Operating instructions

TECHNICAL DESCRIPTION
4-8
Instruction as to the frequency of the required signal is latched into a programmable array device,
which generates a repetitive digital output sequence recurring at the required frequency. The
digital output from the programmable array is transferred as a stream of 13 bit parallel data into an
EPROM. This holds look-up tables containing shape details of sine wave and square wave
signals. Output from the EPROM is as 8 bit, parallel information, corresponding to the selected
shape and at the repetition frequency instructed by the information latched into the programmable
array device.
The EPROM output is applied to a digital to analogue converter which, by converting each digital
value to the corresponding analogue level, produces the required signal. The mean output level
from the digital to analogue converter is constant. The analogue output is filtered by a 50 kHz
low-pass active filter, to remove any spikes created by the digital generation process. The output
from the filter has a peak to peak level of 5.7 V.
Output level is controlled by a digital to analogue converter, configured as a digital level control
circuit. The constant level AF signal is applied to the reference voltage level input, while the
digital input register is supplied with the data relating to the level of output signal required. The
output from the digital to analogue converter will be a replica of the signal at the reference voltage
terminal but proportionally reduced, relative to the data values at the digital input terminals.
After buffering, the AF generator outputs are switched to the AF output drivers for impedance
matching, prior to routing to the AF GEN OUT connector on the front panel. The output level
available at the AF GEN OUT connector is infinitely variable from 0.1 mV to 4.0 V RMS.
Note: The AF generators are also used as modulation sources for receiver testing. Therefore the
options available to the user when carrying out duplex tests are :-
Both generators to modulate the transmitter under test, receiver test signal not modulated.
Both generators to modulate the receiver test signal, transmitter under test not modulated.
One generator to modulate the transmitter under test and one to modulate the receiver test signal.
Data generator
The data generator is a ‘D type’ flip-flop, used to control AF generator 2 when pulsed tones are
required as modulation, such as for Digitally Coded Squelch applications. The Q1 output of the
data generator IC is used as the A13 address line on the lookup table EPROM. The data generator
will thereby directly switch this line, which is arranged to cause full output or no output. The
result is that data applied to the flip-flop D2 input will appear as tone pulses at the output of AF
generator 2.
When not required to produce data, the data generator is configured as a transparent flip-flop, with
the A13 address line functioning normally.