Operating instructions

TECHNICAL DESCRIPTION
4-11
Third frequency changer/mixer
A 3rd frequency change is made to the signal to achieve a final IF of 10.7 MHz.
The local oscillator signal for the 3rd mixer is provided by the voltage controlled oscillator on the
2nd and 3rd oscillator board A10. This oscillator runs at 90 MHz and for normal transmitter
testing is locked to the output of a 90 MHz reference oscillator on the 2nd and 3rd oscillator board
A9/1.
Bandwidth filters
The 10.7 MHz output from the 3rd mixer passes to the 10.7 MHz IF and Log amplifier board
A4/1.
Filters on this board provides the bandwidth filtering for the signal path in the Tx TEST mode and
also for the SPEC ANA mode. The bandwidths provided are 3 MHz, 300 kHz, 30 kHz, 3 kHz and
300 Hz, using crystal filters controlled by signals from the microprocessor.
A direct, unfiltered path is also provided.
After filtering, the signal is passed to the demodulators on the audio processor board (B1/1 or
B1/2).
Demodulators
AM demodulator
The demodulators are located on the audio processor board (B1/1 or B1/2). The IF signal, from
the IF and Log amplifier board A4/1, is fed to a phase splitter circuit which provides anti-phase
outputs for the AM demodulator. The outputs from the phase splitter are taken to the positive and
negative inputs of the balanced demodulator. The demodulator is also fed with balanced,
unmodulated inputs, obtained from the modulated 10.7 MHz signal, by way of a phase-splitting
limiter.
The demodulator produces a balanced output equal to the difference between the two input
signals. This will have an AF component equaling the modulation signal and a DC component
relating to the level of the IF signal. The modulation is fed to the AF switching circuits and the
DC component used for automatic gain control.
FM demodulator
FM demodulation is performed by a pulse width discriminator operating at 700 kHz. The signal
for this is obtained by mixing the 10.7 MHz output from the limited signal fed to the AM
demodulator with the 10.0 MHz reference frequency, and passing the resultant signal through a
low-pass filter.
The 700 kHz signal containing the FM information is fed into a both halves of a dual monostable
flip-flop. By putting this signal to A1 and B2 inputs, the Q- outputs from the monostables will be
in anti-phase. By summing the two output signals the resultant signal will be a true representation
of the modulation.
SSB demodulator (optional)
The single sideband demodulator circuits are all contained on the SSB demodulator board A15.
When the SSB demodulation function is selected the 1st local oscillator of the Service Monitor is
set to a frequency which will produce a final IF of 10.625 MHz (from an input frequency equal to
the carrier frequency) rather than the 10.7 MHz IF produced for all other modes. The IF signal for
the SSB board is obtained from the 3rd mixer on A3, through a 10.7 MHz low-pass filter on A4/1.
It is applied to a variable gain circuit which is used to control the input level for the SSB
demodulator circuits.