VHF COMMUNICATIONS 2197(:/'------------'-""-"""""""==~

Mdtjaz Vidmar, S53MV

23cm PSK Packet RadioTransceiver for 1.2 Mbit/sUser Access

I.WilY Bl-('HASE I'SKM ODULATION'!

Upgrading the packet radio network 10

higher data rates also requires usingmore efficient modulatio n and demodu­lation techniques both to reduce thesigna l bandwidth and 10 increase theradio range of the system. In part icular,inefficien t modems coupled 10 standardFM transcei vers haw to be replacedwith custom-designed radios for datatransmission.

Considering the bandwidth and TXpower available to radio amateurs. it isnecessary to switch to coherent demodu­lation techniques at data rates around100 kbtr s in terrestrial packet radio andat even lower data rates in satellitecommunica tions.

One of the simplest forms of digitalmodulation that can be demodulated in acoherent way is biphase PSK. The usualamateur approach to implement biphase

PSK. is to usc already existing equip­ment like linear rransvcncrs or SSBtransceivers coupled to custom-designedmodems operating at an intermediatefrequency. While th is approach may beacceptable fo r sate llite work, it is rathercomplex and inconvenient for conven­tional terrestrial packet radio.

On the other hand. professionals devcl­oped very simple and efficient digitalradios like GSM cellular telephones.Professionals also found out that theycannot use the frequency spectru m effi­ciently with narrowband FM radios; allnew cellular phone system use high­speed TDMA tech niques or evenspread-spectrum modulation.

If we rad io amateurs want to improveour digital communication, it is there­fore necessary to develop and build newequipment. The only place for obsoletenarrowband FM equipment is in amuseum!

Maybe PSK modulation is not consid­ered "ery e fficient by many amateurs,since it is used on satellites at data rates

74

VHF COM MUNICATIONS 2/97

of only 400 bit's or 120 0 bit/so On theother hand, in Slovenia (55) we installedour fi rst 1.2 Mbit/s PSK links in 1995,operating in the 13cm amateur band at2360 Mllz. This equipment proved veryreliab le and the PSK links never failed,even when the 70cm and 23cm 38.4khit/s links were out due to heavysnowfall in the 1995/96 winter.

The 13crn PSK 1.2 Mbit/s link trans­ceiver used in these links (shown inWeinhcim in September 1995) was onlythe first attempt towards a dedicatedPSK radio. The l3cm transmitter wassimpl ifie d by using direct PSK modula­tion on the output frequency, but thel j cm receiver is stil l using a doubledownconvcrsion followed by a conven­tiona l, if squaring-loop, PSK demodula­tor.

The co nstruction of this transceiver isnot simple, there arc several shieldedmodu les and especially the double­conversion receiver requires lots oftuning.

2.IlIRECT-CONVERSION PSKDATA TRANSCEIVER

Similarly to an SSB transceiver, a PSKtransceiver can also be built as adirect-conver sion radio as shown inFig.l . The Costas-loop demodulator canbe extended to include most of theamplification in the receiving chain.Since such a receiver does not requirenarrow band pass filters, the constructionand alignment can be much simp lified.In addition, some receiver stages canalso be used in the transmitte r (like thelocal oscillator chain) to further simplifythe overall transceiver.

A direct-conversion I'SK receiver alsohas some problems. Limiting is gener­ally not harmful in the signal amplifier,however it increases the noise in theerror amplifier chain. In pract ice, theloop bandwidth has to he decreased. ifno AGe is used and both ampl ifiersoperate in the limit ing regime. It is also

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75

VHF COMMUNICATIONS 2/97

Most of the problems of a d irect­conversion l'SK receiver can be over­come in a so called "zero-IF" PSKreceiver, as shown in Fig.2. Incidentally,a zero-IF PSK transce iver requires verysimilar hardware 10 a direct-conversionPSK transce iver. The main difference isin the local oscillator. A zero-IF I' SKreceiver has a fixed-frequency, free­running local oscillator. while the de­modulation is only perform ed <I ller themain receiver gain stages.

their dynamic response is totally unpre­dictable above 1 kHz. The latter may beenough for full-duplex, continuous-car­rier microwav e links, but it is insuffi­cicn t for CSMA packet radio, where avery fast signal acquisition is required.

