Operating instructions
TECHNICAL DESCRIPTION
4-12 46882-311D
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 routeing circuits.
De-emphasis filter
The output from the discriminator passes through a 163 kHz low-pass filter and then the 750
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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.
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 B1/2. This filter is switched into circuit to make comparison
measurements against the un-filtered 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.
Rx test mode
The Rx TEST mode uses the signal from the RF signal generator circuits, as an input signal for the
receiver under test. The AF signal, produced by the demodulator within the receiver, is analysed
within the Service Monitor to produce results of the test.
Selecting the Rx TEST mode does not disable the functions of the Tx TEST mode. The receiver
circuits remain active at their last settings. The AF generators will also continue operating at their
last settings unless either is enabled as a modulation generator. Calling up one modulation