Rx Front End - Motorola CP185 Basic Service Manual

2-2 Theory Of Operation: Major Assemblies

2.2.1.1 RX Front End

UHF1 : 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 (C426, C427, C428, C429, C445, C446, L409, L410 and L411) and passes through
the transmit/receive switch (CR301) and a varactor-tuned 2-pole pre-selector filter (C351, C361,
CR307, CR314, L320 and L324, ) before routed to an RF amplifier (Q301). The pre-selector filter is
an 8 step Band-shift filter, and the frequency shifting is controlled by varactor diodes (CR307 and
CR314) 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 (C355, C379, L323 and L328) which is also a band
shift filter configured to provide steeper low-side attenuation. The 2 varactor diodes (CR305 and
CR313) with 8 frequency steps are also controlled by the CPU.
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 (C426, C427, C428, C429, C445, C446, L409, L410 and L411) and passes through
the transmit/receive switch (CR301) and a varactor-tuned 2-pole pre-selector filter (C351, C361,
CR307, CR314, L320 and L324) before routed to an RF amplifier (Q301). The pre-selector filter is an
8 step Band-shift filter, and the frequency shifting is controlled by varactor diodes (CR307 and
CR314) 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 (C354, C355, C379, L322, L323 and L328) which is
also a band shift filter configured to provide steeper low-side attenuation. The 3 varactor diodes
(CR304, CR305 and CR313) 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 (C426, C427, C428, C429, C430, C445, C446, L409, L410 and L411) and passes the transmit/
receive switch (CR301) and a varactor-tuned 2-pole pre-selector filter (C301, C304, C305, C307,
C308, CR302, CR303, L301, L302, L303 and L304) 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 varactor
diodes (CR302 and 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 (C315, C354, L308, L309 and
L311 ) which is also a band shift filter configured to provide steeper low-side attenuation. The 2 units
of 6 step frequency varactor diodes (CR305, CR307) are also controlled by the CPU.