Tektronix 494AP service manual O a o O o O o o o

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amplifiers provide approximatety +10 dBm to the 2072 MHz 2nd Converterand +g dBm to the Reference Mixer.

Sinc€ the	two buffersare..nearly
2nd Converter			identical,
			'Oain		only the
	bufferis described.			is provicled
					by
Q1011. printed elements provide input and ortpui
matching..
imp:91199		Out-of-banddampingis provided
by R1011in serieswith
Dc is btocked		a t/4 wavelengih
				snortedstub.
	by G1014			t1+wavetength
		and C101t. i
open stub is usedat the outputto reflect
Converter's				oneof the 2;d
imagefreguenciesat 4ZS4
				MHz (the other
bufferdoes not use nor need this stub). Collectorbias
for Q1011is providedthroughRtfiz; 11011,the 1/4
wavelength shorted stub. and R1011. The 114
wavetengthshorted stub is groundedthroughC2Oi1
(C2011,Cl0lg, and L1011 aie also used for decou-
pling). Coilectorvottage
		is determined
				by dividerR101g
and R2013; this controts the dc te6dback to the
collector-basejunctionof eloll.			The bias networkis

decouptedfrom the RF path by L1014. DiodeCR2013 protectsthe baseof el01 1 from excessivereversebias if the +12 V supptyfails.

2200 MHz Reference Board (Diagram t4)

_ Th," circuit generatesharmonicsof the 100MHz input. The 22nd harmonicor 2200MHz is used by the ReferenceMixer. The input 100MHz signalls apptieO through€^matching,network lconsistiig of LiOg4, L1025, C1096,C1029, and Clb2S) to i oinerential amplifier (01024 and e2024). The €mitters of this ampfifierare ac coupledthrough C2026,reducinglow frequencygain and ensuring balancedoperation. A snap-ofrdiode (CR2014)is driven by the amplifier,via transforner T2015, to generatemuitipteharmonicsof the 100MHz signatinctudingthe 22ObMHz reference. The outputpassesthrougha 3 dB attenuator,for isola- tion, to the ReferenceMixer circuit.

22A0 MHz Reference Mixer (Diagram i4)

Signalsfrom the 2200MHz Referencecircuit are filteredby a printed22AOMHz bandpassnner. OioOei CR1011and CR1012are the switchingelementsof a single-balancedmixer. The microstripLscillatoroutput is appliedto CR1011and througha 1/2 wavelength derayrineto cRl012. The derayrineshifti theosciilator signal 180 degrees so both diodes switch together. Mixingthe 2200MHz with the osciilatorZ1g2Mfiz sig_ nal producesthe differencefrequencyof 1g MHz. This 18 MHz signatis fed througha 3Z MHz tow_passfitter to th€ 16-20MHz phaselock circuit. Thelow-passfilterpley9lls llwanted products,such as 82 MHz(product of 2100MHz and 21g2MHz), from passinginto the phaselockcircuit.

Theory of Opera{on - 4g4Ll4g4ApServlce,Vol. 1

16-20 MHz Phasetock Board (Diagram 13)

This board contains regulatedpow€r supplies,a 16-20MHz (18 MHz nominal)voltagecontrolledoscilla- tor with linearizingcircuitry, and a phase/frequency detector circuit. lts main function is control of thi 2182MHz MicrostripOscillator. Th€ entire circuit board is housed in a magneticshield to reduce spurious efiects of external ac fields. All power supply and con- trol inputs enter thg circuit board via feedthroughcapa- citors in the housing wall. All connectionswith the 1ic^loyave circu_rlryare through feedthroughcapacitors c2200 through c22a4.in the floor of the ho=using.

The +15V, -lSV,and +gV supplyinputsare re- regulateddown to +12V, -12V,and *5.2 V by regula- tors using operationalamplifiers.lC U2O2Sprbvidesa stable -6.2V referencethat is fitteredby R201gand C2015and amptifiedby U20t6Bto produte the -i2V supply. fC U20168 uses emitter-foilowere2024 to increasethe current capabilityof the supply. Resistor R2013 ensures sufficientbase drive, wiriie collector resistor R2025 r€duces pow€r dissipationin e2024. DiodeCR2019protectsthe base-emitterjunctionduring power supply shutdown. Feedback resistors R2016 and R2017 set the gain of U2O16Band control the -12V, +12 V, and *5.2 V supptyvottages.

..^-fE-12Vsuppty is apptiedto invertingamptifier U2016Ato produce the +12 V supply, and inverting amplifierU1017to producethe +S.2V suppty. The out- put circuitryfor the +12V and +5.2 V suppliesare simi- larto the -12V suppty.

DifferentialamplifferV2072Aaccepts th6 2nd LO sweepvoltages. One input sensesthe sweep voltage while the other inputsensesthe groundpotentiatat the Sweepboard. Sweepsensitivityis adjustedby select- ing resistor R2070. fn wide spans, the sweep signal passesthroughparall€lresistorsR2AE2and R20g3. ln narrow spans, R2082 may be switched out by 02094, which reducesthe weep sensitivityby a factor of ten. Whenthe TTL signatto 02026 is high,e2076 is tumed off, R2086holds the gate of e2094 to -lSV. e2094 is turnedoff, and R2082is switchedout. This reducesthe sweep sensitivity. Whenthe TTL signal is low, e2026 saturateswith the collectorstighflyabove 0 V, O20g4 turnson, and full sweepsensitivityis restored.

AmplifierU2072Bacceptsthe 2nd LO tune vottag€. the Tune board sensesthe groundpotentialof the 16-

20MHz PhaseLock board and floats the tune voltage. Tunesensitivityis adjustedby selectingresistorR2Ol2.

The sweep and tune signalscombine at the sum- ming node input of a non-linearshapingamplifier.The non-linearityof the shapingamptifiercompensatesfor

the non-lineartuningof the referenceoscillatorvaractor to givea lineartuningcharacteristicfrom 16 to 20 MHz, The shapingfunction is producedby a resistor-diode

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