Receiver Front End - Motorola P100 Series Detailed Service Manual

5-2 435 – 480 MHz UHF2 Theory of Operation: UHF2 Receiver
5.2.1

Receiver Front End

UHF2
Incoming RF signals from antenna are first routed through the harmonic filter (L409 – L411,
C426 – C429, C445, and C446) and antenna switch (CR301), part of the transmitter circuitry, before
being applied to the receiver front end. The receiver front end consists of preselector filter,
RF amplifier, post-selector filter and a single-balanced mixer.
The preselector filter is a varactor-tuned 2-pole design using discrete elements (C351 – C361, L320,
and L324) in a series/shunt resonator configuration. It is a band-shift filter and the frequency shift is
controlled by varactor diodes CR314 and CR307, which are connected to the microprocessor. It is
configured to provide steeper attenuation above the passband for improved spurious rejection when
low-side local injection is used. The frequency is separated into 8 steps and controlled by CPU
(435 – 480 MHz).
The output of this filter is matched to the base of RF amplifier Q301 which provides 13 dB of gain.
The output of the RF amplifier is applied to the post-selector filter. The post-selector filter designed
using discrete elements (C354, C355, C379, L322, L323, L328) in a series/shunt resonator
configuration. It is a band-shift filter and the frequency shift is controlled by varactor diodes CR304,
CR305, and CR313, which are connected to the microprocessor. It is configured to provide steeper
attenuation above the passband for improved spurious rejection when low-side local injection is used.
The frequency is separated into 8 steps and controlled by CPU (435 – 480 MHz).
The output of the post selector is connected to the single-balanced mixer consisting of components
L329, L333, Q306 and Q307. 1st local signal generated from VCO is filtered by injection filter
(C325 – C327, L310, and L331) to remove second harmonics. The converted 1st IF frequency at
mixer passes through L331 and matches the 45.1 MHz IF signal to pair crystal filter (FL301).
UHF2 R&TTE
Incoming RF signals from antenna are first routed through the harmonic filter (L409 – L411,
C426 – C429, C445, C446) and antenna switch (CR301), part of the transmitter circuitry, before being
applied to the receiver front end. The receiver front end consists of preselector filter,
RF amplifier, post-selector filter and a double-balanced mixer.
The preselector filter is a varactor-tuned 2-pole design using discrete elements (L320, L324, C351
and C361) in a series/shunt resonator configuration. It is a band-shift filter and the frequency shift is
controlled by varactor diodes CR314 and CR307, which are connected to the microprocessor. It is
configured to provide steeper attenuation above the passband for improved spurious rejection when
low-side local injection is used. The frequency is separated into 8 steps and controlled by CPU
(435 – 480 MHz).
The output of this filter is matched to the base of RF amplifier Q301 which provides 13dB of gain.
The output of the RF amplifier is applied to the post-selector filter. The post-selector filter designed
using discrete elements (L323, L328, C355, C363, and C379) in a series/shunt resonator
configuration. It is a band-shift filter and the frequency shift is controlled by varactor diodes CR305
and CR313, which are connected to the microprocessor. It is configured to provide steeper
attenuation above the passband for improved spurious rejection when low-side local injection is used.
The frequency is separated into 8 steps and controlled by CPU (435 – 480 MHz).
The output of the post selector is connected to the double-balanced mixer consisting of components
L329, L333, and CR316. 1st local signal generated from VCO is filtered by injection filter (L310, L331,
C365, C386, and C387) to remove second harmonics. The converted 1st IF frequency at mixer
passes through L331 and matches the 45.1 MHz IF signal to pair crystal filter (FL301).