Tektronix 494AP service manual

o a o

I		Becausethe grd harmonic
o							of 719 MHz is lockedto
	the	2182MHz reference, the						tuning range ot tne
I	719MHz osciilatoris ontyone third oithe tuningrange
	of the referenceSince.				.th:rgngeis 4 MHz,
t	the 719MHzosciilator								'ttreringe-ot
			is 71g Jt.gg trlnz.
t
	The 719MHz		VCO (O20i4)						uses a		Cotpitts
	configuration,
o		with a printedcircuit quarter-wavelength
	transmissionline resonator,to							achieve high specir"t
o	purityand good thermatstability.
								Correction
	appliedto varactorctiode									voliageis
I						CRldlt (whichis connected
	at the midpointof the transmissionline resonator)to
t	vary ths r€sonantfrequency						of				jine
								the transmission
	ov€r a 1.5 MHz range. A tunabletransmission
o											line(atso
	printed)adjacentto the printedresonatorcompensates
	for	variationsin componenttolerancesand resonator
o
	dimensions-This adjustable
						transmissionlineis cut, at
o	factorycalibration,to the correct lengthfor properVGO
	operation.A scalewith minordivisionsevery										iMHz is
o	printednext to th€ adjustabre
	The						line to aid in caribration.
o		outputfrom the oscillator
	of the quarter-wavelengthis extract€dnearone end
	inductorsand applied					line through two printed
t				to output amplitiersthrough a
	powersplitter.
o	The 719MHz VCO				is enabtedor disabted,under
a	microprocessor
		control,				dependentupon the frequency
	bandbeing.anatyzed,
o			by the tF SELECTline. Whenthis
	lin€ is low, e2017 is cut off, which turns e2016 otr.
o	This, in turn, cuts off transistore8015 (which is the
	current source for		oscillator transistor e20t+;, tnus
o	disablingthe 719 MHzoscillator.
O	From the oscillator,the *6 dBm 719MHz output
a	signalis applied,throu-gha power dividerconsisting
	resistors Rl021,		Rl022, and					Rl02O, to			of
											tsolation
o	amplifierQ1021. From the other side of this power
o	9iut9gr,the signat is apptied to an output amptifier
	(02021)for transmissionto the 929MHz 2nd Converter
o	Mixer circuit. A second isolation								amplifier (OgO21),
		in configuration,provides
t	Il.e^n!i911							isotationbetweenthe
	719MHz oscillatoroutputand any undesiredHarmonic
	Mixerproducts.
a	The829MHz Harmonic
o						Mixer producesnot onlythe
	required25MHz differencefrequency,but also many
a	higher order
	frequencies,intermodulationproOucts. Two of these
t		744 MHz and 6g4 MHz, are 25 MHz from
	the	719MHz oscillator frequency. The								isolation
o	amplifiers,01021 and e3021,provid6sufficientattenua_
	tion in the reversedirectionto preventthese products
a	from gettinginto the 829MHz mixer to produie spuri_
	oussignals.
o	To provide maximum
I
						reverse attenuation
											in each
	amplifiercircuit, external RF feedback is kept to a
o	m,inimum.An output matching
	of capacitor						LC network,consisting
a		C1025plus a printedinductor
									for e1021,
	and capacitorC3021plus a printedinductorfor e3021,
o	presentsan optimumloadimpedance
									to the collectorof
o

a o o

Theory of Operation- 4g4ful4g4ApServlce,Vol. 1

each transistorto allowmaximumpowertransferto the attenuatorthat precedesthe harmonicmixer. An input LC matching network consisting of capacitors Ct0'23, 919?,plus a printedinductorfor al02i and capacitors C3023,CgA22,ptusa printedinductorfor e302i, estab_ lishesthe 50 ohm inputimpedanceto eachtransistor.

A3 dB attenuatorconsistingof resistors R3021, R3022, R2021, and R3023, at the output of isoiation amplifier Q3021, provides a non- rehective source impedanceto the mixer. Withoutthe attenuator.mixer conversionloss couldvaryfrom unitto unit.

The 829MHz HarmonicMixer, consisting of diode CR2021, inductor L2A14,and a half-wavelength(at2182 MHzl transrnissionline, produces thE difference frequencybetweenthe third harmonicof the 719MHz 9:9,11?tolfrequency (nominalty2157MHz) and the 2182MHz reference frequency. Note that the 2182MHz sfgnal is suppliedtrom tfre 21g2MHz 2nd LocalOscillatorthroughcoaxialconnectorp237andthe power divider, consistingof resistors R1021,R1029, and R1022.to a 112wavelengthtransmissionline. The VCO input to the mixer switchEsdiode CR2021at a 719 MHz rate, The 2182MHzreferenceacts as the RF and is appliedto the diode from the transmissionline. The resultant 25 MHz int€rmediate frequency is diplexedfrom the mixerthroughth€ 1OOMHz tow-pass filter consistingof capacitorC3014and inductorL{jfi4. (DiodeCR2021is mountedon printedcircuitboardcut_ outs to relieve any nec€ssity of bending the diode leads. Lead bending may fracture the diode case.) fnductorL2014providesa bias returnpathto allowthe diodeto switchat a 719MHz rate.

From the 829 MHz HarmonicMixer, the signal is appliedthroughthe abovementionedlow-passfiltertocascaded amplifiers Ul053 and Ui0448. These amplifiersboost the €2 dBm mixer output signalto a level appropriateto drive the phase/frequencydetector. lC amplifier(U1053)containstwo differentialamplifiers in cascade; amplifier lC U1044 contains only one differentialamplifierand acts as a bufier. When the loop is first acquiringlock, such as at power-on,thenominal25 MHz lF may be as high as 34 MHz. Two stagesof amplificationare necessaryto ensureenough gain tor the phase/frequencydetectorto drive the lF back to 25 MHz;the bufferis necessaryto provideECL levelsto the detector.

The secondinputto ths phase/frequencydetectoris the 100 MHz signal,from the referenceoscillatorin the 3rd Converter,via two amptifierstag€s, U1022Aand U10228, and a divide-by-fourcircuit, U1036A and U10368. The 100MHz signatis divideddown to a 25 MHz reference for application to the phase/frequencydetector. Two stagesof amplification are usedto isolatethe 100MHz referencebusfrom sig- nals generatedin the local oscillatorsectionof the 2nd Converter.This stable25 MHz referencesignalis used

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