Receiver Front-End - Motorola Commercial Series Service Information

2-2 THEORY OF OPERATION
2.1 Receiver Front End
Incoming RF signals from the antenna are first routed through the harmonic filter and antenna
switch, part of the transmitter circuitry, before being applied to the receiver front end. The receiver
front end consists of a preselector filter, RF amplifier, an interstage filter, and a double-balanced first
mixer.
The preselector filter is a fixed-tuned 4-pole design using discrete elements (L1-L4 and C1-C9) in a
series/shunt resonator configuration. It has a 3 dB bandwidth of 44 MHz, an insertion loss of 2 dB
and image attenuation of 37 dB at 226 MHz, with increasing attenuation at higher frequencies.
Diode CR1 protects the RF amplifier by limiting excessive RF levels.
The output of the filter is matched to the base of RF amplifier Q21, which provides 16 dB of gain and
a noise figure of 2 dB. Operating voltage is obtained from the 5R source, which is turned off during
transmit to reduce dissipation in Q21. Current mirror Q22 maintains the operating current of Q21
constant at 6.7 mA regardless of device and temperature variations, for optimum dynamic range
and noise figure.
The output of the RF amplifier is applied to the interstage filter, a fixed-tuned 3-pole series-coupled
resonator design having a 3 dB bandwidth of 54 MHz and insertion loss of 1.8 dB. This filter has an
image rejection of 40 dB at 226 MHz, with increasing attenuation at higher frequencies.
The output of the interstage filter is connected to the passive double-balanced mixer consisting of
components T41, T42, and CR41. This mixer has a conversion loss of 7 dB. High-side injection from
the frequency synthesizer is filtered by L40-L41 and C40-C44 to remove second harmonic energy
that may degrade half-IF spurious rejection performance. The injection filter has a 3 dB bandwidth
of 52 MHz and an insertion loss of 1.5 dB. The filtered injection signal is applied to T42 at a level of
+6 dBm.
The mixer output is applied to a diplexer network (L51-L52, C51, R51) which matches the 44.85
MHz IF signal to crystal filter FL51, and terminates the mixer into 50Ω at all other frequencies.