TECHNICAL DESCRIPTION
4-13
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 unfiltered path.
The additional distortion frequencies enabled by Option 29 are provided by an additional board
that plugs into the audio processor board B1/2. It provides two additional notch filters and the
necessary circuits to switch between them.
Each filter is very similar to the standard 1 kHz filter but with different component values required
for the different notch frequencies.
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 analyzed
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
generator will allow the other to function as an AF generator for the Tx TEST mode, but using
both as modulation sources will remove both from Tx TEST use.
RF generator
The RF signal is produced within the monitor by a fractional N controlled oscillator with a range
of 1,280.1 MHz to 2,280.0 MHz. This is located on the RF generator oscillator board A13/1. The
oscillator and its control system are of a complex design in order to meet strict requirements of
frequency stability, low phase noise and very fast settling after a change of frequency. The output
from this oscillator is fed to the RF generator mixer board A12/1, where it is combined with a
signal taken from the output of the second local oscillator on A9/1.
The mixer output is filtered by a 1050 MHz low-pass filter to remove all unwanted frequencies,
before passing to the RF output amplifier board and AM modulator board A6/1.
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