Abstract

A 56 KILOBAUD RF MODEM

Dale A. Heatherington, WA4DSY

This paper describes a 56 kilobaudsynchronous RF modem with a 70 kHzbandwidth. The modulation is bandwidthlimited MSK generated by a digital statemachine driving two digital-to-analogconverters, and two double balancedmodulators. The carrier phase is shiftedplus or minus 90 degrees for each bit.Demodulation is accomplished withstandard quadrature detector chip bu:various coherent methods can be used foroperation at lower signal to noiseratios. The design is relatively simpleand easily reproduced.

Design Philosophy and Goals

To simplify the designconstruction and to allow operationvarious bands, the modem operates atpower (1 mW) in the 28-30 mHz range.3

andonlowThe

final operating frequency ana poweroutput are obtained by selecting anappropriate transverter module. Goodperformance has been obtained with theMicrowave Modules MMT 220/28s and MMT432/28s. These transverters provide 5 to7 watts of output power in the 220 or 432mHz band. These units require a smallmodification to improve theirtransmit/receive switching time. Acapacitor in the T/R switching circuitmust be changed or removed.

Modulation

Many different modulation methodswere tried in order to find one with themost desirable characteristics. BPSK wasnot used primarily because it has largeamplitude variations (zero to full power)and uses more bandwidth than the chosenmethod. Amplitude variations are aproblem because they must be passed un-changed by the transmitter. Any non-linear amplifier stages will causespreading of the spectrum andinterference to adjacent channels. Thiseffect is the same as "flat topping" inSSB transmitters.

FSK was not used because it cannotbe demodulated with a coherentdemodulator and it is difficult to designand build a stable frequency modulatedoscillator capable of large linearfrequency shifts (28 kHz).

The chosen method is a bandwidthlimited form of minimum shift keying(MSK) . It is slightly different from theordinary textbook example of minimunshift keying. Minimum shift keying isbasically FSK with precise control ofseveral parameters. The frequency shiftis exactly l/4 the baud rate and thephase of the carrier shifts exactly 90degrees during each baud interval. Theamplitude is constant. Unfortunately. MSKproduces many unnecessary sidebands whichmake the signal very wide. When MSK isfiltered with a bandpass filter to

68

eliminate the unwanted sidebands, thecarrier phase no longer changes byexactly 90 degrees and the amplitude isno longer constant.

The method used in this designremoves the unnecessary sidebands andmaintains the 90 degree phase shiftduring each baud interval at the expenseof about 3.5 dB of amplitude variation.Also, when the signal is detected usingany kind of FM demodulator, the highfrequency components appear to beboosted. This necessitates the use of asimple de-emphasis network following theFM demodulator. This network alsoreduces high frequency noise whichresults in improved performance.

Bandwidth limited MSK characteristicsinclude:

26 dB bandwidth is 1.25 Hz/Baud.

Uses slightly less bandwidth thanBPSK.

Error rate vs carrier to noiseratio performance comparable toBPSK when a coherent demodulatoris used.

Has much less amplitude fluctuationthan BPSK.

Can be demodulated with severaltypes of detectors.

. quadrature detector

. discriminator

. differential phase detector

Modulation Hardware

Modulation is accomplished with twodouble balanced modulator chips (type1496). One modulator is called the " I "(in phase) modu:iator and the other iscall the "Q" (quadrature) modulator. (SeeFig. 1) . The carrier frequency isgenerated by a crystal oscillatoroperating in the 28 to 30 mHz range. Thecarrier drives the I modulator directlybut is phase shifted by 90 degrees beforedriving the Q modulator. Modulationwaveforms are stored in an EPROM chip.These waveforms are read out by a digitalstate machine into two digital to analogconverters (type DAC-08). The analogwaveforms are filtered with simple threepole active low pass filters to removedigital sampling noise before being sentto the I and Q modulators. The outputs ofthe modulators are combined, amplified,and output to -the transverter module.Signals genera-ted this way areunconditionally stable in terms of phaseshift and frequency deviation.

Almost any modulation type can begenerated with this hardwareconfiguration because the digital statemachine has complete control of theamplitude and phase of the carrier. Themodulation characteristics are defined bythe data stored in the EPROM. The sameEPROM also contains the code to run thestate machine and NRZ to NRZI converter.

Data rates from 1 to about 120kilobaud are easily generated bychanging the baud rate crystal and/or thedivide ratio in ,the baud rate circuit.Six resistors in the low pass filtersalso need to be changed for different

. Costas loop - data rates.

