Tektronix 494AP service manual

This synthesizeruses a variablemodulusprescater to dividethe VCOfrequencybeforeis processeO "+N" counter,such as shown in Figure7-29.by the

The vari_ able.modulusprescaleris iontroilej by a mocluluscon_ trol input. Whenthe line is hightne pr6scateris a divide by P+l and whenthe tine is l6w the divisioncnange"to P. A commontype of.pre.scaleris a *1g1li. A cy:crcof systemop€rationstartswith all programmable

counters loqOe!.1n9*OV to count. The'var]ablemodutuspres- calerinitiailydividesby p+1.

Two programmable

dividersare used with this sys- te-T,qoth triggeredby the prescaleroutput. One is a +N, yith N beinga relativelylargenumbei,tne otneris a "+A",whereA is a smallnumOlr. Onepossiblestate includesA - 0.

.	The operationof this system is as foilows. The
lower case letters representvariables,
		the upper case
!:,:t:,r"^pr-"sentthe programmedvalues. At the begin_
ntng of -th€cycle, p-p1l		, a_A, and n_N. After p+1
pulsesfrom the VCO, one pulse
		is applied			"a,,
and *n'counters and			to the
		,,n,,'decr"".e			1
!3 -			by
	t-1,n - N-1). Thiscontinuesuntita_ 0 at which
time the moduluscontrolline changes				_ p
		stateand p
while n - N-A. The counting co-ntinues				_	O.
			until n

Both the nnoand "a,'counters now return to the pro- grammedcondition.The total numberof pulsesapplied from thEVCOis:

Ntotar- (p+l)A + p(N-A) : A + pN

Both N and A are programmablesuch that

F,"o] (A +_PN)FrerThis. leads-to a possiblechannel spacingof Fr"y,obtainedby changingA by 1.

A functionalblock,diagram of the AuxiliarySyn- thesizer is shown adjacent to the schematicin the diagramssection. The VCO (02071)is configuredin a Colpittsoscillatorcircuitwith ttrelnductanceas a three turn air core coil with feedbackprovidedby C2072and C2071. Coarse tuning is accomptistreOwitn C1O7O, while the voltagecontrolof the frequencycomesfrom the varactordiode,CR2O68.This diodejrovides a fre_ quency shift of over 30 MHz from a voitageswing of +5V to +11 V, whichis ampteovertaptorine 20MHz tuning range, The output power of the oscillator is

0 dBm into 50O. The oscillatoris biasedso it can be turnecfoff and on rapidly.

The VCO is turned off by turning e2076 on. In operation,the synth€sizeris iurned off during periods when informationis presentedon the CRT. Synthesis and counting is done during retrace time to prevent possibleinterferenceon the displayfrom any radiated energyfromthe synthesizer.

Theoryof Operailon- 4g4Al494ApService,Vot.1

-TheVCOoutputis split by a resistivepowerdivid€r. One output drives transistor U2O5g,w'trictrprovidei f7 lAm of signatoutput to the HarmonicMixer. This deviceis biasedto a 20 mA collectorcurrentby transis- tor Q2055.

The other VCO output drivesa low gain amflifier, Q2049,which is biasedby transistorozdst. Heiative feedback,in the form of emitterdegenerationandlhunt currentfeedback,sets and stabilizesthe gainto ensure stabilitywith regardsto spuriousoscillations.The out_ put of Q2049,to drive the variabtemodulusprescaleris 0 dBm. The variabtemodulus prescalertj3ost, is a +92133tC that featuresan ECL input with a TTL or CMOScompatibleoutput.

. The major circuit of the synthesizeris U4041,a large-scale-integrationCMOSdevice,which is usedfor frequency synthesis apptications with a variable modulusprescaler.The devicecontainsthreeprogram- mablecounters;a +N, a moduluscontrolcounter+A, and a referencedividerthat dividesan inputfrom a cry- stal controlled source or other referencetrequency down to a desiredfrequency.This devicehas a speeO comparableto TTL. lt also containsa phase-frequency detectorwhichdrivesan externaltoopfilter. U+Oit wiit acceptdatafor N, A, and R inputsfrom a 4-bitdatabuswhile a 3-bit addressbus sel€ctsthe informationto be loaded. DatacontainEdon instrumentbus linesDB4 to DB7 is loadedwhenthe enablelinegoeshigh. Address informationis containedon instrumentbus lines DB0, DB1, and DB2. The appropriateg2 latches are also containedwithinthis lC.

-Theoutput from ths phase/frequencydetector in U404'lis a chalnof puls€ signalsat the ieterencefre- quency. The pulses contain both ac and dc com_ ponents. The ac part of the signal causesreference sidebandsto appear on the VCO output.These side_ bandsare suppressedby two activeloopfiltersconsist_ ing ol U2040Aand U204OB.The detectoroutputs,CR and.dv, which are similar but with reversedpotaritrj, apply differentialtyto the input of U204OA. Stight differencesin pulsewidth betweenthe two outputsgin- eratea dc voltage. This is furtherfilteredby an active low-passfilter,U20408,to suppressfrequenciesabove 20 kHz. Thenit is appliedto the varactordiodeCR2O6g in the 200-220MHzVCO.

U2MOAis an integratorwith a seriesresistoradded to the feedbackcapacitor. This controtsthe slope of the loop gain at gain crossover. To provideadditional suppressionof the referencesidebands,an RC active two pole filter, U20408,is added. Cutofffrequencyis about 20 kHz. The loop fitter (U2040A)anctthe VCO providBthe dominantpoles that determineth€ system response, A damping factor near unity providesthe stability.Additionalfilteringin the form of passivecom- ponents, with a high frequency cutoff, are added betweenthe output of U203gBand the varactordiode

7-81