Instruction manual

TM 11-6625-3017-14
switch to SET CAL and check that the
meter reads SET. If necessary, adjust the
f.m. calibration by inserting a screwdriver in
the SET CAL F.M. preset and adjusting
until the meter reads SET.
CAUTION Do not attempt to SET CAL on the 1.5 kHz
f.m. deviation range. Noise in f.m.
measurements
To accommodate signals with up to 100%
amplitude modulation the maximum r. f. input level, as
indicated by full-scale deflection on the black meter arc,
is set at 6 dB below the maximum amplitude which the i.
f. amplifier can handle. When making measurements
on f.m. signals including f.m. noise, and f.m. on a. m.,
this extra level capability of the i. f. amplifier can be
used with advantage to reduce the internally generated
noise to a minimum. This is useful when measuring
noise on signal sources or when measuring deviations in
the 5 kHz range on the 200 kHz bandwidth so that errors
due to noise are minimized.
The optimum r. f. level is set by monitoring the i.
f. signal at the I. F. OUT socket and adjusting the r. f.
input level for the maximum undistorted i. f. signal.
Alternatively, if no means of monitoring the i. f. signal is
available, it is sufficient to increase the r. f. input level
until the noise signal being measured is at a minimum, or
if deviation is being measured, until the deviation reading
is at a minimum.
2.6 MEASURING AM. DEPTH
After carrying out the preparation procedure in
section 2.4:
(1) Set the A. M./DEV RANGE switch (below the
meter) to the 30% or 100% a.m. depth range
according to the expected modulation depth of
the signal under test.
(2) Set the oscillator RANGE switch in accordance
with table 2.1. For detailed instruct- ions on
oscillator arrangements, see section 2.10. In
this section, it is assumed that the internal local
oscillator will be used.
(3) Set the Function switch to TUNE OSCILLATOR
AND ADJUST LEVEL. Adjust the OSCIL-
LATOR TUNE dial to a frequency of 1.5 MHz
above the input frequency. Tune for peak
deflection and then adjust LEVEL to give a meter
deflection in the black R. F. LEVEL arc,
preferably towards the lower end for best mixing
and hence minimum distortion.
(4) Set the Function switch to A.M. - SET FREQ.
Slightly readjust the OSCILLATOR TUNE control
until the meter deflects to the SET line. Avoid
spurious settings - the correct one is the closest
to the original setting obtained in (3).
(5) Set the Function switch to SET A.M. Adjust the
adjacent SET A, M. control until the meter reads
on the SET line. Note that this setting can be
done with or without modulation since the
adjustment is to the mean level of the i. f.
signal. However, appreciable even harmonic
distortion can affect the setting accuracy and, in
such circumstances, it is preferable to set up
without modulation.
(6) Set the Function switch to A.M. PEAK. Read the
percentage modulation depth at the peak
(7) Set the Function switch to A.M. TROUGH. Read
the percentage modulation depth at the trough.
When the peak and trough readings are
unequal, distortion is present in the input signal.
2.7 MEASURING F.M. ON A.M.
To measure the spurious f.m. on an amp-
litude modulated source, proceed as for f.m.
measurement, section 2.5.
A certain amount of spurious f.m. is introduced by
the instrument itself. When the carrier is deeply
modulated, at high modulating frequencies, this should
be taken into consideration, but below 40% depth in the
audio range of frequencies the spurious deviation is not
very significant. At 80% depth and 1 kHz frequency, the
maximum spurious deviation is typically less than 150 Hz
referred to the 15 kHz bandwidth.
Tables 2.2 and 2.3 give typical results which should
be used for general guidance purposes only .
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