Operating instructions

TECHNICAL DESCRIPTION
4-16
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 1st local oscillator, located on A7/1, is made to sweep through the frequency range selected
on the display. Any signals which fall within the sweep range will pass through the IF circuits of
the Service Monitor.
The input frequency of the Service Monitor is made to sweep through the frequency range selected
on the display by sweeping the 1st 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 3rd
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 digitized 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 center 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 3rd mixer
stage. A ramp voltage is generated within AF generator 2 on the audio processor board (B1/1 or
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 3rd mixer, located on
the 2nd and 3rd 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.
AF test mode
The AF TEST mode requires a signal source to apply to the circuit or equipment under test and a
measuring facility to analyze the resultant output signal.
The signal source is provided by the audio generators as for the Tx TEST mode and the AF
measuring circuits configured as for the Rx TEST mode. The RF generator and IF local
oscillators are disabled. The power metering and modulation metering functions are also disabled.