Operating instructions

TECHNICAL DESCRIPTION
4-14 46882-311D
digital to analogue converter. This device provides fine level control of the RF signal by applying
a DC level to the AM modulator as described earlier.
Pre-emphasis filter
A 750
µ
s pre-emphasis filter is included on the audio processor board and is switched into the FM
modulation path when selected as part of the Rx TEST set-up.
AF measuring circuits
The AF output signal from the receiver under test is fed into the AF INPUT connector of the
Service Monitor. This signal is directly fed to the audio processor board and applied to a switched
divide by 2/divide by 20 circuit. It is then routed through two paths, one to the AF filtering and
measuring path, the other to the oscilloscope input circuits.
Oscilloscope function
When used in the Rx TEST mode, the oscilloscope displays the AF input signal fed to the AF input
connector.. The signal is passed through a sensitivity control circuit for scope calibration before
following the same path as used in the Tx TEST mode.
Dx test mode
The Dx TEST mode uses all the circuit elements described in the proceeding sections. The
technical description is valid for all three modes. The display presentation allows parameters
applicable to transmitter testing and receiver testing to be seen simultaneously.
Spectrum analyzer mode
Data to be presented as the active trace on the spectrum analyzer display is obtained through the
following circuits:-
The input frequency of the Service Monitor is made to sweep through the frequency range selected
on the display by sweeping the first local oscillator, located on A7/1. Any signals which fall within
the sweep range will pass through the IF circuits of the Service Monitor.
The output from the third mixer is amplified and filtered on A4/1, then fed to a logarithmic
amplifier. The 10.7 MHz signal is detected to produce a DC signal, the level of which is relative to
the level of the RF signal input. The DC voltage is digitised by an analogue to digital converter on
the Microprocessor board B2/1. The digital value obtained is stored in RAM at a memory location
which is related to the sweep position and to the value.
Each time the display is refreshed, the contents of each display location address will provide the
latest data, thus updating the display. The refresh rate is approx. 11 per second.
Look and listen function
When the look and listen function of spectrum analyzer mode is selected, the operation is modified.
In order to demodulate the centre frequency of the span, the signal fed to the demodulator must be
obtained from a fixed frequency point. As the frequency span in the look and listen function is
limited to 2 MHz, the frequency sweep can be applied at the third mixer stage. A ramp voltage is
generated within AF generator 2 on the AF board B1/2 and applied to the 90 MHz voltage
controlled oscillator on A10. The 10.7 MHz IF signal will thus have swept over the selected range
to produce a Frequency vs Signal Level display.
The signal fed to the modulator meter is obtained from the Look and Listen third mixer, located on
the second and third local oscillator board A9/1. The demodulated AM or FM signal is amplified,
to provide a low impedance output which is fed to the loudspeaker. It is also fed to the
ACCESSORY connector on the front panel.
A 90 MHz reference oscillator on A9/1 provides the local oscillator signal for this mixer. It also
provides a 90 MHz reference for the 90 MHz voltage controlled swept oscillator on A10.