 YES 51

MZ7410C DM0520LW1 7 – 18 DC – 3 YES 51

MZ7420C DM0520HW1 7 – 18 DC – 3 YES 51

DBL2–18 SBB0218LR5 2 – 18 0.01 – 0.5 YES 67

DCM4–26 TB0426LW1 4 – 26 0.5 – 8 NO 63

DCMX12–18 DB1218LW1 12 – 18 DC – 2 NO 13

DCMX18–26 DB1826LW1 18 – 26 DC – 2 NO 19

DCMX4–26 DB0426LW1 4 – 26 DC – 2 NO 17

DCMX4–8 DM0408LW2 4 – 8 DC – 2 NO 41

DCMX8–12 DM0812LW2 8 – 12 DC – 2 NO 45

DM1–18A DB0118LA2 1 – 18 DC – 0.5 NO 5

DM1–2A DM0104LA1 1 – 2 DC – 0.5 NO 31

DM1–4A DM0104LA1 1 – 4 DC – 0.5 NO 31

DM1218A DB1218LW1 12 – 18 DC – 0.5 NO 13

DM2–4A DM0204LA1 2 – 4 DC – 0.5 NO 33

DM4–8A DM0408LA1 4 – 8 DC – 0.5 NO 41

DM8–12A DM0812LW2 8 – 12 DC – 0.5 NO 45

DME4–40 SBE0440LW1 4 – 40 DC – 0.5 NO 89

DMS1–26A DB0130LA2 1 – 26 DC – 0.5 NO 21

M2–18 DB0218LA1 2 – 18 DC – 0.5 YES 7

M2–26 DB0226LA1 2 – 26 DC – 0.5 YES 15

TBM4–18 TB0218LA1 2 – 18 0.5 – 8 YES 55

TBM4–18 TB0218LA1 2 – 18 0.5 – 8 YES 55

AVANTEK

WATKINS-JOHNSON

RHG



126

PRODUCT INDEX

MODEL NUMBER PAGE NUMBER MODEL NUMBER PAGE NUMBER

DA40208LC7 97 DMX0518L 49

DA40502LC7 97 DMX0618L 49

DA40818LC7 97 DMX0716L 47

DB0118LA2 5 DMY0207L 37

DB0130LA2 21 DMY0418L 47

DB0218LA1 7 DMY0518L 49

DB0218LW2 7 DMY0618L 49

DB0226LA1 15 DMY0716L 47

DB0418HE1 9 DSS0818 99

DB0418HW1 11 FDM0325HA1 29

DB0418HW6 9 FDM0325HA2 29

DB0418LE1 9 SBB0218LR5 67

DB0418LW1 11 SBB0618LR5 67

DB0418LW6 9 SBE0440LW1 89

DB0426LW1 17 SBE0818LA1 87

DB0440HW1 25 SBE0818LM2 87

DB0440LW1 25 SBE1015LM2 87

DB1218HW1 13 SBF0208LW2 75

DB1218LW1 13 SBF0405LW2-A 71

DB1826LW1 19 SBF0405LW2-B 71

DB3336LW1 23 SBF0506LW2 73

DBF0611HI2 79 SBF0506LW3 73

DBF0812HI2F 81 SBF0618LW2 85

DBF1800W3 69 SBF0810HI3A 77

DM0052HA2 27 SBF0810HI3B 77

DM0052LA2 27 SBF1215HI3 83

DM0104LA1 31 SBF1215HI3A 83

DM0104lA3 31 SBF1215HI3B 83

DM0204LA1 33 SBW1822LG1 91

DM0204LW2 33 SBW2226LG1 93

DM0208LW2 39 SBW3337LG2 95

DM0408HA1 41 TB0208LA1 53

DM0408HW2 41 TB0208LW2 53

DM0408LA1 41 TB0218LA1 55

DM0408LW2 41 TB0218LW2 55

DM0412LA1 43 TB0226LA1 59

DM0412LW2 43 TB0226LW2 59

DM0416LA1 43 TB0426LW1 63

DM0416LW2 43 TB0440LW1 65

DM0520LW1 51 TBR0218LA1 57

DM0812LW2 45 TBR0218LW2 57

DM0818LW1 51 TBR0226LA1 61

DMA0208LA1 39 TBR0226LW2 61

DMX0207L 37 TIM0206HI2 35

DMX0418L 47



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IndexTable Of ContentsIntroductionGeneral InformationMixer TerminologyModel Selection GuideMixer CircuitsMixer SubsystemsSpecifying Downconverters & DemodulatorsFax Back Form

Questions & AnswersBalanced Schottky Diode MixersMESFET Mixers

Mixer Design ReferencesCross ReferencesProduct Index

Documents

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