D I G I T A L 1

STATE MACHINE 8 BIT DAC _

XP x-5 + x-17+ 1 <I CHANNEL)

IATA b SCRAMBLER ’ p NRZ to NRZI

c d

CLOCK 4

56 kHz l

CLOCK

GENERATORA

2970

ntiz OUTPUT

TRANSVERTER

29 nHz 1

XTAL OSC

T R A N S M I T T E R D A T A E N C O D E R

A:YD MLJDULATUR

F i g . 1

69

Demodulation Hardware Quadrature Detector

Several methods of demodulation canbe used. Since the carrier phase changesby exactly 90 degrees during each baudinterval, a Costas loop demodulator couldbe used. The signal also has a frequencyshift of l/4 the baud rate which allowsconventional FM or FSK demodulators to beused.

In the interest of cost,simplicity, and fast signal acquisition,a conventional quadrature FM demodulatorwas used in this design (see fig. 2)The Motorola MC3359 chip was chosen fogthis task. It's more than just an FMdemodulator. It also includes anoscillator, mixer, limiter, and severalother functions which were not used.

Costas LoopReceiver Bandpass Filter

The Costas loop is anintelligent phase lock loop which locksan oscillator to the phase and frequencyof the received carrier even with thepresence of phase modulation. T h ereceived signal is multiplied by thelocally generated carrier to recover theoriginal modulation. The main advantageof the Costas loop is its ability tooperate at low signal to noise ratios.The disadvantages are its complexity andslow lock-up time. A Costas loopdemodulator was built for this projectbut temporarily shelved because of its60 ms lock time and large number ofparts. It did have a 5 dB S/N advantageover the quadrature detector that will bedescribed below.

The receiver bandpass filterwas a major stumbling block at thebeginning of this project. It had to be70 to 80 kHz wide with low group delayvariations. No "off the shelf" filterscould be found at any price. Customcrystal filters had very long lead timesand high prices. The solution was simpleand cheap: a 3 section L-C bandpassfilter operating at 455 kHz. It consistsof three 50 uH slug tuned coils and 5capacitors. The cost is about $4.00.Another filter was required for the 10.7mHz IF stages. Since its major purposeis image rejection, the response shapewas not too critical. A Radio Shack FMbroadcast band receiver IF filter had thedesired characteristics. It's 280wide, small, and cheap ($1.00).

BAND PASS ’ MIXER BAND PASS

FC=45S kHzBW=70 kHt

LOCAL asc. LOCAL 0.x.Ftn + 10.7 + 10.245 nHz

Eye pattern

Law PASS

FC= 40 kHtIDATA

1 NRZ ’

CLOCK

HIGH PASS

LIMITER LATCH

DIFFERENTIATORSAMPLE

INTEGRATOR vcll

RECEIVER DEMUDULATURDATA DECODERCLClCK RECUVERY F i g , 2

NOISE CARRIER DETECTDETECTOR

kHz

70

Data and Clock recovery

After demodulation the signalmust be processed to recover the clockand data. The data is detected with atracking data detector, then convertedfrom NRZI to NRZ format. It is thendescrambled and sent to the outputconnector on the modem. Clock isrecovered with a sampled-derivative phaselocked loop circuit. This circuit alignsthe active clock edges with the center ofthe incoming data bits.

data peaks (centeras) to zero crossings.This works because ,the rate of change iszero at the peak of the data bit. Thissignal is then f,urther processed andcompared to the phase of the VCO. Theresult is an error voltage which is thenintegrated and applied to the VCO controlvoltage input. The VCO phase locks tothe centers of the incoming bi,ts. Locktime is about 5 milliseconds in thisimplementation.

Carrier Detector

Tracking Data Detector

A data detector is basically aanalog comparator. Its threshold is setexactly halfway between the voltage levelof a " 1 " and a " 0 " . It outputs a 7" ifthe input is higher than the thresholdand a 'ON if it's lower. There is aproblem when the carrier frequency of theincoming signal changes. The voltagelevels of the ones and zeros change sothe threshold is no longer exactly half-way between them. This causes an in-crease in errors. One common solution,which doesn't work very well, is to ACcouple the output of the demodulator tothe detector, This is fine if the shortand long term average of the number ofones and zeros is equal. This idealcondition cannot be guaranteed even if ascrambler is used. A much bettersolution is to put some intelligence inthe detector so that it averages thevoltage level of the ones separately fromthe average of the zeros then subtractsthe two averages to obtain the idealthreshold level. This circuit doesn'tcare about the ratio of ones to zeros aslong as there is a reasonable number ofeach. A scrambler is used to make surethere is a reasonable number of both onesand zeros. The circuit will compute thecorrect threshold if the input signalcarrier frequency is anywhere within theexpected range of the ones and zeros, inthis case plus or minus 14 kHz. Themaximum frequency offset that can betolerated is actually limited by thebandwidth of the receiver filter. Inthis implementation the error rate startsto increase slightly with frequencyoffsets greater than 5 kHz.

