4-8 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.3.3 Crystal Filter
One sideband of the DSB signal from the modulator is selected by the
crystal filter to give an SSB signal. During transmit, the C rail is at 0 Volts,
and forces a low output from IC1b for maximum AGC to V2 and V3. Thus,
none of the transmit signal passes through the IF Amplifier.
4.3.4 Mixers, IF Filters, and IF Amplifier
(Refer Dwg 04-02450)
The SSB signal from the crystal filter is applied to J4 on the RF Mixer and
Dual Synthesizer PCB. As the C rail is at 0 Volts in transmit, the noise
blanker is disabled, switching V23 on and V24 off.
V16 and V17 are switched as a source follower selected by the C and D
rails, via diodes D35 to D37. The output is applied to the double-balanced,
diode-ring mixer formed by T5, T6, and D31 to D34. Here the signal is
mixed with the output of VCO2 (43.352 to 43.350MHz) to produce the
45MHz IF.
The 45MHz IF signal is amplified by V9 switched as a grounded-gate
amplifier by C and D rails, via diodes D26 to D29. The amplified output is
passed to the crystal roofing filter (Z1) to remove the unwanted mixer
products.
V7 and V8 are switched as a source follower by the C and D rails, via diodes
D22 to D25. The output is passed to the double-balanced, diode-ring mixer
formed by T1, T2 and D13 to D16. Here the 45MHz IF signal is mixed with
the output from VCO1 to produce the required channel frequency. This
signal is filtered by the 28MHz low-pass filter then passed by D12 to the
Power Amplifier input.
The receive band-switched filters are switched off while transmitting.
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