Selectivity Test.

the space between module Al0 and the

ceiver test setup (para 102 and fig. 79.7),

brass wheel-like part of the coupling.

except disconnect test cabIe No. 9 from

the 1066A output- and connect it to the

(3) Adjust RT-505/PRC-25 to a frequency

50-kc higher than its current setting.

AN/USM-44 output. Monitor the RF

(4) Momentarily set the RT-505/PRC-25

voltage at A21J3 with RF Millivoltmeter

function switch to OFF, then back to

Hewlett-Packard Model HP-411A (HP-

ON, and verify receiver audio output by

411A).

observing the oscilloscope and the TS-

(2)

Adjust the AN/USM44 for a 41.00-mc cw

723A/U (used as a vacuum-tube voltmeter)

output at approximately 100 UV.

indications.

(3)

Tune the AN/USM-44 for a maximum

(5) Repeat the procedures given in (3) and (4)

indication on the HP-411A.

above until a frequency setting produces

(4)

Increase the RF output of the AN/USM-

no audio output within 3 seconds, or

44 to the point where limiting just starts

until a frequency of 31.95 mc is reached.

(where an increase in the AN/USM-44

Record the maximum frequency that pro-

output does not increase the reading on the

duces an audio output,

HP-411A.)

(6) Adjust the RT-505/PRC-25 to 31.50 mc

(5)

Reduce the AN/USM-44 RF output for

and repeat the procedures given in (3) and

an indication on the HP411A that is 6

(4) above except reduce the frequency by

db below the limiting level. Sweep the

50 kc for each successive step until the

ANT/USM-44 frequency across the selec-

audio output is not obtained or until a

tivity pass band and select the frequency

frequency of 31.00 mc is reached. Record

that provides the highest peak. Readjust

the minimum frequency that produces an

the AN/USM-44 output level so the

audio output.

selected peak is 6 db below the limiting level.

(7) Withdraw the decoupling tool and rotate

(6)

Determine the pass-band irregularities and

the kilocycle frequency selector to 50

the 6-db-down points. The pass-band

to re-engage the coupling.

irregularities should be within 2 db and

(8) Adjust the RT-505/PRC-25 to 51.50 mc

the 6-db-down points should be at least

and disengage the coupling ((2) above).

32-kc apart. Use the AN/USM-26 to

(9) Repeat the procedures given in (3) and

determine the AN/USM-44 frequency at

(4) above until a frequency produces no

the 6-db points. Disconnect test cable No.

audio output within 3 seconds or until a

9 from the dummy antenna input and

frequency of 51.95 mc is reached. Record

reconnect cable No. 9 to the AN/USM-26

the maximum frequency that produces an

input. Increase the AN/USM-44 output

audio output.

level sufficiently to drive the AN/USM-26.

(10) Adjust the RT-505/PRC-25 to 51.50 mc

(7)

Set the AN/USM-44 to the frequency of

and repeat the procedures given in (3) and

the highest peak of the selectivity pass

(4) above except reduce the frequency for

band and to an output level 6 db above the

each successive step until the audio output

noise level. (Read the noise level on the

is not . obtained or until a frequency of

HP-411A with the AN/USM-44 output

51.00 mc is reached. Record the minimum

at zero; then increase the AN/USM-44

frequency that produces an audio output.

RF output until a 6-db increase is indi-

(11) Withdraw the decoupling tool and rotate

cated by the HP-411A.)

the kilocycle frequency selector to 50 to

reengage the coupling.

(8)

Increase the AN/USM-44 output level 60

(12) Evaluate the test results. At both test

db and locate the two frequencies that

frequencies (31 .50 and 51.50 me), the mini-

produce the same output level (6 db above

mum acceptable catching range is 650

noise) as read on the HP-411A ((7) above).

kc with at least 250 kc on each side of the

Determine and record the frequencies by

test frequency.

use of the AN/USM-26. The two re-

f. Selectivity Test.

corded frequencies are the 60 db-down

(1) Perform the selectivity test with the

points. They should be less than 100-kc

equipment connected in a standard re-

apart.

TAGO

8344-A