Tektronix 494AP A o a o o o o o o, Otr, Switchandamplifier, 829MHz2nd Conv, 2O72MHz2nd Conv

o a o a o o

O

I Ia

a

to

O

a o o o

t

a o a

O

a a o o o

to

O

o

t

o o a o a o o o o o o

_ The 3 dB pad helpsensurea consistent5Oo inter- face for the 829MHz Band-passFilter.

. T!" 450 MHz High-passFilter btocks the tower lF signalsgeneratedwithinthe mixer.

.	The mixer generates several intermodulationpro-
ducts of the 829MHzRF and 71gMHz LO signats.The
mixer diodes are transformer
					driven Oy a targe ampli-
tucre{+12dBm) 719 MHz signartrom t'herocit osciita-
tor.	.This large signal driveJthe diodei in					and out of
conduction,switchinglhe.lower
					amplitudeg29MHz sig_
nal on and off at a 71g MHz rate,to generatethe mixer
producJ:: Ontythe differencetrequeicy of 110MHz is
passedthroughthe A00MHz Low-pasj filter. The sum
productof 1548
	MHz, is reflected
					back to th€ mixer by
the 829 MHz Band-passFilter, in-phasewith LO har-
monics,to increasethe energyof ine 110MHz signal.
A..printedde-layline, Uetweenitre S2gMHz Band_pass
Filterand 1.3 GHz Low-passFilters,							the phase
delay, The net rssult of					,'image
				this	"ontrot"
					enhancem€nt,,
							is
low conversionloss and good intJr-modulation
tion performance.		The OdB					distor-
					pad reduces the image
enhancement
	effect and permitsthe use of non-critical
line lengthsand filter characteristics.
loss,from829MHzto					Overallconversion
					g.5	dB, includ-
			1.10MHz,is about

ing 2 dB from the 929MHz Band-passFitterandg dB from the attenuator.

The 300 MHz Low-passFilter btocks LO, RF, and higherfrequencyproducts.

110 MHz lF Select The 110MHz lF Selectcircuit {1?3161selectsthe 110MHz tF signatfrom eitherthe 829MHz 2nd Converteror the 2OlZ-UAz2ndConverterfor transmissionto the 110MHztF Amptifier.

The 110 MHz tF signatfrom the 929MHz 2ndCon_ y919r-boa( is apptieddirecflyto thE selectcircuit. The 1'f0 MHz lF signalfrom the iaZZu{z Convert€rboard is appliedto the selectcircuitvia p233 and a controiled amplifier(Ot012/Oi011).Switchingbetweenthe two lgft:_ is done by CR2011,CRtOt2, CR2013,and cR1015 .

DiodeCR2011is turnedon whenthe tF SELECTline to the 110MHz lF SElectis low. Thls steers the 110MHz lF signal,from the g29MHz 2nd Converter 1119:-tothe_outputport. At the same time CR2012, CR2013,and CR't01Sturnon and el011 turnoff to iso- lalg_theoutput port from any spurioussignalsfrom the 2072MHz 2nd Converter.

Whenthe lF SELECTtinegoeshigh,etOtl turnson and CR2012,CR201g,andCBl01Stirn otr to ailowthe 110MHz lF signal,from the 2O72MHz2nd Converter, to be apptiedto the outputport. SeriesdiodeCR2011 also turns off to preventsignal loss into th€ inactive

Theory of Operation- 4g4[l4g4Ap SeMce, Vot. 1

829 MHz 2nd Converter. Becausethe 7l g MHz LO is alsoturnedoff by the state of the lF sELECTline,isola_ tion for the 829 MHz 2nd Converteris nol critical wlien the converteris inactive. The switchand amplifierlogic is summarizedin TableZ-2.

TabteZ-2

SWITCHANDAMPLIFIER
			SELECTION
lF Select	Serles	Shunt	110MHz
Llne	Swltch	Ampllfier	lF Source
High	On	otr	829MHz2nd Conv.
Low	otr	On	2O72MHz2nd Conv.

The diodes are used as the basic switchel€ments. They presentonly a few ohms of series resistanceto RF signalswhenfonrvardbiased,with currentof several milliamps. When reverse biased,the diodEs present €s_sentiallyan open circuit. The control signal from 02015 is connectedin a series path throughthe four diodes (CR2011,CR2012,CR201g,anOCAtOtSl anC inductorsL2011,L2019,and L2019.Thus,onlya small currentis requiredto fonarardbiasall four diodes. This biascurrentis also usedto turn off el01 1.

DiodesCR2012and CR2013are incorporat€dinto a pi+!pj_matching network,consistingof L2011,L2O1O, and C2012. Thereforeboth switchesshuntthe signalat moderatelyhigh impedancepoints. In addition,when the switch diodes are turned on, parallelresonance betweenL2011 and CzAl2 presentsvirtuallyan open circuit to signals passed by switch CR2O11.Switch diode CR2013is tocatedat th€ high impedancenode createdby the series resonanceof L2019and C2017. DiodeCRl015direcilyshuntsthe outputfromel01 l.

Transistor Q1011 operates as a common-ernitter ampfifierfor the 110MHz tF signatfrom the 20Z2MHz 2nd Converter. lts gain and impedancematchare con- trolledby feedbackresistorsR101i and Rl012. Resis- tors R1013and R1018attenuatethe outputby approxi- mately6 dB for enhancedcontrolof impedancematch and stabilitycharacteristics.

TransistorQ1012 maintainsa constantdc current through01011. Dc Coilectorcurrentfor 01011is set at approximately15 mA. Collectorcurrentfrom e1011 developsa voltageacross Rl017. Transistorel012 then comparesthis voltagewith the fixedvoltageof the voltagedivider,Rl015 and R1016.Any variationin the collector current of Ql 011 is sensed by e1012, and ofiset by a resultant change in the base curent of Q 1 0 1 1

When th€ control current from e2015 (throughthe switchingdiodes)developsa voltageacrossRl017 that exceeds the control limits of 01012, it effectively removesthe base-biasfrom e101i and turns el 01I

7-15