Technical description
HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual 4-7
The outputs of the flip-flop pins 3 and 4 produce complimentary pulses that are
connected to the FET gates of V204 and V205. These gate out the noise bursts
from the 455 kHz signal. With V204 on and V205 off, the IF signal is passed to
the sideband filter Z202. When a noise burst is present, V204 is switched off and
V205 is switched on, blocking the IF signal for the period of the gate pulse.
The average DC component of V201’s collector current is used as an automatic
gain control to IC201. It is applied via R206 to pin 5. This ensures that only the
noise burst signal triggers the monostable. The AGC decay constant is set by the
components C204 and R205.
In transmit, IC201 is disabled (see page 4-4, Receive/Transmit switching).
455 kHz filter and IF amplifier
1 04-02972 and 04-03135 Sheet 2
The IF signal from the noise blanker gate V204 is applied to a 2.5 kHz ceramic
filter Z202. The filter removes the unwanted sideband, adjacent channels and
other unwanted products generated by the second mixer. The wanted sideband,
centred at 455 kHz, is passed by the filter to a two-stage gain controlled amplifier,
consisting of V207, V208, and its associated components.
The IF amplifier is broadly tuned to 455 kHz by the tuned circuits L202/C219 and
L203/C223.
Demodulator
1 04-02972 and 04-03135 Sheet 2
The output of the 455 kHz IF amplifier is passed via coupling capacitor C224 to
the input of a double balanced mixer IC203 pin 1. The IF signal is mixed with the
local oscillator that is applied to pins 8 and 10 to produce an audio output at IC203
pin 6. The local oscillator operates at 456.5 kHz for USB conversion and
453.5 kHz for LSB. For details of operation of the local oscillator see page 4-18,
455 kHz local oscillator for USB/LSB.
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Local oscillators VCO1 and VCO2 shift by a total of 3 kHz when switching from
USB to LSB. This enables filter Z202 to be used for either sideband.
Automatic gain control
1 04-02972 Sheets 1 and 2
An Automatic Gain Control (AGC) is applied to a number of stages in the
receiver. This is used to increase the dynamic range of the receiver and to maintain
an almost constant audio output level for a large variation of input signals.
When the receiver is at maximum gain, the AGC at the output of IC205/A pin 1
(TP205) (see Sheet 2) is nominally 5.5 V. This is set by the resistor divider R257
and R258 in conjunction with the gain setting resistors R254 and R259.
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