Antenna Switch; Receiver Front End - Motorola ASTRO XTS 3000 Service Manual

Antenna Switch

Receiver Front End

Switching between the standard and external antenna ports is
accomplished with the external mechanical switch that is actuated by
a plunger, located on the accessory connector.
An electronic PIN diode switch steers RF between the receiver and
transmitter. The common node of the switch is at capacitor C101. In
the transmit mode, RF is routed to the anode of diode CR104. In
receive mode, RF is routed to pin 1 of U201. In transmit, bias current,
sourced from U504, pin 21 is routed through PIN diodes CR104 and
CR102, biasing them to a low-impedance state. Bias current returns to
ground through U504, pin 20. In receive, U504, pin 21 is pulled down
to ground and pin 20 is pulled up to B+, reverse-biasing diodes CR104
and CR102 to a high impedance.
For the purposes of this discussion, the receiver front end is defined as
being the circuitry from the antenna switch to the output of the IF
crystal filter. The 800MHz front end converts the received RF signal to
the 1st IF frequency of 73.35MHz, while at the same time providing for
spurious immunity and adjacent channel selectivity. A review of the
interstage components of the front end will now be presented, with
emphasis on troubleshooting considerations.
The received RF signal is passed through antenna switch input
matching components C101, L105, and C114, tank components C106
and L103 (which are anti-resonant at the radios transmitter
frequencies), and output matching components C103 and L104. Both
pin diodes CR102 and CR104 must be back-biased to properly route
the received signal.
The stage following the antenna switch is a 50-ohm, inter-digitated,
three-pole, stripline preselector (U201). The preselector is positioned
after the antenna switch to provide the receiver preamp with some
protection to strong signal, out-of-band signals.
After the preselector (U201), the received signal is processed through
the receiver preamp, U202. The preamp is a dual-gate, GaAs MESFET
transistor which has been internally biased for optimum IM, NF, and
gain performance. Components L201 and L202 match the input (gate
1) of the amp to the first preselector, while at the same time
connecting gate 1 to ground potential. The output (drain) of the amp
is pin 7, and is matched to the subsequent receiver stage through
components L204 and C222. A supply voltage of 5Vdc is provided to
pin 3 through RF choke L203 and bypass capacitor C204. The 5-volt
supply is also present at pin 4, which connects to a voltage divider
network that biases gate 2 (pin 5) to a predefined quiescent voltage of
1.2Vdc. Resistor R202 and capacitor C203 are connected to pin 5 to
provide amp stability. The FET source (pin 3) is internally biased at
0.55 to 0.7Vdc for proper operation with bypass capacitors C201 and
C202, connected to the same node.
The output of the amp is matched to a second three-pole preselector
(U203) of the type previously discussed. The subsequent stage in the
receiver chain is the 1st mixer, U205, which uses low-side injection to
convert the RF carrier to an intermediate frequency (IF) of 73.35MHz.
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