Operating instructions

TECHNICAL DESCRIPTION
4-15
Modulators
The Rx TEST signal can be either AM or FM modulated.
The selected modulation signal, is obtained from either or both of the AF generators, from the data
generator or from an external source. It is conditioned and level corrected within the modulation
control circuits on the audio processor board (B1/1 or B1/2).
Frequency modulation is applied by injection into the fractional N control loop, thereby
influencing the generated frequency. When the signal is FM modulated, the AM modulator is held
in a 0% modulation state.
AM modulation is applied to the signal using a pin diode modulator, located within the output
amplifier circuits on A6/1. The output level of the Rx TEST signal is also controlled using the pin
diode modulator.
The signal controlling the attenuation level of the pin diode modulator, is a composite signal
containing the amplitude modulation component obtained from the modulation source and a DC
component representing the required output level.
They are combined into one modulation signal using a digital to analogue converter on the audio
processor board (B1/1 or B1/2).
RF amplifier
The signal level from the AM modulator is increased by 50 dB by a three stage RF amplifier
before passing through the output control attenuator to the RF switching unit and the RF output
sockets.
Audio generators
The audio generators on the audio processor board (B1/1 or B1/2) are used to provide AF
Modulation to the RF test signal generated on the RF generator oscillator board A13/1. The
operation of the generators is as described for the Tx TEST mode but rather than the output level
being called up as an output voltage, this is specified as a modulation level. The necessary
calculations are performed by the Service Monitor's software.
The DATA GENERATOR facility is available in the Rx TEST mode.
Modulation control
The output from each generator is taken through selector switches to a summing amplifier at the
input to the modulation control circuits. The signal from an external modulation source or from
the modulation options circuits is also summed in at this point. An external modulation source can
be connected to the EXTERNAL MOD IN connector on the rear panel. This is passed through a
level control circuit similar in design to that in the AF generator circuit.
The summed modulation signal is taken through a modulation correction level control circuit, then
routed to either the FM attenuator D to A converter or to the RF level D to A converter. The latter
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 µs pre-emphasis filter is included on the audio processor board and when selected, is
switched into the FM modulation path as part of the Rx TEST set-up.
AF measuring circuits
The AF output signal from the receiver under test is connected to the AF INPUT connector of the
Service Monitor. This is routed to a switched
÷2/÷20 circuit on the audio processor board (B1/1
or B1/2), then split and routed to the AF filtering and measuring path and to the oscilloscope input
circuits.