TECHNICAL DESCRIPTION
4-12
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 output signal will equal the difference in frequency between the original carrier
frequency and the original sideband frequency. Either an upper sideband signal or lower sideband
signal will produce an output. This will equal the frequency of the applied modulation. The
demodulated signal is fed to the input of the audio routing circuits.
De-emphasis filter
The output from the discriminator passes through a 163 kHz low-pass filter and then the 750 µs
de-emphasis filter before being routed by the demodulation selection switches. The de-emphasis
filter can be by-passed as part of the test set-up.
RF counter
A sample of the 10.7 MHz IF signal is taken from one output of the phase splitter/limiter and
supplied to the IF counter circuit, also located on B1/2.
This counter measures the mean frequency of the IF signal and the result is written into memory.
By making a calculation using the reading from the RF counter and the division ratios from the
three local oscillator control loops, the instrument software can establish the mean frequency of
the transmitter output.
Measurement ranging circuits
The evaluation of RF level, modulation depth, modulation deviation, distorted levels etc. are all
made using a metering circuit on the microprocessor board B2/1. Before the various levels can be
measured, each must be conditioned so as to bring the minimum and maximum levels of each
parameter within the range of the metering circuit. The signals to be measured are all brought to
the analyzer ranging circuits on the audio processor board B1/2. Signals relating to RF power
input to the instrument and power readings from an auxiliary power head are passed through
various switched gain amplifiers before leaving the board to be measured.
Measurements to the demodulated AF signals are made after filtering to the selected AF filter pass
band. The AF filters are located on the audio processor board and offer the choice of 300 Hz-
3.4 kHz band-pass; 15 kHz low-pass; 300 Hz low-pass and the full bandwidth of 50 kHz.
The selected filters are switched into the AF circuit before the ranging amplifiers.
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