Instruction manual

TM 11-6625-3017-14
Fig. 3-2. Operation of discriminator
From this point onwards, all sections are common
to f. m. and a. m. and it is necessary to return to the i. f.
amplifier and consider the a. m. sections of the
modulation meter before proceeding to describe these
final sections.
A.M. sections
From the i. f. amplifier, a. m. signals are passed in
the appropriate position of the Function switch to the a.
m. detector. A diode detector is employed, preceded by
an i. f. amplifier stage, the gain of which is variable by the
SET A. M. control to standardize the carrier level at the
detector. When the carrier level is correct, the d. c.
output from the detector produces a reading at the SET
line on the meter.
After detection, the signal is fed via a 50 kHz low-
pass filter to the A M. /DEV RANGE switch and other
common a. m. /f. m. sections.
The d. c. component of the detected a. m. signal is
fed direct to the meter in the SET A. M. and TUNE
OSCILLATOR positions of the Function switch. In the
former position the d. c. component, which is
independent of a. m., is used as already stated to set
the carrier level at the detector, while in the latter
position, when the i. f. is fixed, it is used to set the mixer
input to the correct level by adjustment of the LEVEL
attenuator and also for oscillator peaking adjustment
during initial setting up of the modulation meter.
A.M./F.M. sections
The A. M. /DEV RANGE attenuator has a total
attenuation of 40 dB. The attenuation in the 1.5 and 5
kHz positions is zero, and in the other deviation positions
increases by 10 dB per step, providing the deviation and
modulation depth ranges on f. m. and a. m. On a. m. only
the 10 and 20 dB steps are used.
5 to 500 kHz deviation ranges:
After attenuation, the signals are passed to the 2nd
1. f. amplifier which contains an output amplifier
arranged to supply approximately 0 dBm into 600 E2 to
the OUTPUT terminals. This stage effectively isolates
the meter circuits from the OUTPUT terminals and
permits de-emphasis networks to be switched in to the
output terminals when required, without affecting the
meter reading. The peak reading meter circuit consists
of an amplifier with push-pull pair to operate the meter
diode in its most linear region.
1.5 kHz deviation ranges:
The signal is passed to the 2nd 1. f.
amplifier which supplies approximately -10 dBm into 600
Ω
to the output terminals. Unlike the other deviation
ranges the meter circuits are connected to the output of
the amplifier and thus the de-emphasis networks will
affect the meter reading.
Calibration circuit
In the SET CAL position of the Function switch, the
calibrator produces a standard crystal controlled
deviation signal, i.e., it produces a similar signal to that
from the limiter, with a peak-to-peak amplitude
corresponding to a
±
200 kHz deviation. For a more
complete description of the calibrator, refer to sect. 3. 8.
It thus provides a means of standardizing the
discriminator and 1. f. circuitry in order that accuracy can
be maintained and also a means of checking for all
possible sources of drift.
3.2 POWER UNIT
(Unit A13--Fig. 7-6 )
Occupying the rear corner behind the oscillator
compartment, the power unit includes two power
transistors fitted to heat sinks on the chassis, a printed
board and electrolytic capac-
3-2