Clock Recovery

For the lowest possible errorrate the clock recovery must be fast,accurate, and have low jitter. The statemachine circuit used in most TNCs todayhas a large amount of phase jitter and isnot suitable for this application. Itsmain advantage is low parts count ( 2chips). The circuit described here is asampled-derivative phase locked loop. Itgets its phase information from the bitcenters, not the zero crossings like mostcircuits in common use. It does this bytaking the derivative (rate of change) ofthe demodulated data. This converts the

The carrier detector circuitworks by measuring the signal to noiseratio of the signal. This is not an easytask because the data resembles randomnoise in most aspects. This designsolves the problem by first taking thederivative of the demodulated signal,then sampling only the zero crossings ofthe derivative with the active clockedge. Since the rate of change at thebit centers should be zero, the output ofthe circuit should be zero. Random noisehas random rates of change at thesampling instants which cause thecircuit to output a random voltage. Thisnoise voltage is rectified, filtered, andsent to a comparator. If the noisevoltage is below a preset threshold, thecomparator turns on the carrier detectsignal at the digital interface and alsoturns on a data gate which allows thereceived data to go out to the interface.An unmodulated signal will also triggerthis circuit since there would be nonoise at the sampling instants to inhibitthe comparator.

NRZI to NWZ Converter

NRZ is a data signaling formatin which zeros a:re represented by acertain voltage level and ones bYanother. NRZI is a signaling format inwhich zeros are re:presented by a changein voltage level while ones are indicatedbY no change. NRZI coded data is notaffected inverting the data voltagelevels or the mark/space frequencies inthe case of FSK. This modem converts theincoming NRZI data to NRZ data with asimple circuit consisting of a "D" FlipFlop and XOR gate.

Descrambler

There are two good reasons adata scrambler was used in this modem.First, it makes the data stream look likea random stream of ones and zerosregardless of the data being transmitted.This characteristic makes the trackingdata detector and clock recovery circuitswork better. Second, it makes the RFspectrum look and sound like band limitedwhite noise. In other words, the RFenergy is spread evenly over the modemsbandwidth and shows no single frequencylines regardless of the data being

?l

transmitted. Any potential interferenceto near by channels is limited to anincrease in the noise floor instead ofsqueaks, squawks, and other obnoxiousnoises. This type of scrambling is alsocommonly used in high speed syncronousmodems for telephone use.

The hardware to implement thescrambler and descrambler is very simpleIt consists of a 17 bit shift registerand two XOR gates. See Fig. 3. Eachtransmitted bit is the result of theexclusive ORing of the current data bitwith the bits transmitted 5 and 17 bitstimes before. To descramble the data itis only necessary to exclusive OR thecurrent received bit with the previous5th and 17th bits. If the data consistof all ones, the scrambler will producea pseudorandom sequence of bits that willrepeat after 131,071 clock pulses orevery 2.34 seconds at 56 kilobaud.

Performance

There are several measures of amodem's performance. One of the mostimportant is its bit error rate. Anywell designed modem should not produceany errors if the signal has no noise ordistortion in it. The true measure of amodem's quality is its performance underweak signal conditions when the signal tonoise ratio is low. These modems weretested to determine their error rateperformance at various signal levels andfrequency offsets. The results aresummarized in the table below.

Errors per 1 million bitsSignal level Freq. offsets in kHzuv dBm 0 +5 -5

.71 -110 3620

Some people may complain thatscrambling violates the FCC ruleconcerning codes and ciphers. It doesnot. The scrambling algorithm ispublished here and is available to anyonewho wants it. For this reason it doesnot actually obscure the meaning of thedata. If all codes and ciphers wereillegal we could not use Morse code,ASCII, or NRZI!

.79 -109 2736

.89 -108 843

1.0 -107 129

1.1 -106 29

1.2 -105 0r

1.4 -104 0

1.5 -103 0

1.7 -102 0

SCRAMBLER DE-SCRAMBLER

F i g . 3 - SCRAMBLED DATA

43261 11280

8490 3110

4694 1510

1680 285

536 77

240 19

23 3

0 0

0 0

72

The table shows that 1.5 microvolts ofsignal are necessary to achieve an errorrate less than 1 per million over a plusor minus 5 kHz frequency range. It alsoshows that the performance is degraded by2 dB if the frequency is offset by 5 kHz.

