TECHNICAL DESCRIPTION
4-11
Second local oscillator
The local oscillator signal for the second mixer has a frequency of 1.280 GHz and is generated on
the second and third oscillator board A9/1. This oscillator is a phase locked loop device, locked to
the instrument's 10 MHz reference oscillator.
The output from the second mixer is passed through a 79.3 MHz band-pass filter, to remove the
unwanted products of the mixing process, before being given 20 dB amplification.
The 79.3 MHz IF signal is split and fed to two independent mixers.
The secondary feed supplies the 90 MHz swept local oscillator board A10. This operates as a
swept oscillator when the spectrum analyzer
Look and Listen function, is active, as described later.
It provides a locked 90 MHz signal for the third mixer at other times.
The primary path is through a band-pass filter with 5 MHz bandwidth, to the main 90 MHz 3rd
mixer. This mixer is on the second and third mixer board A3.
Third frequency changer/mixer
A third frequency change is made to the signal to achieve a final IF of 10.7 MHz.
The local oscillator signal for the third mixer is provided by the voltage controlled oscillator on
A10. This oscillator runs at 90 MHz and for normal transmitter testing is locked to the output of a
90 MHz reference oscillator on the second and third oscillator board A9/1.
Bandwidth filters
The 10.7 MHz output from the third mixer in the Tx TEST path, passes from the second and third
mixer board A3 to the 10.7 MHz IF and Log amplifier board A4/1.
Filters on this board provides the bandwidth filtering for the signal path in the Tx TEST mode and
also for the SPEC ANA mode. The bandwidths provided are 3 MHz, 300 kHz, 30 kHz, 3 kHz and
300 Hz, using crystal filters controlled by signals from the micro processor.
A direct, un-filtered path is also provided.
After filtering, the signal is passed to the demodulators on the audio processor board B1/2.
Demodulators
AM demodulator
The demodulators are located on the audio processor board B1/2. The IF signal from the IF and
Log amplifier board A4/1, is fed to a phase splitter circuit which provides anti-phase outputs for
the AM demodulator. The outputs from the phase splitter are taken to the positive and negative
inputs of the balanced demodulator. The demodulator is also fed with balanced unmodulated
inputs, obtained from the modulated 10.7 MHz signal, by way of a phase-splitting limiter.
The demodulator produces a balanced output equal to the difference between the two input
signals. This will have an AF component equaling the modulation signal and a DC component
relating to the level of the IF signal. The modulation is fed to the AF switching circuits while the
DC component is used for automatic gain control.
FM demodulator
FM demodulation is performed by a pulse width discriminator operating at 700 kHz. The signal
for this is obtained by mixing the 10.7 MHz output from the limited signal fed to the AM
demodulator, with the 10.0 MHz reference frequency and passing the resultant signal through a
low-pass filter.
The 700 kHz signal containing the FM information is fed into a both halves of a dual monostable
flip-flop. By putting this signal to A1 and B2 inputs, the Q-outputs from the monostables will be
in anti-phase. By summing the two output signals the resultant signal will be a true representation
of the modulation.
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