Boonton Electronics 71A, 71AR - Explanation of Controls and Adjustments

Boonton Electronics 71A, 71AR

40 pages

Save Page as PDF

To Next Page

To Next Page

To Previous Page

To Previous Page

Loading...

2.5

Explanation

of

Controls

and

Adjustments

2.5.1

Front

Panel,

Models

7lA

and

71AR

RANGE

-

pF!~:

Seven-position

switch

selects

measuring

ranges

having

full

scale

values

of

1,

3,

10,

30,

100,

300,

or

1000,

with

units

of

measurement

being

either

picofarads

or

microhenries,

as

determined

by

the

position

of

the

"L/C"

selector.

C ZERO:

Provides

means

for

balancing

out

capacitance

across

the

"TEST"

terminals

(up

to

approximately

7

pF)

contributed

by

exposed

terminations

of

connecting

cables,

test

fixtures,

etc.

Its

range

may

be

increased

by

attach-

ing

an

external

capacitance

to

the

"DIFF"

posts,

as

out-

lined

in

paragraph

3.7.

NOTE

This

is

a

dual

ratio

control

which

adjusts

a

variable

air

capacitor

having

a

full

360

0

of

rotation.

It

will

be

noted

that

for

about

270

0

of

rotation

of

the

"c

ZERO"

con-

trol,

it

turns

very

easily.

Beyond

this

point

the

torque

requirGment

increases

abruptly,

indicating

a

shift

from

the

36:1

"fine"

ratio

to

the

6:1

"coarse"

ratio.

COMP

ADJ:

The

front

panel

"COMP

ADJ"

control

adjusts

a

variable

air

capacitor

which

negates

stray

capacitance

from

the

high

postto

ground

(such

as

those

existing

be-

tween

the

inner

and

outer

conductors

of

coaxial

cable)

to

eliminate

its

effects

on

measurements.

This

control

will

tune

out

approximately

100

pF.

As

a

rule

of

thumb,

the

amount

of

"HI"-post-to-grourrlcapacitance

required

to

cause

1%

of

full-scale

error

is

somewhat

greater

than

two

times

the

full

scale

value

of

the

range

for

all

ranges.

Thus,

it

can

be

seen

that

the

amount

of

"HI"-post-to-ground

capacitance

encountered

in

the

great

majority

of

cases

will

be

of

minor

significance,

and

require

balancing

only

on

the

lower

ranges.

However,

when

working

on

the

30

pF

range

or

lower,

the

"COMP

ADJ"

adjustment

should

be

checked

when:

(a)

The

instrument

is

initially

put

intq

service.

(b)

Any

change

is

made

in

the

connection

system.

(For

example,

going

from

remote

measurements

to

local

measurements

or

vice

versa,

changing

connecting

cables

or

test

fixtures,

etc.).

2-4

7lA