11.2.1 Receiver Front End; 11.2.2 Receiver Back End - Motorola P100 Series Detailed Service Manual

11-2

11.2.1 Receiver Front End

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,
C361, CR307 and CR314) in series/shunt resonator configuration. It is a band-shift filter and the
frequency shift is controlled by varactor diodes CR307 and CR314, which are connected to the
microprocessor. It is configured to provide steeper attenuation above the pass band for improved
spurious rejection when high-side local injection is used. The frequency is separated into 8 steps and
controlled by CPU (403 – 447MHz).
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, C379, C363 and C355) 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 high-side local injection is used. The frequency is
separated into 8 steps and controlled by CPU (403 – 447MHz).
The output of the post selector is connected to the double-balanced mixer consisting of components
L329, L333, CR316. 1st local signal generated from VCO is filtered by injection filter (L310, L331,
C325, C326, C388, C386, C365 and C387) to remove harmonics. The converted 1st IF frequency at
mixer passes through L333 and matches the 45.1MHz IF signal to crystal filter (FL301).

11.2.2 Receiver Back End

The 1st IF signal is amplified about 15dB by IF amp Q303. The output of the IF amp is connected to
IF IC (U201).
1st IF frequency (45.1MHz) and 2nd LO frequency (44.645MHz) are mixed in U201. The second
mixer converts the 45.1MHz high IF frequency to 2nd IF frequency (455kHz).
Additional IF selectivity is provided by two ceramic filters (CF1, CF2). The wider filter 455FW is used
for 25kHz channel spacing, and the narrower filter 455HW is used for 12.5kHz 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 side 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 acts 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.
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403 – 447 MHz UHF1 Theory Of Operation: UHF1 Receiver