MSF LockedFrequencyReferenceThe first of a two part feature by Andy Talbot, G4JNT

o ASGB

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half output when the carrier is keyed off everysecond. The gating output of the lone de­code r is used to control a sample-and-holdcircu it cons isting of a 4066 analogue switchand capacitor.

The vollage at this point is an acc uratemeasure of the phas e relationsh ip be tweenthe two signal s, and is displayed on a frontpane l meter to show the loop lock state. Aloop filler integ rates this output and uses it tocontrol ' he VCXO frequ ency . Since a secondorder loop is used , Ihe mean phase relation­ship is exactly 90~ and the meter clearlyshows when the loop is in lock by silti ng lirmlyat mid scale.

An ana logous system to this reference isused in colour television receivers to regener­ate the colour subcarner. at4A3MHz, from aburst reference transmilled just afte r the linesynchronising pulse. In th is case the gatingsignal is generate d by delaying the line

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Fig 1: MSF Rel emnce, bloc:k d iagram.

calibration simple. Since the prime purpose01the Irequency standard was for calibratingmy 10GHz equipment, a limes-9 multipliersection was added to give a 108MHz outputwhich could be used to generate a comb 01harmoni cs detectable up to 10GHz.

The frequency reference is buill in fou rmodules ,each one 01which could be used forother purposes.A block diagram of the Refe r­ence is shown in Fig 1. The MSF signal isreceived off air by a ferrite rod antenna andhead ampl ifier , mounted remote ly. Any at­tempt to place the ante nna near to the dividerci rcuits will lead to pick up of the VCXO signaland the loop locking to itsell rather than MSF.An NE567 lone decoder provides a gatingsignal corresponding to the carrieron/off key­ing. The received signal is squared up in anl M393 comparator, and applied to an Exc lu­sive OR gate used as the phase detector forthe Pll.

A 12MHz crystal cscma-tor is vollage tuned over alimited range of approxi­mately Sppm by the varicapacross th e crys ta l. The12MHz output is applied totwo bipolar tripler stages togive the 108MHz signal,anda portion is squared up anddivided by 200 in a TIL di­vider chain. The resulting60kHz output is applied tothe other input of the phasecomparator,

The output 01the phasecomparator has a meanlevel correspondi ng 10 thephase difference betweenthe two signals when thecarrier is on but settle s to

Fig 2: Program mable Frequency Divider , bloc:k d illgra m.

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WHEN OPERATING narrow­band on the microwavebands,freq uency settin g canbecomeaproblem.Typically.crystal oscillators used in

uansveners can, wit h a certain amount ofcare, achieve 0.5 parts per million (ppm)accuracy when temperaturecompensation isused. However, at 1QGHz this slill corre­sponds 10 5kHz uncertainty, nearly two SSBbandwidths! Freque ncy co unters are avail­able that can give accuracies of 1 part in 10"but theseare expensiveand notusuallyavail­able when wanted.

Some years ago a frequency source basedon the 200kHz long wave transmission fromOroilwich was published . This sulfe red fromres idua l phase modulalion on the carrier anda good 'note' was dillicull 10 obtain whe nmultiplied 10 10GHz.soceeen.ireDronwichIransmission has moved to 198kHz and thecarrier is phase shift keyed at 25Hz du ringsome limes 01the day, making a reliable andclean trequency standard much more difficult10 achieve .

It was dec ided to adopt a ditl erent ap­proach.The MSFStandard Frequ ency Trans·mission trcrn Rugby on 60kHz is well knownlor transmill ing time and date informationwhich is easilydecoded .The carrier frequencyis related to mternancnat Sta ndards, and isaccurate to better than 1 in 10'°, This co-re­sponds to an error at 10GHz of just 1Hz! Intact accuracy can be degraded by the trans­mission path atdusk and dawn ,due to Dopplershif1 caused by the height 01the Ionospherechang ing, but this is unlikely to be noticeab le.

