Tektronix 494AP service manual Ou-Too---l, Precision, ExternalCabtes

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o			Theory of Operaton - 4g4A/4g4ApService,Vol. .l
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o		j--"ou"'-'T'oo"---l
o	RF INPUT				RF OUTPUT
a	Q+r				+Fo
a	h		TUNINGSCREW		I
a			,I		lh
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a	CAVITY 1	CAVITY 2	CAVITY 3	CAV!TY 4
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o					2727-1o2A
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o		Figute 7-2.Equivalentcircuit of the four-cavity filter.

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2072MHz RF from.the

four-cavity fitter (FL14)

feedthroughcapacitorC182)is low. This causesthe

entersthe mixer, where it

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is switchedon anOoff at a

outputfrom U1014Ato be at +14 V andthe outputfrom

2.182MH2rate by the the mixer diodes. Both mixer

U10148to be -14V. Diodes

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diodesare tumed on and off by the 2lg2 MHz 2nd

LO

CR1O14and CR101gare

therebyreverse-biased.

signal. The

difierence frequency of

110MHz is

potentiometer

Thus,the seriesresistancesof

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separatedfrom the other mixer productsby a low-pass

R1019 plus resistor R1014,and poten_

tiometerRl010 plus resistorR1017,provideforuvard

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filter for use as the tF output. Attnougnth! diodesare

bias to the diodes. The potentiometers

connectedfor opposite polarity,both are tumed on at

ancethe biaslevels.

are set to bal-

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the same time becauseof ths 1g0 degreephase shift

delay.line in.the input path to one of t[e

diodes. Note

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thatpair the diodesare matchedand mustbe replacedas a

In op€ration where the mixer is not active,the lF

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lf onefails.

SELECTsignal is high. This reverses

Ul0'14outputsand

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the statesof the

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At the output of the mixer. the two inductorsand

foruvardbiasesdiodesCRl014and

CR1018. With these

diodes conducting, resistors

one capacitorform a low-passfilter that passes

R1014,

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tenuated

unat_

R1016,R1017,and R1018form two voltage

110MHz signatto the g2gMtiz

2nd Con-

dividersthat set the reversebias,to the mixer diodes.

verter,via

plg2.

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coaxialconnector

Dc_blocking

capaci-

at 5 V. This effectively

tors at the three inputs to the mixer, keep tne diode

turns the mixer off and attenu-

atesthe 110MHz signalby about55 dB.

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bias from being applied

to the RF and locat oscillator

lines.

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The bias

circuit,

consists of operational

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amplifier

which

Ul014 and the associated

Precision

components,estab-

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lishesth€ bias for the mixer diodesand atso provides

ExternalCabtes

the meansfor effectivelyswitchingthE mixer oif lunder

The externalcablethat connectsthe four-cavityfilter

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control of the microcomputer). When the mixer is

output to the mixer RF input (W140)and the external

active,each diode has approximately

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2 mA of forward

cable that connectsthe 2nd LO to the mixer LO input

bias. For this condition,the tF SELEbTsignalfrom the

W222)are both criticattengthcabtes.

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Z Axis/RF

lnterface circuits lappti-eO through

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7-9

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