3.ZERO-IF PSK DATATRANSCEIVER

(t' - - - - - - - - - - - --'-"'-""===""'-"""­very difficult to have both amplifiersDC coupled as required by the theory. IfAC coupled ampl ifiers are used, ran­domisation (scrambling) has to be ap­plied to the data stream and someadd itional noise is genera ted. However,in a well-designed d irect-conve rsionPSK receiver, the signal-to-noise ratiodegradation due to AC coupling can bekept sufficiently small.

Buildi ng a real-world, direct -convers ionPSK receiver one shou ld also cons iderother unwanted effects. For example, theCostas-loop demodulator includes veryhigh-gain stages . Unwanted effec ts likeAM modu lation on the vr.O or FM­to-A M conversion in the multiplierstages can lead 10 unwan ted feedbackloops. However, the mos t cri tical com­poncn t seems to he the veo. In apract ical microwave PSK transceiver,the veo is buill as a VCXO fo llowedby a multiplier elwin. Although thestatic frequency-pull ing range of funda­mental-resonance and third-overtonecrystals is sufficien t for th is application,

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76

VHF COMMUNICATIONS 2197

A ze ro-If PSK rece iver includes aquad rature mixer that provides twoo utput signals I' and Q' with the sameban dwidth as in a di rect-conversion RX.The s ignals r and Q' conta in all of thein form ation of tbc input RF signal, butthey do not represent the demodulatedsigna l yet.

S ince the zero- lf' RX contains a trcc­running LO, its phase is certa inly notmatched 10 the transmitter. Further, ifthe re is a difference between the fre­quc ncics of the transmitter and of thereceiver. the phasor represented hy theI' and Q' signals will rotat e 'II a ratecorresponding to the difference or thetwo fre quencies.

1'0 demodulate the information. the I'and Q ' signals have to be fed to a phaseshifter to counter-rotate the phasor. Thephase shifter is kept synchronised to thecorrec t phase and rate by a Costas-loopfeedba ck. Since the who le Costas-loopdemodulator operates at high signalleve ls and at relat ively low frequencies,it ca n be built with ine xpensive74HCxxx logic circuits thai require notunin g at all !

A zero-IF PSK receiver requires linearampli fication of the r and Q ' signa ls.Limiting of the rand Q' signals is veryharmful to the overa ll signa l-to-no iserat io. If the zero-It' amplifiers arc ACcoup led . data randomisat ion (scram­hling) is required. On the other hand, azero-If PSK transceiver docs not in­clude any critical stages or unstablerc-edb ack loops and is therefore easilyreproducible.

Search ing for a simple PSK transce iverdesign, I attempted to build both adirect-conversion and a zero-If I)SK

transceiver for 23cm. Th e 23cm bandoffers suff icient bandwidth for 1.2 Mbit!50 operation. Further, the who le trans­ceiver can be buill on conventional,inexpensive gfassfibre-epoxy laminateFR4. Finally, the propagation losseswithout optical visibility arc sma ller inthe 23cm band than at higher microwavefrequencies .

A direct-co nversion I)SK transceiver for23cm proved very simple. The signaland error amplifiers used just oneLM311voltage comparator each. ooerar­109 as a limiting ampl ifier. The onlylimitation of this transceiver was theVCX O ,

Due to th e unde fined dynamic responseof the VCXO, the capturing range ortheCostas-loop KX was only about ~ 1i-5kHz. Further, even this Fig.ure washard ly reprod ucible, since even twocrystals from the same manufacturingbatch had a quite d ilfcrcnt dynamicresponse in the VC XO.

A zero-IF 23cm ?SK transceiver rc­sultcd slightly more complex. due to thelinear IF amplificat ion with AGC andthe add itional Costas-loop demod ulator.On the other hand, the zero-Itt 23cmI)SK transceiver res ulted fully reproduc­ible, since there are no crit ical parts orunstable circuits built in.

Since the addi tional complexity of thezero-If transceiver is in the IF part,using on ly cheap components and notuning points, it docs not add much tothe overall comp lexity of the trans­cerver.