It was decide d to produce a general pur­pose frequ ency standard that had sufficientlygood Short term stability to be mull iplied tomicrowave frequencies. In addition, towerIrequencies would be provided that co uld beused as the timebase lo r freque ncy countersand osci llosco pes il requir ed.

Direct multiplication 01 the 60kHz signa lwould not serve any useful purpose. No use­ful frequency couldbe produced forumeoases.and the concept of directly mulliplying by172800 to 10GHz is mind boggling! In anycase , the carrier is pulsed at 1Hz and aninte rmill ent outpu t would result.

The solution was to employ a 12MHz volt­age controlled crystal oscillator which is di­vided down t060kHz and phase locked totheMSF transmission. 12MHz was chosen forseveral reasons, Firstly, a simple to achievedivis ion ratio 01200 gives the 60kHz VCXOsignal. Secon dly, aJilrequencies in the nar­row-band segments 01the microwave bandsare (currenlly) multiples of 12MHz, making

RADIO COMMUNICATION April 1994 39

M SF LOCKED FREQUENCY REFERENCE

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unecese flyback pulse.To provide a range of other useful frequen­

cies. a switchable . or programmable, divideris also included, The block diagra m of this isshown in Fig 2. By selecting lhumb-wheelswitches. the 12MHz signal can be passedthrough between 0 to 7 divide-by-2 blocks. 0to 6 owioe-by-s blocks and a possib le divide­by-a .By this means 112 ditferent frequenciesdown to 1Hz can be produced ,by selling totaldivision ratios from 1 to 12,000,000. All the'useful' cnessucnas 1MHz.25kHze tccanbeobtained, as well as such frequencies as93,75kHzand375kHz.Who knows. they maybe useful one day!

CONSTRUCTIONNO DETAILED CONSTRUCT ION details aregiven as constructors will probably want totailor the design to their own requirements,Separate units such as the u-es-s multiplierand the programmable divider could be usedon their own if desired .

The comple te circuit diagram is given inFigs 3, 4, 5 an d 6.The MSF head amplifie r isbui lt in a remote box and suppl ied with powervia the coaxial cable . In spite of being atnearly audio frequencies, RFtechniquesneedto be used here because of the low signallevels in this area . Input and output signalsshould be kept separate as the high gain inthis block can easi ly cause osci llation. Theferrite rod antenna is a long wave unit, withextra capacitance added. and provides theonly filtering needed . Bandwidth is around1kHz, but depends to a large extent on thesize of rod employed . The bigger the ferriterod the larger the signal received.

The reference genera tor is made in threeblocks , The VCXO and nmes-s multiplier areon one board, which contains all the real RFcircuitry . Layout should be suitable for thefrequencies invo lved , and ideally a printedcircuit board should be used. However, anyother method of RF construction, such as the'ugly' technique could be employed.

Fig 3: 60kHz Head Ampll tier.

A second board contains all the sectionsneeded for the PLL and the divide·by-200circuitry. together with the MSF input compa­rator and tone decoder. Any 'fogc ' type con­struction such as point-to-point wiring is suit­able. Care Should be taken to keep the two60kHz signals separate, as otherwise theloop will leekonto its own internally generatedsignal, and ignore MSF completely. Eachintegrated circuit should be by-passed with aO. l /IF capacitor at its pins, and ground con­nections should be as thick and direct aspossible. If the l 08MHz outpu l is not re­quired, the VCXO could also be incorporatedon this board.

The same comments on decoupl ing andlayout apply to the programmabledivider; thisshould be kept away from the RF board as thelow frequency signals could modulate thecrystal oscilla to r in the event o f anyleakthrough .

ADJUSTMENTSTHERE ARE A NUMBER of adjustments 10be set up and constructors are advised 10follow the procedure given .

Firstly the VCXO is buill and the 108MHzmultiplier aligned, Monitor the voltage on theemill er ofTR2 and peak L1 • L3 tor a figure 01O.7Vorhigher. Then monitoring the emiller olTR3, peak L4 and LS. Finally , using a diodedetector , peak L6 tor maximum output. An

output level of around 1OdBm Should be eas­ily achieved , enough to drive a 1N23 typediode in a comb generator to l OmA.

