HP 8481A - Service and Maintenance; Principles of Operation

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HP

8481A/82A/83A

08481-90049

Service

Service

instructions

consist

of

principles

of

operation,

troubleshooting,

and

repairs.

Test

equipment

which

meets

or exceeds

the

critical

specifications

in

table

3

may

be

used

in

place

of

the

recommended

instruments

for

troubleshooting

the

Power

Sensor.

Note

Hf

Check

your

warranty.

Opening

the

Power

Sensor

will

void

warranty.

Principles

of

Operation

For

the

following

discussion,

refer

to

fi

gure

4

Schematic

Diagram

and

fi

gure

5

Operational

Amplifier.

The

operational

amplifier

is

made

up

of

the

Power

Sensor

input

amplifier,

A2Q1,

and

the

fi

rst

amplifier

stage

in

the

power

meter.

For

additional

information

on

thermocouple

sensors,

refer

to

HP

Application

Note

64-1,

“Fundamentals

of

RF

and

Microwave

Power

Measurements”.

The

A1

Bulkhead

Assembly

provides

a

50

ohm

load

(HP

8481A

and

8482A)

or

a

75

ohm

load

(HP

8483A)

to

the

RF

signal

applied

at

the

RF

INPUT.

The

rf

signal

is

coupled

through

a

dc

blocking

capacitor

and

absorbed

by

the

thermocouples

which

generate

a

dc

voltage

proportional

to

the

rf

input

power.

The

dc

voltage

is

routed

from

the

thermocouples

to

the

input

amplifier

on

gold

wires

to

reduce

undesired

thermocouple

effects.

The

gold

wires

pass

through

ferrite

beads

A2E1

and

A2E2

which

are

located

in

the

black

plastic

block.

(See

fi

gure

2.)

The

ferrite

beads

increase

the

self

inductance

of

the

gold

wires

causing

this

portion

of

the

wires

to

provide

the

properties

of

an

rf

choke.

The

result

is

to

minimize

rf

feedthrough

to

the

A2

Input

Amplifier

Assembly.

The

dc

output

from

the

bulkhead

assembly

is

applied

to

the

two

fi

eld

effect

transistors

(FETs)

in

A2U1.

These

transistors

function

as

a

sampling

gate

or

chopper.

The

sampling

rate

is

controlled

by

a

220

Hz

square

wave

supplied

by

the

power

meter.

The

amplitude

of

the

sampling

gate

output

(at

pin

3

of

A2U1)

is

a

220

Hz

square

wave

proportional

to

the

power

input.

The

sampled

220

Hz

ac

output

is

applied

to

the

input

amplifier

A2Q1

which

is

the

input

stage

for

an

operational

amplifier

(figure

5).

The

ac

gain

of

the

operational

amplifier

is

approximately

700.

A

dc

feedback

voltage

from

the

power

meter

Auto

Zero

circuit

is

coupled

to

the

input

of

FET

A2U1Q1

to

set

the

zero

level.

The

voltage

is

developed

across

the

voltage

divider

consisting

of

A2R1

and

the

series

resistance

of

the

thermocouple

AlTCl.

When

the

Power

Sensor

is

used

with

an

HP

Power

Meter,

the

short

to

ground

at

Jl-K

(Mount

Resistor)

causes

the

power

meter

to

automatically

select

the

proper

measurement

range

of

-30

to

4-20

dBm.

With

the

HP

435

series

Power

Meter

this

short

serves

no

function.

15

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