Simrad HT50 Assembly Instructions Manual page 8

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The signal is then applied to mixer TR2 where it is mixed with the VCO output, 21.4
MHz below the signal frequency. The output of the mixer, first Intermediate
Frequency (IF) at 21.4MHz, is matched into the crystal filter network Xtal 1 and Xtal
2 by CH3, C26 and R8. The crystal filter provides isolation from adjacent channel
interference. The crystal filter output is matched to the second IF stage IC4 by CH4
and associated capacitors.
IC4 houses a second mixer amplifier and demodulator. The second mixer mixes
the output of the crystal filter with the output of a crystal controlled oscillator,
operating at 21.85MHz, TR10. The output of the second mixer produces the
second IF, 450KHz, which is filtered by a ceramic filter CF1 to provide further
isolation from adjacent channel interference. The signal is then demodulated in
IC4 and the audio is fed to the control PCB via R61 and C93. The combination of
values of R61, C92 and C96 provide signal de-emphasis.
The audio from pin 9 is also amplified by a high gain amplifier within IC4 to detector
SD2 to provide a voltage proportional to the received signal strength to provide a
noise squelch signal. The squelch signal is fed to an A-D converter on the
microcontroller for comparison with the selected squelch level.
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