Antenna Switch - Motorola ASTRO XTS 3000 R Detailed Service Manual

Antenna Switch

Receiver Front End
Two antenna switches are part of the radio circuitry. One of the
switches is a mechanical connector that attaches to the external
antenna bushing. It switches between the radio antenna and a remote
antenna. Switching is accomplished by a plunger located on the
accessory connector. With a remote antenna installed, continuity
between the radio antenna and the RF input line is broken; continuity
is made from the remote antenna to the radio RF line.
The second switch is a current device. It is a pair of diodes (CR108/
CR109) that electronically steer RF between the receiver and the
transmitter. In the transmit mode, RF is routed through transmit
switching diode CR108, and sent to the antenna. In the receive mode,
RF is received from the antenna, routed through receive switching
diode CR109, and applied to the RF amplifier, U1 (UHF), Q1 (VHF). In
transmit, bias current, sourced from U101, pin 21, is routed through
L105, U104, CR108, and L122 in VHF, and L105, CR108, and L122 in
UHF. Sinking of the bias current is through the transmit ALC module,
U101, pin 19. In the receive mode, bias current, sourced from switched
B+, is routed through Q107 (pin 3 to pin 2), L123 (UHF), L121, CR109,
and L122. Sinking of the bias current is through the 5-volt regulator,
U106, pin 8.
The RF signal is received by the antenna and coupled through the
external RF switch. The UHF board applies the RF signal to a low-pass
filter comprising L126, L127, L128, C149, C150, and C151. The VHF
board bypasses the lowpass filter. The filtered RF signal is passed
through the antenna switch (CR109) and applied to a bandpass filter
comprising: VHF— L11 through L14, CR1 through CR9, C4, C2, and
C3, or UHF—L30, L31, L32, L34, L35, CR6 through CR9, C1, C2, and
C3. The bandpass filter is tuned by applying a control voltage to the
varactor diodes in the filter (CR1 through CR9 in VHF and CR6
through CR9 in UHF).
The bandpass filter is electronically tuned by the D/A IC (U102), which
is controlled by the microcomputer.The D/A output range is extended
through the use of a current mirror: transistor Q108 and associated
resistors R115 and R116. When Q108 is turned on via R115, the D/A
output is reduced due to the voltage drop across R116. Depending on
the carrier frequency, the microcomputer will turn Q108 on or off.
Wideband operation of the filter is achieved by retuning the bandpass
filter across the band.
The output of the bandpass filter is applied to a wideband GaAs RF
amplifier IC, U1 (RF AMP), on the UHF transceiver board. The VHF
board uses an active device for RF amplification (Q1). After being
amplified by the RF AMP, the RF signal is further filtered by a
second broadband, fixed-tuned, bandpass filter consisting of C6, C7,
C8, C80, C86, C87, C88, C97, C99, L3, L4, L5, and L30 (VHF); or C4
through C7, C88 through C94, C99, and L11 through L15 (UHF) to
improve the spurious rejection.
The filtered RF signal is routed through a broadband 50-ohm
transformer (T1) to the input of a broadband mixer/buffer (U2). Mixer
U2 uses GaAs FETs in a double-balanced, Gilbert Cell configuration.
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