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4.DESIGN OF TIlE ZERO-IF23CM PSK TRA NSCEIVER

In this article J am therefore going todescr ibe the above ment ioned successfuldesign of a zero-If PSK data trans ­ceiver . The transceiver is built on sevenprinted circuit boards, fo ur o f which (theRF part) arc insta lled in metal shieldedenclo sures. The RF part is built mainlyas mic rosmp circuits on O.8mffi thickg lassti bre-epoxy laminate FR4,

Sub-ha rmonic mixers are used both inthe tran smitter modulator and in thereceiver quadrature mixer . Sub-harmonicmixers with two antiparallel diodes aresimple to build . Since the LO signal is

at half of tile RF frequency, RF signa lsare easier to decou ple and less shieldingis required. Finally, it is vel)' easy tobuild two identical sub-harmonic mixersfor the receiver quadra ture mixe r.

The whole transceiver therefore requiresa single loca l osc illator ope rating at halfof the RF frequency, or at about 635MH/. lo r ope ration in the 23cm amateurband . The local osc illator including acrystal osci llator and multiplier stages isshown in Fig.3. The LO module is builton a single-s ided PCB, as shown inFig.4 and Fig.S.

To speed up the Txm.X switching, thereceiving mixers are powered on and arereceiving the 1.0 signa l all of the time ,On the other hand , the LO signalfeeding the modulator has to he turned

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off to avoid any interference duringreception, Therefore , the LO signal isfed to the receiving mixers throug h adirectional coupler located in the 1270Ml lz PSI( modulator mod ule as shownin Fig..6.

On ly a small fraction of the LO ptm er(-20dB) is fed to a separation amplifierstage (Bf Pl lB ), The 635 MHz HPFensures a good residual carri er suppl'es-

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VHF COM MU NICATIONS 2197~.--------------''''''-''''''''='''''''''''''~

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Fig. l l : I~ F Frun t l':1It1 Component Overlay

Since the termination impedances o f thesub-harmonic mixers depend on the LOsigna l power I the d ifference ports ofboth the quadrature (RF) and in-phase(LO) power splitters have to be termi­nated to ensure the correct phase and

of the 23cm PSK trans­ceiver. shown in Fig.9. ·includes a 'I X power am­plifie r with a CLYSpower GaA sFET to boostthe T X o utput power to

about IW (-30 dBm). aPIN diode antenna switch( B AR 6 3 ·0 3 W a n dBARXO) and a receiveRF am pli fie r with aUfP [8 1. T he latte r hasabout 15dB gain. but thefollowing 1.27 Gl lz BPJ.'has about 3dB passbandloss. The RF front end isalso buill as a microstripcircuit em <I double-sidedPCB .... shew n ill Fig. tuand Fig . l l .

the quadrature I/Q mixerfor 1270 Mll z, sho....n inHg. 12, includes an addi­tional gain slage at 1.270 117 (2f,dB MMIC INA­03 1!~.1 ). two bandpass fil­ters at 1.27 (J llz (3dBinsertion loss each), aquadrature hybrid for theRF signal at 1.27 G117,on in-phase power splitterfor the 1.0 signal at 635MI ll, two ident ical sub­

harmonic mixers (two HATl 4-099R ­Schottky quads) and two identical IFpreamplifiers (two 11 (7 199).

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serve as a groundplanc for the micros ­trip ci rcuit. The RF signa l lm ses in theFR4 la minate are rather high at 1.'27Gllz. For example. the 1.27 GHz BPFhas a passband insertion loss o f about5dB. On the other hand . a ll of themicrostrip bandpass fillers are designedfor a bandwidth o f more than IO~/o ofthe ce ntre frequency and therefore re­quire no tuning considering the laminateand etching tolerances.Th e RF front end

J'ig. IO: RF Front End I'en - actua l site SO.\ 401Jl1llu.Smm double-sided FR4

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AC-coupl ed stages has to be set suffi ­ciently low. At a data rate of 1.2 Mbit/s.a convenient choice is a lower frequencylimit of l kl lz. The latter allows all ofthe lime constants in the range of l ms(IX/RX switc hing time!) and causes adistortion of about 4% of the amplitudeof the IF signal.

