Instruction manual

TM 11-6625-3017-14
actual reading will depend on frequency tolerances in the
source and local oscillator output. Switch to DEV +.
(5) Modulate the carrier to the reference deviation and
select the appropriate range on the A. M./DEV RANGE
switch.
(6) Read the external meter and record the dB level.
(7) Remove the modulation from the signal under test.
Turn the A.M. /DEV RANGE switch to its most sensitive
range and then increase the sensitivity of the external
meter to give a convenient reading. Record the new
level, which is given by the reading on the external meter
plus 10 dB for each step of the A.M. / DEV RANGE
switch.
(8) The signal-to-noise ratio referred to the selected
bandwidth in which the measurements were made is
given by the difference between the levels measured in
(6) and (7).
A.M. noise
(1) Connect an external meter to the OUTPUT terminals.
Apply r. f. input at a suitable level.
(2) Adjust the OSCILLATOR TUNE dial to a frequency
1.5 MHz above the carrier and tune for peaking, as in
section 2.6
(3). Then adjust LEVEL to the top end of the black arc
on internal meter. (3) Switch to position A.M. SET FREQ
and adjust the oscillator until the meter reads SET.
(4) Switch to crystal control and check that the meter still
reads near to the SET mark - the actual reading will
depend on frequency tolerances in the source and local
oscillator output. Switch to A.M. PEAK.
(5) Modulate the carrier to the reference depth and
select the appropriate range on the A. M./DEV RANGE
switch.
(6) Read the external meter and record the dB level.
(7) Remove the modulation from the signal under test.
Turn the A.M./DEV RANGE switch to its most sensitive
range and then increase the sensitivity of the external
meter to give a convenient reading. Record the new
level, which is given by the reading on the external meter
plus 10 dB for each step of the A. M./ DEV RANGE
switch.
(8) The signal-to-noise ratio is given by the difference
between the levels measured in (6) and (7).
2.10 OSCILLATOR ARRANGEMENTS AND
OPERATING NOTES
The same procedure applies for both f.m. and a.m.
measurements. The basic requirement is for a stable
frequency source 1.5 MHz higher than the signal under
test, and this can be obtained from three sources:
(a) The internal permeability-tuned variable oscillator
usable at all frequencies between 4 and 1000 MHz.
(b) The internal crystal controlled spot-frequency
oscillator, usable with selected crystals over the range of
20 to 1000 MHz.
(c) An external oscillator with an output of 100 mV
across 50 U.
2.10.1 Variable oscillator
Choose the appropriate range on the RANGE switch.
Check that the CRYSTAL selector is at OFF.
Rotate the TUNE dial with the coarse and fine
controls to a setting 1.5 MHz above the frequency of the
input signal. Note that there are meter peaks 1.5 MHz
above and below the carrier frequency; always tune the
oscillator to the higher frequency for correct polarity
sense of the Function switch.
Adjust the fine control from time to time, to bring
the meter reading to the SET mark when switched to the
SET FREQ position. It is permissible to tune to another
internal oscillator harmonic as this will not affect the
readings obtained.
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