Technical description
4-12 HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual
When the level of the speech input applied to the microphone amplifier is greater
than ≈3.5 mV P (compression threshold), the output swing will exceed the
comparator window (±0.25 V). This produces negative going output pulses from
the comparator IC205/A and B pins 1 and 7. These pulses lower the DC charge on
capacitor C212. The effect is that the voltage will be reduced on the FET gate of
V202. This increases the resistance of the FET and lowers the gain of the
microphone. This puts the microphone amplifier into compression. The output
level remains constant for a further 30 dB increase in input level before
overloading occurs.
Under normal speech conditions, the time constant (consisting of C212 and
R211), sets the talk power ratio and produces a high average transmission power
level. When data transmission is required, the time constant is extended by
switching in capacitor C213 in parallel to C212 (TALK PWR OFF line). This is
achieved by a command from the microprocessor IC101 [1 04-02976 Sheet 1] to
set the output port of IC114 pin 10 low, thus grounding the negative side of C213.
When lower power is required for CB channels, the drive level to the PA is
reduced by the microprocessor IC101 setting IC114 pin 9 low. This causes FET
V201 (see Sheet 2) to switch on and apply R203 in parallel to resistors R205 and
R206. This action reduces the comparator window to ±0.125 V, resulting in a
reduction of 6 dB in the audio output from the microphone compressor amplifier
IC204/B.
Modulator
1 04-02972 and 04-03135 Sheet 2
The transmitter audio output of IC204/B is passed via P102 pin 4 to the
RF/Exciter PCB P201 pin 4. The output is then fed to the input of the balanced
modulator IC202 pin 1. The modulator is enabled when DC is applied via the
transistor switch V7 and R238 (Sheet 1) to the bias input pin 5.
The audio is mixed with the local oscillator of 456.5 kHz (453.5 kHz for LSB)
and is applied to pin 8 and 10 (see page 4-18, 455 kHz local oscillator for
USB/LSB). This produces a DSB output at pin 6. From there it is passed via D204
to the 455 kHz sideband ceramic filter Z202 pin 1.
455 kHz filter and first mixer
Filter Z202, with a bandwidth of 2.5 kHz, filters out the unwanted sideband
leaving only the wanted sideband at pin 3. From there it is fed via the noise limiter
gate V204 (disabled in transmit) to a tuned auto transformer T201. It is fed from
T201 via a resistor matching attenuator network R48 and R49 [1 04-02972 and
04-03135 Sheet 1] to the input of the first balanced amplifier/mixer IC8 pins 12
and 13.
The first mixer IC8 (see Sheet 1) is enabled by operating transistor switch V7,
applying DC via R46 to pin 4 and bias current via R50 to pin 11. The filtered "B"
rail is applied to the mixer output pins 3 and 14 via the centre tap of output
transformer T4.
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