8525B/8528 Technical Service Manual 3-1
Publication No: 15-02036 Issue 6
3 Brief Description
3.1 General
This description should be read in conjunction with Block Diagram
03-00636.
The same frequency conversions are used in both transmit and receive
modes, therefore many circuits are common to both modes of operation.
Signal routing is determined by switching and control voltages according to
the mode selected.
3.2 Receive
The RF signal from the antenna passes through the PA low-pass filters, then
via the transmit/receive relay and broadcast filter on the PA PCB, to the
switched high-pass filters on the RF, Mixer, and Dual Synthesizer PCB.
Filters are automatically selected according to the frequency band in use.
The band-pass filtered signal is applied to the first mixer. This double-
balanced, diode-ring mixer, driven from VCO1, converts the signal up to a
45MHz IF signal.
The 45MHz signal is amplified and filtered with a 20kHz wide ‘roofing’
filter. The output of this filter is mixed with the output of VCO2 in another
double-balanced, diode-ring mixer. This produces a second IF of 1650kHz.
The second IF signal is passed to the IF amplifier and noise limiter. The
noise limiter is designed to suppress impulse noises such as motor vehicle
ignition noise. The limiter operates by 'gating' the IF signal path for the
duration of the noise pulses.
The crystal filter on the Audio and 1650kHz IF PCB reduces the bandwidth
to 2.5kHz before passing the signal to the 1650kHz tuned AGC controlled
IF amplifier.
The signal is then demodulated to provide an audio signal which is fed via
the mute gate to the digital volume control and audio PA where it is
amplified before being passed to the speaker.
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