Operating instructions

OPERATING MANUAL 2945A
46882-311D 4-13
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.
Modulators
The Rx TEST signal can be either AM or FM modulated.
The selected modulation signal, from either or both of the AF generators, from the data generator
or from an external source, is conditioned and level corrected within the modulation control
circuits on the audio processor board 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. 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 B1/2
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 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
instrument 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 and is passed through a level
control circuit of the same type as that in the AF generator circuit, incorporating a digital to
analogue converter, with the signal input to the voltage reference point and the level set by the
value on the digital input.
The summed modulation signal is taken through a modulation correction level control circuit
before being routed to either the FM attenuator digital to analogue converter or to the RF level