Of course, the AGC time constantshould also be in the same range around1ms. Such a fast AGC can on ly be

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amplitude relationships. Consi dering themanufact uring tolerances of the micros­trip PCB shown in Fig.13 and Fig.l4, theamplitude matching is usua lly within 5%and the phase shin is within +/- Sde­grees from the nominal 90 degrees .

A zero-IF receiver requires a dua l IFamplifier with two identical ampl ifica­tion c hannels I but a single. commonAGC . Since De-coupled amplifie rs cannot be built the lower frequency limit o f

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applied to low gain stages to avoidunwanted feedback. A simple technicalsolution is to use more than one AGC inthe IF amplifier chain. The T/Q dualamplifier shown in Fig. 15 has threeidentical dual amplifier stages and eachof these dual stages has its own AGecircuit using MOS trans istors (4049UB)as variable resistors.

The I/Q dual amplifier module alsoincludes two identical lowpass filters on

the input (that define the receiver band­width) and two phase inversion stageson the output to obtain a four-phaseoutput signal (+ 1, +Q, -I and -Q) todrive the following phase shifter. Thel/Q dual amplifier is built on a single­sided PCB as shown in Fig. 16 andFig.l7 .

The Costas-loop 1/Q PSK demodulatoris built entirely using cheap 741IC.x.\.\logie as shown in Fig.]R. The four-

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phase input signal (...1. +Q, .J and -Q)feeds a resistor network that generates amuhiphase system with a large number(16) o f phases. Two 74 HC·W67 ana­101::ue switches are then used to selectthe de sired signal phase . T he inputs ofthe two analogue "electors are o ffset by4 to provide the required 90 degreephase shift between the signal and errorou tputs.

Both the signa l and error are firs t fedthrough two lowpass filte rs (10 suppressthe 74HC4067 switching transients) andfinally to two LM3 11 vo ltage compara­tors to ob tain TTL-l evel signa ls. Thesignal and error arc then multiplied inall EXOR gate and feed a d igital yeo.Th e d igital VCO includes a 6 .144 MH".clock osci llato r and two 74 HC 19 1 up!down counters.

89

VHF COMMUNICATIONS 2/97

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The up/down co ntrol is used as theveo cantrilI input. I f the latter is at alog ical ZERO, the up/do wn co unterrotates the two 74HC4067 switchesFORWA RD with a frequency o f Zdkl Iz,lf the input is at a logic ONE, theup/do wn counter rotat es the two74 11C4067 switches HACKWARD ",,' illla frequency of 24 kll z. Fina lly, i f thecontro l input toggles. the result dependson the ONIO FF ratio o f the co ntro lsignal. At 50%, duty the 7411 C4067switches stay in the same position.

T he overall circuit therefore operates asa first-order, Costas phase-locked loopthat is able to correct carrier-freq uencyerrors between -24 kllz and +24 " li z.The loop gain is de fined by the dividingrat io of the 74HC19 l up/down cou ntersand the clock frequency . If a widerca ptur ing range is desired. the clockfrequency can be incre ased up 10 20MHz, but the resulting higher loop gainalso increases the phase noise!

T he Cos tas-loo p demodulator is built ona doubl e-sided PCB as shown in Fig .19

lind Fig .20 . The circuit includes ils own+5 V regulator and an output stageca pable (If feeding a 7511 cable with thel•'modulated I{X data.

The overall PSK transce iver requires afew addit ional interface circuits (shownin Fig.21) including a supply voltageswitch and a modu lato r driver. Themodulator driver includes a lowpassfilter to dec rease the h igh-ordersidelohcs of the modulation spectrum.The supply switch interface is built on asingle-sided PCB il.S shown in Fig.22and Fig.23.

T he overall PSK transceiver is enclosedin an aluminium box with the dimen­sions o f 320mm (width) X l75nun(depth) X 32 mm (height). The location(If the single modules is shown inf' ig.24 . The lour RF modules are addi­tionally shielded in small boxes made ofO.Smm thick brass sheer as shew n inFig.25 . The groundplane of the reus isso ldered along all four sides to the brassframe 10 ensure a good electr ica l con­tac t.

