CT Systems Wavetek 2500 - Page 42

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3.4.4

Receiver

IF

Bandwidth

and

Symmetry

This

test

will

determine

the

IF

filter

bandwidth

and

the

symmetry

<whether

or

not

the

high

and

low

cutoff

points

are

correct).

Since

a

too

narrow

bandwidth

will

cause

distortion,

and

a

too

wide

bandwidth

will

cause

noise,

this

test

is

a

good

troubleshooting

aid

in

finding

cause

of

high

noise

and

distortion.

If

the

Bandwidth

of

the

IF

filter

is

correct,

but

is

offset,

(i.e.,

high

cutoff

too

low

or

low

cutoff

too

low)

the

result

will

be

distorted

recovered

audio

in

the

form

of

positive

peak

clipping.

Refer

to

Figure

3-6

for

the

following

test

procedure.

1)

Connect

the

"RF IN/OUT"

jack

of

the

Model

2500

to

the

RF

input

(antenna

jack)

of

the

receiver

under

test.

2)

Select

GEN

mode.

3)

Program

the

receiver

frequency

in

MHz

using

the

numerical

keys

followed

by

the

ENTER

key.

4)

Connect

receiver

audio

to

"SINAD

AUDIO

IN".

5)

Set

RF

LEVEL

and

vernier

controls

to

1

mV.

6)

Select

DEV

, 1 kHz

source,

and

adjust

1 kHz

level

control

for

5

kHz

deviation.

7)

Select

the

AC

pk

VOLTS

function.

8)

Adjust

receiver

squelch

to

full

open

position.

9}

Connect

high

resolution

VOM

Cin

DC

mode)

to

AGC

or

limiter

test

point

of

Receiver.

10)

Reduce

RF

output

by

3 dB

(30%).

Make

note

of

AGC

voltage.

Increase

RF

output

back

to

1

mV.

11)

Select

RF

step

and

program

the

Model

2500

for

a

100

Hz

(.

OOOlMHz)

step

size

using

the

numerical

keys

followed

by

the

ENTER

key.

12)

Select

GEN

and

use

the

UP

arrow

key

to

increase

the

RF

frequency

until

the

AGC

voltage

noted

~n

Step

10

is

obtained.

Record

this

as

the

"high"

frequency.

13)

Use

the

down

arrow

key

to

return

to

the

RF

center

frequency

and

then

continue

decreasing

the

RF

frequency

until

the

AGC

voltage

noted

in

Step

10

is

obtained.

Record

the

"low"

frequency.

The

difference

between

the

high

frequency

and

the

low

frequency

is

the

3 dB

bandwidth.

The

symmetry

is

good

if

the

low

frequency

is

off

from

the

center

the

same

amount

as

the

high

frequency.

2500-MANUAL

PAGE

30

SECTION-3