4-4 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.2 Receiver
4.2.1 Input Filters (Refer 04-02452)
The receiver input signal is passed through the PA low-pass filter selected
by the PA band line, the transmit/receive relay K6 and the 2MHz high-pass
broadcast filter to the RF, Mixer & Dual Synthesizer PCB.
Option LF adds a 1nF capacitor C111 in parallel with C47, reducing the
broadcast filters passband to 1.6MHz. D3 and D4 provide protection against
excessive input signals.
4.2.2 Mixers, First IF Amplifiers and Filters (Refer 04-02450)
The receive input signal enters the PCB at pin 1 of J1. Five high-pass filters
are used to complete the bandpass filter characteristics and are switched
according to the frequency band in use by grounding the appropriate select
line (the band 2.0 to 3.1MHz is not filtered on this board). From the selected
filter, the received RF is passed via a 28MHz low-pass filter (C32 to C37
and L15 to L18) to the first mixer.
The first mixer is a double-balanced, diode-ring mixer consisting of
transformers T1 and T2, and diode ring D13 to D16. The 0.25MHz to
30MHz signal is mixed with the output of VCO1 (45.25MHz to 75MHz) to
produce the first IF signal, at 45MHz.
FETs V7 and V8 are switched as a grounded-gate amplifier through diodes
D22 to D24 by the C rail being high and D rail being 0V. The amplified
output is passed, via a crystal roofing filter Z1 (approx. 20kHz bandwidth)
to FET V9.
FET V9 is switched as a source follower by the C and D rails through diodes
D26 to D29. The output is applied to the second mixer, formed by
transformers T5 and T6, and diode ring D31 to D34. The 45MHz signal is
mixed with the output of VCO2 (43.350MHz to 43.352MHz) to produce the
second IF signal at 1650kHz. This signal is passed to both the second IF
amplifier and the noise blanker.
FETs V16 and V17 are switched to form the second IF amplifier as a
grounded gate amplifier by the C and D rails through diodes D35 to D38.
To Next Page
Loading...
Need help?
General
