Variable oscillator

TM 11-6625-3017-14

actual reading will depend on frequency tolerances in the

(7) Remove the modulation from the signal under test.

source and local oscillator output. Switch to DEV +.

Turn the A.M./DEV RANGE switch to its most sensitive

range and then increase the sensitivity of the external

(5) Modulate the carrier to the reference deviation and

meter to give a convenient reading. Record the new

select the appropriate range on the A. M./DEV RANGE

level, which is given by the reading on the external meter

switch.

plus 10 dB for each step of the A. M./ DEV RANGE

switch.

(6) Read the external meter and record the dB level.

(8) The signal-to-noise ratio is given by the difference

between the levels measured in (6) and (7).

(7) Remove the modulation from the signal under test.

Turn the A.M. /DEV RANGE switch to its most sensitive

2.10

OSCILLATOR

ARRANGEMENTS

AND

range and then increase the sensitivity of the external

OPERATING NOTES

meter to give a convenient reading. Record the new

level, which is given by the reading on the external meter

The same procedure applies for both f.m. and a.m.

plus 10 dB for each step of the A.M. / DEV RANGE

measurements. The basic requirement is for a stable

switch.

frequency source 1.5 MHz higher than the signal under

test, and this can be obtained from three sources:

(8) The signal-to-noise ratio referred to the selected

(a) The internal permeability-tuned variable oscillator

bandwidth in which the measurements were made is

usable at all frequencies between 4 and 1000 MHz.

given by the difference between the levels measured in

(b)

The internal crystal controlled spot-frequency

(6) and (7).

oscillator, usable with selected crystals over the range of

20 to 1000 MHz.

A.M. noise

across 50 U.

(1) Connect an external meter to the OUTPUT terminals.

Apply r. f. input at a suitable level.

2.10.1 Variable oscillator

(2) Adjust the OSCILLATOR TUNE dial to a frequency

Choose the appropriate range on the RANGE switch.

1.5 MHz above the carrier and tune for peaking, as in

section 2.6

Check that the CRYSTAL selector is at OFF.

(3). Then adjust LEVEL to the top end of the black arc

Rotate the TUNE dial with the coarse and fine

on internal meter. (3) Switch to position A.M. SET FREQ

controls to a setting 1.5 MHz above the frequency of the

and adjust the oscillator until the meter reads SET.

input signal. Note that there are meter peaks 1.5 MHz

above and below the carrier frequency; always tune the

(4) Switch to crystal control and check that the meter still

oscillator to the higher frequency for correct polarity

reads near to the SET mark - the actual reading will

sense of the Function switch.

depend on frequency tolerances in the source and local

oscillator output. Switch to A.M. PEAK.

Adjust the fine control from time to time, to bring

the meter reading to the SET mark when switched to the

(5) Modulate the carrier to the reference depth and

SET FREQ position. It is permissible to tune to another

select the appropriate range on the A. M./DEV RANGE

internal oscillator harmonic as this will not affect the

switch.

readings obtained.

(6) Read the external meter and record the dB level.

2-8