The next stage is to bring the ferrite rodonto 60kHz.The useofan oscilloscope makesthis a relatively straightforward job. With thehead amplifier unpowered, connect the cscu­loscope across the tuned circuit and, makingsure the ferrite rod is at right angles to thedirection of Rugby, adjust the preset capaci­tor until a 60kHz signal is seen which ispulsing on and off . Depending on the particu­lar ferrite rod employed. it will probably benecessary to slide the coils along the rod toobtain resonance.

On the South Coast I am able to obtain asignal of 5to 1OmVpeak-to-peak (p.p). If lhehead amplifier is now powered , a signal ofmore than O.5V p-p sinewave should bepresent on the feeder. If a distorted signalmuch greater than this is present. the gain ofthe head amplifier should be reduced byadjusting the preset.

With the MSF signal applied to the PhaseLocked l oop board, ensure that a 60kHzsquare -wa ve output from th e LM 393comparator is present on pin 2 of the NES67.During breaks in carrier, noise should causea random output at this point. Adjust the 10kpreset until the LED flashes on and off withthe received carrier. This completes the tonedecoder selli ng up.

Set the lock I free switch so thai the VCXO

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40 RADIO COMMUNICAnON April 1994

MSF LOCKED FREQUENCY REFERENCE

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Iree-runs an d adjust VA ' for a VCXO luningvoltage of 3V. If the divide·by·200 circuitry iswor king correctly. the phase meter shouldnow be swinging bet ween 0 and FSD. If not.it is poss ible that the beattrequency betweenthe VCXO and the rece ived signal is too highfor Ihe meter to indicate.

Adjust the VCXO preset. C ' , to bring thisbeat freq uency as near as possible to zero .The error should be less Ihan 0.1Hz, ie acomp lete cycle Irom zero to lull scale andback to zero should take longer than tenseconds . Set the switch to Lock, dischargethe 470IJF capaci to r, C' , by momentar ilysho rting it, and watch the me ter . Aller three 10four swings. the amplitude of the osc illationshould have died away and the meter needlewill be resting at mid scale, this is the 90 ·phase point an d the loop is now locked.

If a digital vol tmeter is available, mon itorthe tuning voltag e ove r a pe riod of one hour,it w ill drill due to the VCXO warming up. andil the vo ltage exceecs a.s vons. C' should beadju sted 10 achieve this va lue when locked.Harmonics of the VCXO can now be listened

for on any band of choice and the ca libration01 receivers etc checked .

II at any poin t in the selling up , the MSFLED stays on , the receiver is picking up theinternally generated 60kHz signal and thehead unit should be moved further Irom themain unit. JIailleads arBsufficienlly decoupled,a dis tance 01 , to 2 metres should be su ffi­cient.

There is no selling up olthe prog rammabledivi de r, except by ensuring an output 01 thecorrect frequency is avai lable lor differentsellings 01the programming switch es.

... to be continued

NEXT MONTHIN PART TWO, G4JNT wil ibe detailinghow to set up the receiver chain with­out using an oscilloscope.

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RADIO COMMUNICATION April 1994 41

MSF LockedFrequencyReferenceThe co nclus ion of a two part feature by A. C. Talbot, G4JNT

Tabte " F/lKl uencie. available l rom program mable di vid er.

