X-2 Technical Service Manual Technical Description 4-5
Publication No: 15-02047
4.2.5 Noise Limiter
The 455 kHz output at pins 3 and 14 of IC8 is applied via C79, R52, C80,
and R53 to the balanced input of high gain amplifier IC13
[C9]
pins 4 and 6.
The balanced output at pins 1 and 8, connect to a tuned transformer T6
(455 kHz) which sends its output to the base of the active rectifier V11.
Noise bursts produce positive going pulses at the collector of V11 and
trigger via V12 the monostable flip-flop IC7/A and IC7/B. The pulse width
is determined by C86 and R60 (nominally 250 µs).
The outputs of the flip-flop pins 3 and 4 produce complementary pulses that
are connected to FET gates V14 and V15
[D11]
and gate out the noise bursts
from the 455 kHz signal.
With V14 on and V15 off, the IF signal passes to the sideband filter.
When a noise burst is present, V14 switches off and V15 switches on,
blocking the IF signal for the period of the gate pulse.
The average DC component of V11 collector current serves as an automatic
gain control to IC13, and flows via R57
[C10]
to pin 5. This ensures that only
the noise burst signals trigger the monostable. C82 and R56 set the AGC
decay time constant.
4.2.6 455 kHz Filter and IF Amplifier
104-02907 Sheet 2
A 2.5 kHz ceramic filter Z301
[B2]
receives the IF signal from the noise limiter
and removes the unwanted sideband signal and other unwanted products
generated by the second mixer. The wanted sideband centred at 455 kHz
passes from the filter to a two stage gain controlled amplifier consisting of
V301
[B4]
and V302 and associated components.
The IF amplifier is broadly tuned to 455 kHz by the tuned circuits
L301/C304 and L302/C308. The output of the IF amplifier (drain of V302)
passes via coupling capacitor C310
[B6]
to the demodulator IC302 pin 1.
The 455 kHz signal converts to audio when mixed with the local oscillator in
the double balanced mixer IC302. The local oscillator is set to 456.5 kHz for
USB conversion and 453.5 kHz for LSB.
Note: Local oscillators VCO1 and VCO2 shift by a total of 3 kHz when
switching from USB to LSB. This enables filter Z301 to be used
for either sideband.
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