Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 123

CIRCUIT DESCRIPTION WJ-8718-19/FE HF RECEIVER
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3.4.15 455 kHz AMPLIFIER/AM DETECTOR (A4A7) (726002)
The 455 kHz Amplifier/AM Detector nfounts on the IF Motherboard, A4
(see paragraph 3.3.4). Figure 3-21 is a detailed functional block diagram of the
455 kHz Amplifier/AM Detector which should be referred to in the following circuit
description. Figure 6-19, 455 kHz Amplifier/AM Detector Schematic Diagram, may be
referred to for greater component level detail, if desired.
The IF input signal is first amplified by two-stage gain controlled
amplifier, The signal is then split to provide three outputs: the IF sample which
operates the FM/CW/SSB Demodulator, the IF output for the rear panel, and the input
to the AM Detector. The AM Detector, which operates at a relatively high level for
good linearity, has its output directly coupled to the AGC module and the Audio
Amplifier.
FET's Ql and Q2 operate as common, source amplifiers with their gains
controlled by a variable voltage applied to gate 2 of each transistor. Inductors LI and
L2 broadly tune the outputs of Ql and Q2 by cancelling any stray capacitance. The
net overall bandwidth of the Ql/Ql amplifier exceeds 50 kHz. Potentiometer R7
between the first and second amplifiers adjusts the maximum gain of the amplifiers and
hence of the whole receiver. Q2 drives emitter follower amplifier Q7. The low
impedance emitter output of Q7 passes through bandpass filter L3, L8, L9, Cll and
C34-C36. This filter establishes the bandpass of A4A7 at approximately 100 kHz. The
filter is terminated by the low impedance base input of Q3.
Transistor Q3 serves as a buffer between the 455 kHz amplifier and its
chree outputs. For signals fed to the FM/CW/SSB Detector (pin 13), Q3 acts as an
emitter-follower stage. For the rear panel IF Output, Q3 feeds the signal to Q4, which
acts as a power amplifier. Transformer Tl supplies a 50 ohm IF outout to the rear
panel, providing a nominal 20 mV IF output for RF inputs greater than 3 V. For the
AM detector, Q3 and Q5 both act as common-emitter amplifiers to raise the IF signal
to a level of several volts which will permit the detector diode, CR3, to perform
linearly. Diodes CR4 and CR5 provide a dc-bias to operate the AM Detector and
emitter-follower (Q6) above ground to establish the proper dc level for the AGC
circuit. The low-pass filter of L7 and C31 suppresses any residual IF signal.
3.4.16 FM/CW/SSB DETECTOR (A4A9) (791599)
The FM/CW/SSB Detector mounts on the IF Motherboard, A4, (see
paragraph 3. 3. 4).Figure 3-22 is a detailed functional block diagram of the FM/CW/SSB
Detector which should be referred to in the following circuit description. Figure 6-20
FM/CW/SSB Detector Schematic Diagram, may be referred to for greater component
level detail, if desired.
For FM reception, this module contains a limiter and discriminator.
Power for these circuits is supplied when the FM detection mode is selected. For CW
or SSB reception, there is a product detector which has its power applied when the
CW, USB or LSB detection modes are selected. Also, when the product detector is
energized, the BFO Synthesizer is enabled and its output is applied to the product
^etector. v
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