Operating instructions

OPERATING MANUAL 2945A
46882-311D 4-11
Third frequency changer/mixer
A third frequency change is made to the signal to achieve a final IF of 10.7 MHz.
The local oscillator signal for the third mixer is provided by the voltage controlled oscillator on
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 second and third oscillator board A9/1.
Bandwidth filters
The 10.7 MHz output from the third mixer in the Tx TEST path, passes from the second and third
mixer board A3 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 micro processor.
A direct, un-filtered path is also provided.
After filtering, the signal is passed to the demodulators on the audio processor board B1/2.
Demodulators
AM demodulator
The demodulators are located on the audio processor board 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 equalling the modulation signal and a DC component relating to
the level of the IF signal. The modulation is fed to the AF switching circuits while the DC
component is 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 first 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 third 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.
After leaving the gain control circuit the signal is then mixed in a double balanced mixer with a
10 MHz signal from the instrument reference oscillator. The output from the mixer will contain a
625 kHz component. This is passed through a 625 kHz low pass filter, to one input of another
double balanced mixer. The second input is fed with a reference signal of 625 kHz derived from
the 10 MHz reference through a ‘divide by 16’ circuit. The output from the double balanced mixer
will be zero when no modulation is present on the input signal. When modulation is applied, the