Rx Back End; Rx Squelch - Motorola CP185 Basic Service Manual

Theory Of Operation: Major Assemblies

2.2.1.2 RX Back End

UHF1 : RF signal from RX front-end is then directed to a Double Balanced Mixer (CR316, L329 and
L333). 1st LO signal from VCO is filtered by an injection filter (C325, C326, C387, L310 and L331) to
remove harmonics.
After passing through a pair of 45.1 MHz Crystal filter, the 1st IF signal is amplified by 15 dB via an
IF amp (Q303) and channeled to IF IC (U201) to be mixed thus producing the 2nd IF Frequency
(455 kHz):
Depending on channel spacing, the 2nd IF frequency passes through the wide (CF1) and/or narrow
(CF2) filters to eliminate undesired signals before being finally demodulated by demodulator in U201
with Recovered Audio as the final output.
UHF2: RF signal from RX front-end is then directed to a Single Balanced Mixer (L329, L333, Q306,
and Q307). 1st LO signal from VCO is filtered by an injection filter (C325, C326, C327, L310 and
L331) to remove harmonics.
After passing through a pair of 45.1 MHz Crystal filter, the 1st IF signal is amplified by 15 dB via an
IF amp (Q303) and channeled to IF IC (U201) to be mixed thus producing the 2nd IF Frequency
(455 kHz):
Depending on channel spacing, the 2nd IF frequency passes through the wide (CF1) and/or narrow
(CF2) filters to eliminate undesired signals before being finally demodulated by demodulator in U201
with Recovered Audio as the final output.
VHF: RF signal from RX front-end is then directed to a Single Balanced Mixer (L329, L333, Q306,
and Q307). 1st LO signal from VCO is filtered by an injection filter (C325, C326, C333, L310 and
L331) to remove harmonics.
After passing through a pair of 45.1 MHz Crystal filter, The 1st IF signal is amplified by 15 dB via an
IF amp (Q303) and channeled to IF IC (U201) to be mixed thus producing 2nd IF Frequency
(455 kHz)
Depending on channel spacing, the 2nd IF frequency passes through wide (CF1) or narrow (CF2)
filter to eliminate undesired signals before being finally demodulated by demodulator in U201 with
Recovered Audio as the final output.

2.2.1.3 RX Squelch

The mute (squelch) circuitry switches off the audio amplifier when no audio is detected from the
recovered audio. The squelch circuit main components are U201 and U202.
U202 will adjust the squelch circuit sensitivity depending on Noise level from recovered audio. Noise
level is amplified by internal amplifier of U201 to help U202 decide the squelch circuit sensitivity. If
the noise level is over the set threshold, the microprocessor mutes the radio.
Note Perform squelch tuning after any RX part replacement. Refer
on page 5-12.
1st IF (45.1 MHz) - 2nd LO (44.645 MHz) = 2nd IF (455 kHz)
1st IF (45.1 MHz) - 2nd LO (44.645 MHz) = 2nd IF (455 kHz)
1st IF (45.1 MHz) - 2nd LO (44.645 MHz) = 2nd IF (455 kHz)
Chapter 5.7: Receiver Tuning
2-3