FRED '" FRED '" FREQ '" FREO '"'000 016 , ""'.000 eo, I 3.7SOk , s c I 62.500k SO,' 000 0" I 240 .000 raa , 3840k '" I 75. 000 k , ea'000 '" , 250.000 " , , ' ''''''' '" I eo.coo, 0 57' .000 0" 300000 , a t ,

''''''' 13' I 93.7SO!< r zo' 000 , 16 320.000 '" I s.coce 0 53 , se.oooe

t "' 000 '" "'000 '" I 6,OOOk t ea , 100,0001< 085

10.000 oa , 400 .000 eo, I 6.250 k 06 , , 120.000k r se12.000 ' " 400 .000 '" z.sooe , s t I 125.0001< SO,16.000 '" 500.000 '" acoo, '" I 150.000k , ea"'000 '" 500000 12' aeoo, 12' I 187 500k '"24.000 ," 625.000 OSO IO.COOk OS, I zoo.oooe 06625.000 0 30 640.000 027 12.()OOk ' "

, 240 .000k , SO"'.000 , au 750.000 r ec 12.5001< 062 I ascocoe 0"" .000 OIS 768 .000 , t r 15.0001< IS ' , 3OO.000k ,"40 ,000 '" 800.000 '" 15,6251< 030 , ars.cooe '"48.000 , 13 960000 '" 16,0001< '"

, 400.000k 0"so.ooc ua , 1000.000 '" 18 .7501< 160 I 480.oook lSISO.OOO , a t 1.200k 12 3 19.2001< t a r I sco.oooe or6<000 '" 1.25<1k cs , eccoce 055 I 6OO.000k 16'"'.000 oa 1.5OOk ," 24 .0001< , a s I zeo.oooe ,"" .000 ," t.sooe '" 25.00llk 0 63 I rooo.ccoe '"100.000 032 1.920k '" 30.""'" 1S 3 , 1.2OOM 16'

120,000 , a a 2.OOOk '" 31.2501< or , , 1.500M '"125,000 0 " 2.400k ," 37.5OOk , s r I 2.000M SO ,128.000 '" 2,500k 052 40.oook 0 " I 2.400M 16'150.000 , 30 accoe '" 49.000k ," , 3.000M t zs160.000 0 25 3. \ 25k 060 5O.000k 06 ' I 6.000M t ze192 000 lIS 3 .200k eo, so.coce ," I 12 .000 M In

x a lvide by 6 se lector Switch AV Orvideby 5 selector Sw,lches B.C,DZ Dividebv 2 seteec- 5WltCheS E,F.G

tally the 1Hz comp on ent caused by the ca rrie rpulsing. The sample and hold significantlyreduces this componenl , but conside rable1Hz sidebands are still p resent. To achievethis. the loop bandwidth has to be significantlyless than O.5Hz an d a figure at O.12Hz waschosen. To calc ulate the values for the inte­grator time constants the characteristics ofIhe VC XO and ph ase detector mus t be know n.The VC XOconslant (Kv) , when divided downto 60 kHz, was measu red and a figure ofO.13Hz I volt obtained.

Th e phase detector figure can be calcu­lated by assuming the output changes be ­tween the supply rai ls. ie from 0 to 5 valls,when the phase var ies from 0 to 180\ Thusgiving a phase de tec tor constant 015 /1t '" 1.4voits zradian. A damp ing factor of 0,7 is usedas giving an optimum compromise betweenloop track ing and lock up charecteristics. Thestandard equations for pha se locked loo plead- lag network time constentsareemployed :

By USING THE INTER NALLYgene rated 60kHz, it is possible toset up the receiver chain Withoutusing an oscilloscope.

Firstl y. se t upthe tone decoderby feeding the 60kHz signal present a t pin 9altha 74HC74 to the MSF input. Adjusl lhe10k preset unlil the carrier LED lights. Nowthe lone decoder itself may tie employed toset up the ferrite rod tuning. Adjusl lhe presetcapacitor and Coi l posrnon unlil the LED isflashingreliably.Inareasof lowsignalstrengththe phase meier could be used to maximisethe rec eived srqnatto noise rauc by adjustingtor maximum deflection et the ma xima andmi nima 01the be at signal.

OPERATIONWHEN FIRST SWITCHING ON, with ireloop settc 'lock', the me ter wi ll be observedtoswing wit h a cycle lime of a few seconds.A fter 3 to 4 swin gs, the amptilude wi ll d ieaway and the meter shou ld sit at mid scaie . Ifthe two signa ls presen t at pins 1 and 2 01the7486 are examined on a dua l beam scope , nomovement between them shou ld be seen.

