Tektronix 494AP service manual BandLevetingCircuitDiagram20, GalnRequired

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o	Theory of Operatlon - 4g4Al4S4Ap Servlce, Vol. 1
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Table 7-4

GAIN STEP COMBINATIONS

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GalnRequired

Data Biis

468A5

468A6

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2

1

0

PinN (10dB)

PinV (20dB)

PinY (10

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ru ots

0

0

1

0

1

1

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20 dB

1

0

0

1

30 dB

1

0

1

0

1

0

0

1

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40 dB

1

1

1

0

0

0

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BandLevetingCircuit(Diagram20)

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VR Mother

Boards {Diagram 18)

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-

The two amplifiers,

in the VariableResolutionBand

The circuits on the VR Mother boards provide

Levelingcircuit,conect gainvariations

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end.

throughthe front

addressand data decoding,band leveling

These band-to-bandvariations are due to

the

plwer supply

control,and

and controlsignalinterfacing

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differentmodulationproductsout of the lst

Converter

to the other

VR boards. The VR Motherboard#1 (A6SA)provides

and lossesthroughthe preselector.

decoupledpower suppliesand interface

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Input (A6849),

linesto the VR

Steps(A68A5),lst

Fitter Setect(A68A4),10 dB Gain

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and 20 dB GainSteps(A6gA6)

Nominalsignal input tevel for band 1 at 100 kHz

boards.

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resolution,in the Min Distortionmode, is _12 dBm.

The VR Mother board #2 (A6gA2)providesaddress

This decreasessornefor

the higherbands. The output

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level is about -2

and data decodingand gain controffor the Band Levef-

dBm. This output level is kept con_

ing circuit. The VR Mother board #2 providespower

stant by

using

microcomputer to adjust

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amplification

the

the

supply voltages and control lines to the VR Mother

throughthis circuitfor eachbanct.

board #1 (A68A1),Band Leveting(A68AZ),2nd Fitter

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Select(468A8),andthe VR post Amptifier(A6gA9).

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The two amplifierstages

on this boardare similarto

Address and data valid lines from the instrument

10dB

gain

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llre

steps circuits. A stage consists of a

address bus are appliedto address decoder U4O2Z.

three-transistorcircuit using a diffeiential pair

con-

Data bit 7 is appliedto the decoder,sselectinputA as

nectedto an emitter-follower.

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alteringthe feedbacknetwork.

The gain is controlledby

a supplementaladdressbit. This bit selectseitheran

address to

latch data for th€

resolution bandwidth

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selection or

an address to latch data for gain step

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.The first stage(e2015,e2019, and e1025) has a

selectionand bandidentification.

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gain range of 13.5

ptN

dB by controiling

Datalatchesu3010and u3017monitorthe databus

diode

the bias of

CR2021in the feedbackloop. Biasfor this diode

at the sel€ctedaddress. Latch u3010 storesthe filtEr

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depend-son-avoltagedivider networkconsisting

s€lect data that controlsth€ lst

and 2nd FilterSelect

of an

array of variablaresistors on the VR Mother board #2,

circuits.

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468A2,with the dividernetworksetectedby the micro-

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computer.

U3017 latches

the

gain

selection and band

identification

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data. Latcheddatabits 0, 1, and2 (output

second.stage(e1031,e1o3g,and

e1041)is

pins 2, 5, and 6) switchtransistorse4095, e3035,and

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.

Jh"

Q4037to controlthe gainswitching

similar,except.the gain

changeis a one step changeof

circuitsin the 1OdB

and 20 dB Gain Step circuitsthroughVR Motherboard

approximately12.5

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dB. This gain step occurs in the

# 1 .

higlrelbands(4 throughi1). tf required,gainchangeis

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activatedby the microcomputer

The output on pins 15, 16, 19, and 12 ol U3017

diodesandtransistore2046.

throughuser-selected

(corresponding

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to databits 3, 4, 5, and 6) are apptiedto

The spectrumanalyzeris normallycalibratedwith

band decoder U3023,an op€n collector decoder. lf

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band1 is selected,pin 1 of u3023goes low andif band

the band 1 gain controlresistor set for rninimumgain.

2 is selectedpin 2

goes low, etc. This outputin con-

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Gain is then addedas requiredfor

the higherbands.

junction

with a 7.5 V referencesource (providedby

-banO

Data bits 3 through6 select gain for each

selec-

operational amplifier U30388 and

driver transistor

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tion.

03036) producesa

voltageat the output of a opera-

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The outputfrom this board is appliedthroughcon_

tional amplifier,U3038A. This voltage is indicativeof

the gain that must be set for each band so the level

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nectorEEto the 2nd FilterSelectcircuit.

remainsconstantat the outputfor all bands.

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7-27

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