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

2-2

2.2.1.1 RX Front End

UHF2 : Receiver Front-end consists of a low pass filter, a pre-selector filter, a low noise RF Amplifier
and a Post-selector filter. Incoming RF signal from the antenna is applied through the Harmonics
Low Pass Filter (L409 – L411, C426 – C429, C445 – C446) and passes through the transmit/receive
switch (CR301) and a varactor-tuned 2-pole pre-selector filter (L320, L324, C351, C361, CR314 and
CR307) before routed to an RF amplifier
and the frequency shifting is controlled by variable capacitor diodes (CR314 and CR307) connected
to the CPU. The filter output is coupled to a 13 dB RF amplifier Q301 which outputs the RF signal to
the post-selector filter (L323, L322, L328, C379, C354 and C355) which is also a band shift filter
configured to provide steeper low-side attenuation. The 3 variable capacitors (CR313, CR304 and
CR305) with 8 frequency steps are also controlled by the CPU.
VHF : Receiver Front-end consists of a low pass filter, a pre-selector filter, a low noise RF Amplifier,
a Post-selector filter. Incoming RF signal from antenna is applied through the Harmonics Low Pass
Filter (L409 – L411, C426 – C430, C445 – C446) and passes the transmit/receive switch (CR301)
and a varactor-tuned 2-pole pre-selector filter (L301 – L304, C301, CR302, CR303, C304, C305,
C307, C308) before routed to an RF amplifier (Q301). The pre-selector filter is a 6 step Band-shift
filter, and the frequency shifting is controlled by variable capacitors diode's (CR302 & CR303)
connected to the CPU. The filter output is coupled to a 13 dB RF amplifier Q301 which outputs the
RF signal to the post-selector filter (L308, L309, L311, C315 and C354) which is also a band shift
filter configured to provide steeper low-side attenuation. The 2 units of 6 step frequency variable
capacitors (CR305, CR307) are also controlled by the CPU.

2.2.1.2 RX Back End

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 (L310, L331, C325 – C327) 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):
1st IF (45.1 MHz) - 2nd LO (44.645 MHz) = 2nd IF (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 (L310, L331, C325, C326 and
C333) 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)
1st IF (45.1 MHz) - 2nd LO (44.645 MHz) = 2nd IF (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.
(Q301). The pre-selector filter is an 8 step Band-shift filter,
Theory Of Operation: Major Assemblies