Operating instructions

TECHNICAL DESCRIPTION
4-12
After leaving the gain control circuit, the signal is then mixed in a double balanced mixer with a
10 MHz signal from the 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 the audio processor board (B1/1 or B1/2).
This 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 Service Monitors software can establish the mean frequency of
the transmitter output.
Measurement ranging circuits
The evaluations 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 (B1/1 or B1/2). Signals relating to RF
power input to the Service Monitor and power readings from optional auxiliary power heads are
passed through various switched gain amplifiers before leaving the board to be measured.
Demodulated AF signals are passed through any selected AF filters before being measured. The
selected filters are switched into the AF circuit before the ranging amplifiers. 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. There are CCITT and CMESS
filters available as options.
Distortion/SINAD filter
The 1.0 kHz active notch filter, used to make distortion percentage and SINAD measurements, is
also on the audio processor board (B1/1 or B1/2). This filter is switched into circuit to make
comparison measurements against the unfiltered path.
Oscilloscope function
The oscilloscope function when used within the Tx TEST mode displays the waveform of the
demodulated signal. The signal for this is taken from the AF filter output and after passing
through level converting amplifier circuits is fed to an analogue to digital converter located on the
microprocessor board B2/1. The digital levels relating to the waveform are written into a digital
signal processor which generates the oscilloscope display. The values are incorporated into the
display and updated continually. The display persistence function is also generated within this
DSP.