Receiver Back-End - Motorola PR860 Detailed Service Manual

Theory of Operation: UHF1 Receiver
Both the pre and post-RF amplifier varactor tuned filters have similar responses. The 3 dB bandwidth
of the filter is about 50 MHz. This enables the filters to be electronically controlled by using a single
control voltage which is DACRx .
The output of the post-RF amplifier filter which is connected to the passive double balanced mixer
consists of T301, T302 and CR306. Matching of the filter to the mixer is provided by C381. After
mixing with the first LO signal from the voltage controlled oscillator (VCO) using low side injection,
the RF signal is down-converted to the 44.85 MHz IF signal.
The IF signal coming out of the mixer is transfered to the crystal filter (FL301) through a resistor pad
and a diplexer (C322 and L310). Matching to the input of the crystal filter is provided by C324 and
L311. The crystal filter provides the necessary selectivity and intermodulation protection.
2.5.2

Receiver Back-End

The output of crystal filter FL301 is matched to the input of first IF amplifier transistor Q302 by L330.
Voltage supply to the IF amplifier is taken from the receive 5 volts (R5). The IF amplifer provides a
gain of about 16dB. The amplified first IF signal is then coupled into U301(pin 1) via C360 and L332
which provides the matching for the first IF amplifier and U301.
Within U301, the first IF 44.85 MHz signal mixes with the 44.395 MHz second local oscillator (2nd
LO) to produce the second IF signal at 455 kHz. The 2nd LO signal frequency is determined by
crystal Y300. The second IF signal (455 kHz) is then filtered by an external ceramic filter FL302
before being amplified by the second IF amplifier within U301. Again, the signal is filtered by a
second external ceramic filter FL303 or FL304 depending on the selected channel spacing. FL303 is
used for 20/25 kHz channel spacing whereas FL304, for 12.5 kHz channel spacing. The simple
circuit consisting of U302, CR312, CR313 and resistors R345, R360, R321 and R324 divert the
second IF signal according to the BW_SEL line. The filtered output of the second IF signal is applied
to the limiter input pin of U301.
The IF IC (U301) contains a quadrature detector using a ceramic phase-shift element (Y301) to
provide audio detection. Internal amplification provides an audio output level around 120mVrms
(@60% deviation) from pin 8 of U301. This demodulated audio is fed to the ASFIC_CMP IC (U404)
in the controller section.
The IF IC (U301) also performs several other functions. It provides a received signal-strength
indicator (RSSI) with a dynamic range of 70 dB. The RSSI is a dc voltage monitored by the
microprocessor, and used as a peak indicator during the bench tuning of the receiver front-end
varactor filter.
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