This data was obtained using a MMT 432/28transverter without a GASFET preamp. Thepublished noise figure for this unit is 3dB. Adding a pre-amp should improve theperformance.

Another important performanceparameter is the delay time fromtransmitter turn-on to valid data at thereceiver. This modem requires a 10 to 13millisecond delay after the transmitteris keyed before data can be transmitted.5 to 7 ms of this time is required forthe crystal oscillator to start andstabilize. The remaining 5 to 6 msallows the distant receiving modem timeto phase lock its clock to the incomingsignal and detect the carrier. This timedelay is not as low as it should be andwork is being done to reduce it. If thedelay were zero a 256 byte packet shouldtake 36.5 ms to transmit. This modemtakes up to 50 ms of transmission time or36% overhead. This overhead percentagecould be reduced by transmitting longerpackets.

Applications

SO, what do you do with a 56,000baud RF modem? One obvious applicationwhich has received much press lately isnetwork backbones. One popular opinionseems to be that high speed modems willsolve network congestion problems. Thisassumes that current firmware andsoftware will run at 56 kilobaud. Itdoesn't! One of the major problems ofthis project was getting packet softwareto operate at this speed. No TNC runningAX.25 will operate at 56 kilobaud. IBMPCS running at 8 mHz can only handleserial port data at 38.4 kilobaud or lesswithout dropping characters. To conducton the air tests it was necessary makemajor modifications to a Z-80 assemblylanguage program called KISS-TNC writtenbY K3MC. This program resides in anEPROM in the TNC. Its main job is toconvert SYNC frames to ASYNC frames andsend them to the serial port on a PC. Aprogram running in the PC is responsiblefor doing the protocol, in this caseNET.EXE by KA9Q, which implements TCP/IP.Although KISS-TNC was successfullymodified for 56 kilobaud, NET.EXE wouldnot run any faster than 19.2 kilobaud.The result was that the PC to TNCinterface ran at 19,200 baud while theTNC to RF modem interface ran at 56,000baud. Needless to say, NET.EXE could notkeep the channel busy. This may not be aproblem because the channel is a sharedresource and should not be hogged by oneuser anyway.

Digital Voicle-

This mlodem is fast enough tocarry real time digitized speech. Twomethods of converting speech to digitalformat are popular. The phone companyuses Pulse Code Modulation (PCM) at a 64kilobaud data rate. Unfortunatly someoneset the upper limit for amateur digitalcommunication at only 56 kilobaud thuspreventing the use of many cheap CODECchips in the market today. They onlywork at 64 KBPS. One good alterative toPCM is continuously variable slope deltamodulation (CVSD). Distorted but intel-ligible speech can be transmitted atspeeds as low as 9600 BPS with CVSD. At56 KBPS it sounds as good as typicalcommunications quality FM. The CVSD chipused in the experiments with this modemis the Motorola MC34115. Its price is inthe $2.00 range. CVSD also works well inthe presence of data errors. Digitizedspeech can be understood with more than10% bit errors. It's quite noisy at thaterror rate and sounds like a weak FMsignal.

Digital Video-

Video frame grabbers are avail-able which digitize and store picturesfrom a NTSC video source in computermemory. Images stored in this manner canbe transmitted digitally. A video frameconsisting of 256 by 256 pixels with 64shades of gray can be transmitted in 7seconds at 56 kilobaud. The same imagewould take more than 5 minutes to sendwith the current 1200 baud standard.

Conclusion

Bandwidth limited MSK is a powerefficient modulation method with reason-able bandwidth requirements and lowamplitude fluctuations. The hardwarerequired for generation and demodulationis simple and reliable. A wide varietyof demodulators can be used depending onthe cost/performance tradeoffs. It is anexcellent choice for high speed RF datalinks.

‘is3

Modulation waveforms

Top trace : I modulationlower trace : Q modulation

Signal constellation

Transmitter X,Y displayX axis : I modulationY axis : Q modulation

Bandwidth limited MSK spectrum20 kHz/div. horz.10 dB/div. vert.

Bandwidth limited MSK spectrum50 kHz/div. horz.10 dB/div. vert.

74

56 kilobaud EYE

Top trace : Receiver EYE patternLower trace : recovered clock

56 kilobaud EYE

Top trace : weak signal EYE patternRF signal level is about 1.4 UV

lower trace : recovered clock