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require any tun ing if theinterconnecting 50n Te­flo n ca b le fr om themo du lato r is exact ly12cm long. Tun ing L3and Ui the output powercan only be increa sed byless than IOOmW. All o fthe other micro strip rcso­nators sho uld not betuned. Finally. the 2501 ~

trimmer in the supplyswitch interface is ad­j ustcd for the maximumT X output power (usually2/3 of the full scale).

VHF COMMUNICATIONS 2/97

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Spe cia l care shou ld be devoted to theassem bly of the mk rcstrip cir cuits Themicrostrip resonators arc grounded atthe marked pos itions using O.6mm thickCuAg wire. The SMD components arcgrounded through 2 .5mm, 3.2mm or5mm diameter hole s ar the markedpusit inm . The holes arc first coveredwith a piece of th in copper sheet on thegroundp fme side. then they arc filledwith solder and finally the SMD part isso ldered in place.

T he assembled PSK transceiver requireslittle tuning. The only modu le that needsto be tuned in any case is the localosc illator modu le. Since most of thestages are just freq uency doublers. it isvel)' difficult to tunc this modu le to thewrong harmon ic.

Th e TX power amp lifier may need sometuning to get the maximum outputp(lwer. As printed on the circuit board.LI in the RF power amplifi er should n OI

S.INTER FACl i\ G T HE 1.2~mIT/S )'SK TRA:>ISCEIVER

Amateu r packet radio interfaces for datarates above 10 0 kbit/s are not verypo pular. One o f the most popular serialinterfaces , the Zilog I H530 sec. onlyinclud es a DPLI. for RX clock recoverythat can ope rate up 10 about 250 kbit/ s.Other integrated circu its, like the oldZ80 S10 , the ~1C6S302 used in the11\C3 or the new MC68360 do norinclude any clock reco very circuit s atall. In addition to the RX d ock recov­ery. data scramblin g/descrambling andsometimes even NRZiN RZI differentia lencoding/d ecod ing have to be providedby external circu its.

The circu it shown in Fig.26 was spe­cially des igned to interface the describedPSK transceiver to a Z8530 s e c .

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VHF CO MMUNICATIONS 2197

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although it will probably work withother serial HDLe controllers as wel l.The circuit includes an interpolationDPLL that only req uires an !:I-limeshigher clock frequency (9.830 " Mll z),a lthough provides the resolution o f a1256 conventional OPLL with a 315MHz clock .

The scremblcrzdescramblcr uses a Yliftregister "iib a linear feedback with

EXO R gales. The scrambling polyno­mial is the same as the one used inK9NGlG3RUH modems:

I+X· · 12+X· · 17

Due to the redundancy in the AX.25data stream (zero insertion and dele­tion), a simple po lynomial scramble r iscompletely sufficient to overcome theAC coup ling limitation of the describedPSK transceivers.

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IVHF COMMUNICATIONS 2197

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95

VHF COMMUNICATIONS 2/97

i"ig.27: Hi t-Synchrunlse r/Scra mhler PCU - actua l slze 120 \ 60mm1.6mm single-sided FlU

Fig.2M: Bit-Syn chroniser/Scrambler Com ponent Overlay

The interface c ircuit also includes 75!lline drivers and receivers. if the PSKtransce iver is insta lled at some d istancefrom the interface. However, connec­tions have to be kept short on the sidetowards the computer serial port. Thedescribed interface only provides onecloc k signal. since it is intended for

simplex operation with the describedPSK transceiver. Of course the DPL L isd isabled during transmission. so that thecircuit supplies a stable clock to the

96

transmitter. The polarity of the clocksignal can be selected with a j umper .When using the 18530 TransceiverC orTRxC cloc k inputs, this jumper shouldbe connected to ground .

The bit-synchronisatiou'scransblcr circu itis built on a single-sided pen as shownin Fig.27 and Fig.28. It only requiresone adjustment, the DeD threshold, andthe latter can only be performed whennoise is present on the RXf\.1 input