8525B/8528 Technical Service Manual Technical Description 4-5
Publication No: 15-02036 Issue 6
4.2.3 Noise Blanker
The 1650kHz IF signal is amplified by the RC coupled stages V15 and V18
and fed to the tuned collector amplifier V20. V21 is an active rectifier.
Noise bursts produce negative going pulses at the emitter of V21 which
drives the monostable flip-flop V25 and V26. The output of V25 and V26
are complementary gating pulses which are fed to FET gates V23 and V24
respectively. With V23 on and V24 off, the IF signal is passed to the crystal
filter. With V23 off and V24 on, the IF signal is blocked.
The average DC component of the collector current of V21 passing through
R65 is amplified by V22 and applied as reverse AGC to V15 and V18. Thus
only impulse type signals produce sufficient rectified output at V21 emitter
to trigger the monostable. C119 sets the monostable pulse width such that
the noise burst has finished before the IF is again enabled.
C rail biases off V15 and V18 in transmit and the noise blanker may be
disabled for test purposes by grounding TP6.
4.2.4 IF Crystal Filter and Amplifier (04-02093)
The 1650kHz IF signal from the noise gate on the RF Mixer and Dual
Synthesis PCB (V23, V24 04-02450) enters the board at P1 and is filtered
by Z1 to pass 1647.2kHz to 1649.7kHz (2.5kHz bandwidth) to the two-stage
AGC amplifier V2 and V3. IF output amplifier V4 drives the demodulator
IC8 and the AGC active rectifier V6. AGC attack time is set by R21/C17
and release time by R24/C17, giving a fast attack/slow release response.
Divider R23/R24 sets the AGC threshold level while the voltage set by the
divider, combined with the gain of IC1a, IC1b and the bias on IC1b positive
input, determines the static AGC voltage. D2 and D3 hold the sources of V2
and V3 positive to enable the AGC voltage applied to gate 2 of these FETs
to go negative with respect to sources for full AGC control.
4.2.5 Demodulator
The IF from V4 is converted to the required audio output by double
balanced mixer IC8. When receiving an RF USB signal, the IF signal is LSB
with an SCF of 1650kHz due to the sideband inversion in the first mixer.
With an RF LSB signal, the IF signal is USB with an SCF of 1647kHz. Thus
the demodulator requires a 1650kHz local oscillator for USB reception and a
1647kHz local oscillator for LSB reception at P2. The oscillators are
described in Section 4.4.5.
The audio output (pin 6) of IC8 passes through a third-order low-pass filter
IC4a and associated components to remove noise above the wanted audio
passband.
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