Instruction manual

TM 11-6625-3017-14
Function switch to SET CAL and, without disturbing the
SET CAL F. M. preset, bring the meter reading to the
SET mark by adjust-ing the internal f. m. preset, AORV3
- see Fig. 4-1. If this has insufficient range select a new
value for resistor AOR15.
(5) This check can also be made at other deviations as
shown in Table 4.2.
(6) If the deviation errors show a general trend in one
direction the presets RV1, RV3, RV5 and RV4
should be adjusted to give the best compromise on
their respective ranges. As RV3 affects all ranges it
should be adjusted first.
Preset Adjust for best compromise
on deviation range
(kHz)
RV3 5, 15, 50
RV4 500
RV5 150
RV1 (on TM8742) 1.5
(RV3 will be usually found to give the best compromise
by adjusting on the 5 kHz deviation range.)
4.3.4 A.M. depth accuracy
Test equipment:- a, f and i.
A. M. depth accuracy, unlike f.m. deviation, cannot
be checked against an internal calibrating circuit.
Therefore, the a. m. Measurement accuracy can only be
checked by making a normal measurement on a signal
of accurately known modulation depth. A depth of about
50% is recommended. It is important that this signal
should have a very low envelope distortion as this can
cause an apparent error in depth indication.
The modulation depth can be checked by means of
a selective level meter as follows:-
(1) Set the 1. f. oscillator frequency to 3 kHz and use it
to modulate the signal generator to a depth of
nominally 50%.
(2) Set up the modulation meter for measuring this
signal as described in Sect. 2.6.
(3) Connect the selective level meter to the I. F. OUT
socket of the modulation meter, tune the level
meter to the carrier frequency of 1.5 MHz and set
its gain to give a 0 dB reference level.
(4) Retune the level meter to one of the 3 kHz
sidebands and adjust the l. f. oscillator out-put to
give a level meter reading of 12 dB below the
reference level. If the sideband levels are different,
adjust for a mean reading of 12 dB. This
corresponds to a modulation depth of 50%.
(5) If the modulation meter reading differs from 50%
when switched to either PEAK or TROUGH, adjust
the internal preset A9RV1 for the best compromise
accuracy between the two readings.
Note: Adjustment of RV1 may affect the accuracy of the
f. m. deviation reading; therefore, after adjusting
RV1, reset the f. m. calibration as shown in Sect. 2.5
(8) (not by adjusting the internal preset).
(6) This check can also be. made. at other modulation
depths, as shown in the following table, in order to
optimize reading errors.
TABLE 4.3
Modulation Depth
A.M. RANGE Modulation Sideband
% amplitude, dB
100 90 -6.94
100 80 -7.96
100 70 -9.12
100 60 -10.46
100 50 -12.04
100 40 -14.02
30 30 -16.52
30 20 -20.00
30 10 -26.02
The higher modulation depths are not recommended
unless an unusually low envelope distortion can be
achieved.
4.3.5 F.M noise
Test equipment:- b, d, h, j and k.
This is measured by comparing the output of the
modulation meter when 5 kHz deviation is applied with
the output when deviation is zero.
4-6