II the sw itch has been set to 'free· an d thesys tem free-running, the n itmay be diflicult orimpossib le subsequen!ly to achieve lock w ith­out switching the unit off and waiting for a fewminutes .rntsebecause, whilst free-running,the integ rator output will probably have rampedup to its ma ximum of around 10 vons.

When the loop is subsequently closed, th isvoltage pulls the VC XO frequency outs ide theloop lock range an d a beat frequency ofl Hzor so may be seen . the loop never being ableto loc k up to this frequency error, The quicksolu tion is to discharg e both 470 J.l F capaci­to rs so tha t they ramp up from zero towardsthe nominal tu ning vo ltage of aro und 3V as ifthe unit had just been turned on .

One other problem that may occur is if theprogrammable divi der (Table 1) is se t to givean output where a close harmon ic fa lls at60kHz. (15, 20 , 30kHz etc) In this case thereis likely to be enough 60kHz teakage to causethe MS F signal to be ov erridd en, and allowthe loop to lock to itself. Should this occur , thecarrier light wi ll remain on and the VC XOfrequen cy w ill e ither ramp to ns ma ximum orminimum values. If le ft unchecked , it is possi ­ble tha t the tuning voltage could rise so highthat subsequent lock cannot be achieved , inthe same way as described above.

The 108MH z ou tput should be sufficient todrive a 1N23 type m icro wave diode to l OrnAor more diode current. This will give strong

enough harmonics to be heard easi ly at10GHz. At this frequency, on an SSB re­ceiver , Ihe received note is 'c lean' but it isunlikely to be complete ly steady, A randomlyvary ing change in the bea t note of some 30 ­100Hz, ov er a period of several seconds. willp robably be observed, du e to several rea­sons. The prima ry one is me characteristicsand signal to noise rat io of the MSF signa l.Interference will perturb the loop operatingpoint and cause a lrequency va riation whilstthe loop tracks the signal. Another cause isinstability in the VC XO. This random variat ionis actually phase noise, although it is dif ficultto think of noise as being at fractions of a HZI

An interesting test is to wa rm the crysta l byholding it whilst listening to the beat note. Asthe crys talwa rms up the frequ ency will quick ly(within 1 - 2 seconds) drift . Over a longerperiod this will be corrected by the loop andme origi nal beat return. Good con structiontecnniquee around the crystal oscillator willminimise this effect.

LOOP DESIGNTHE PHASE LOCKED LOOP has a verydemanding specifica tion , It has to reject to- " .

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46 RADIO COMMUNICATION May 1994

Choosing a capacitor value of C :: 470)J Fgives the necessary values of the resistorsaround the integrator to meet these timeconstants:

Taking me nearest preferred values givesthe values shown in the circuit diagram. Afinal network is added at the integratoroulpulto give further attenuation at 1Hz. A timeconstant of t.zs. formed by 56k and 22~IF

gives a further 19dB reduclion in ttus compo­nent. whilst me time constant is tast enoughnot 10 altect the loop tracking perlormance.Wilhoulthis extra filter. around 10Hz of Ire·quencyshilt (at 10GHz) was noted every timethe carrier was swncneo en. Wilh lhe filter noshift was discernible.

Amateur RadioTechniques (RSGB)

by PIf' Hlfwke r, G3VA

A"l'''''t ol 11~ "" .""" A~"ge sok>c1lOfl 01(1"""1....ss and dg"'OOs. informat"", on ant",,,,,,>I'ILlS.ooo<tr"" lIo,,"1 n.nlS r""" RMC"",'s popularTach...,., ' Top.cs' oaluro. 366 pallOS

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••••

Technical Topics Scrapbook1985-89 (RSGB)

by p",Hawker. G3VAArepnnl0lellll", IT_ lrom1965-119on<:1u"","e,",Ih "" 1tI<lo. , Inv"l_ lor "'p"m""nt. rs on<!c:or>str""lors, 340 t><\9C'5

2) An output level of t 10 dBm at 96MHz hasbeen obtained from the multiplier , bychang ing the capacitors across the tunedcircuits to give a qcadrupler followed by adoubler stage.

