Motorola P100 Series, Detailed Service Manual, Page: 152 / 336

8-2 136 – 174 MHz VHF Theory of Operation: VHF Receiver
8.2.1 Receiver Front End
Incoming RF signals from antenna are first routed through the harmonic filter (L409, L410, L411,
C426, C427, C428, 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 single-balanced mixer.
The preselector filter is a varactor-tuned 2-pole design using discrete elements (L301, L302, L303,
L304, C301, C304, C305, C307) in a shunt or series resonator configuration. It is a band-shift filter
and the frequency shift is controlled by varactor diodes CR302 and CR303, which are connected to
the microprocessor. It is configured to provide steeper attenuation above the passband for improved
spurious rejection when high-side local injection is used. The frequency is separated into 6 steps and
controlled by CPU (136 – 174 MHz).
The output of this filter is matched to the base of RF amplifier Q301 which provides 15dB of gain.
The output of the RF amplifier is applied to the post-selector filter. The post-selector filter designed
using discrete elements (L307, L308, L309, L311, C315 and C354) in a series/shunt resonator
configuration. It is a band-shift filter and the frequency shift is controlled by varactor diodes CR305
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 6 steps and controlled by CPU (136 – 174 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 (L310, L331, C325, C326, C327, C333 and C365) 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).
8.2.2 Receiver Back End
The 1st IF signal is amplified about 15 dB by IF amp Q303. The output of the IF amp is connected to
IF IC (U201). 1st IF frequency (45.1 MHz) and 2nd LO frequency (44.645 MHz) are mixed in U201.
The second mixer converts the 45.1 MHz high IF frequency to 2nd IF frequency (455 kHz).
Additional IF selectivity is provided by two ceramic filters (CF1, CF2). The wider filter 455 FW is used
for 20 kHz and 25 kHz channel spacing, and the narrower filter 455 HW is used for 12.5 kHz channel
spacing. These two ceramic filters may eliminate undesired signal and demodulated by demodulator
in U201. N/S_SW which connected to microprocessor is used to select the wide and narrow band.
The mute (squelch) circuit switches off the audio amplifier when no audio is present. The squelch
circuit consists of U201 and U202 and their associated components. The noise signal from pin 9 of
U201 is used to control the squelch circuit sensitivity of U202. The noise passes through filter, and is
amplified by internal amp of U201. The amplified noise act as a DC voltage to control the mute
system. So if the noise level is under the threshold voltage, the microprocessor (U101) un-mutes the
radio. If the noise level is over the threshold voltage, the microprocessor mutes the radio. The
squelch level is tuned in the factory. When a component or a part in the RX system is replaced, the
squelch must be re-tuned using the Tuner.