Marconi Instruments TF 2502 - Chapter 3 TECHNICAL DESCRIPTION; Circuit Analysis

Marconi Instruments TF 2502

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Technical description

3.1

CIRCUIT

ANALYSIS

R.

F.

Power

Meter

TF

2502

consists

of

an

in-line

thermocouple

detector,

a

diode

detector

and

a

terminating

load

as

shown

in

Fig.

3.1.

IN-LINE

DETECTOR

with

THERMO-ELEMENT

L~------~~'~"-A_D------~--.

;

~

TC1

THERMOCOUPLE

M2

lWATTSl

Fig.

3.1

Functional

diagram

of

r.f.

power

meter

A

smallpercentage

of

tlie

incident

power

is

absorbed

by

the

heater

of

the

thermo-

couple

and

the

resultant

rise

in

temperature

is

monitored

by

a

series

of

bimetallic

junctions,

TCl.

The

thermo~electric

e.

m.

f.

thus

produced

is

then

measured

by

a

d.

c.

millivoltmeter,

M2,

calibrated

in

terms

of

total

power

in

watts.

Power

applied

to

the

thermo-element

produces

a

current,

I,

in

the

heater

resistance,

R5,

with

a

resultant

heat

dissipation

of I2R5.

The

thermocouple

output

voltage

is

consequently

proportional

to

12

since

its

e.

m.

f./

temperature

response

is

nearly

linear.

Hence

the

detector

gives

a d.

c.

output

proportional

to

true

mean

power,

and

independent

of

the

waveform

of

the

power.

The

input

voltage

is

sampled

by

a

capacitance

probe,

rectified

by

diode

MR1

and

monitored

by

meter

M1

which

has

an

arbitrary

0-100

calibration.

Potentiometer

RV3

adjusts

the

sensitivity

of

the

meter.

This

circuit

gives

a

fast-response

indication

and

also

provides

a

demodulated

output

at

the

MOD

OUT

jack,

JKA.

for

external

monitoring.

2502 (1)

13