There is further scope for oplimising theloop feedback function. If the loop bandwidthcould be narrowed still further, it wou ld beless susceptible to noise and interlerence(but more so to oscillator slability). 11 wouldthen probably be necessary to emp loy non­linear techniques to improve lock up l ime andpull in range.

If it is intended to use this unit portable, itmay be necessary to temperature stabilisethe crystat oscillator to preven t n movingoutside lhe pull in range at temperature ex­tremes. One way to do this would be to usethe crysta l heaters supplied by the RSGB'sMicrowave Component Service [Note 2).

NOTESNo components list has been pro­vided as this article is intended as asource ot ideas, rather than a per­fectly reproducib le construct ionproject,

2 The ASGB MicrowaveComponentsService can be contacted clo Mrs PSuckling, G4KGC, 314A NewtonRoad, Aushden, Northants NN10OSY; tel: 0933 411446.

MSF LOCKED FREQUENCY REFERENCE

A2 = 12/ CRl =t1 / C

CONCLUSIONS ANDFURTHER MODIFICATIONSTHE FREQUENCY REFERENCE generatedis more lhan good enough lor 24GHz narrow­band work in CW bandw idths! The unil hasbeen designed so lha l different parts may beused separately as individual constructorswish. II other ouipu tfrequencies are desiredthe following mocmcauons could be made:

1) A 30MHz VCXO could be used instead of12MHz, with a division ratio of 500 to give60kHz. This would give access to 10MHzand 5MHz. not available from the unitdescribed.

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Cont"n l$ Codas :I ~ PCB Mo~nled PaMs Only2 : PCB Only3 =Case Mounted Parts4 • Roady Punc/lod Ca se5 .. Case Vn-Punc~ed

Des c, iptlon RadCom Part no P, lce

RSGB Morseman MMPCB £10.00Morseman EPROM MMEPROM £5.00GW41-1WR 12V l A PS U (MayIJ~"" 91) 99137 £325ICOM IC725173$Conlroller (0Cl 92) ICREMPCB £10.00IC7251735 ClIlr EPAOM EPRO MICOM £5.00Wob bulator INov92) WOBB £4 95Wob bulator ready built ABWOBB '0'S imple Spec1rum AnaPys", INov89) I 189SSA £16,00OSe<lloscope Prolle TeSI"r INov 91) OSCP RO £450G3TSO $·lland l,anse" ,. o, ISe p 88) 15007 £28 00G3TXO 3-llan<l Transe" ,,,,,' IFebiMar 891 TX0 7 £2350G3lS0 Mlnillturo 80m l evr IJ~nlJuVAug 9 1) G3T50 MIN £8 00G4WIM 5OI70MHz lranse.",."r (May ' Aug 1990 ) WIM 10 £52002m noIse e h,mnalor ("1"921 2MTRRF " .00VII,""'te koyor ("arly8051 VLTKEY £6 00WhII" Rose Aaee,ve' (Feb 90) WAMAtN [4 ,25W~lle Rose Plug.m con verters (eac ll) WACONV £2 00W~,le Rose Case WRCAS E £15 ,75G3PCJ 160m Transceiver (JarYFeb 93) TOP I60 £7,50O"eClIOfl FInde r (TT Apr 91) VI-1FDF £375AF Osc liialof (Se!> 90) AFOSC [4,95SynlhOslsor (J uVA~9 92) SYNCPCB ""

Add [ 1.50 to a ll pnce8 10' 1'081"9 " and pac king

Available trom'Badger BOllrdS

87 Blac kbe,ry Lane ,Fo ~ , Oa ks ,Sulton Coldliold , B74 4J F. Te l: 021 353-9326

RADIO COMMUNICATION May 1994 47