Antenna Switch; Receiver Front End - Motorola ASTRO Digital Saber Service Manual

Antenna Switch

Receiver Front End

Two antenna switches are part of the radio circuitry. One of the
switches which is located in the radio casting is mechanical. 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 SB+, 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 comprised of: 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
comprised of: 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-CR9 in VHF and CR6-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 on or off Q108.
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 broad-band, 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.
Via a broadband 50-ohm transformer, T1, the filtered RF signal is
routed to the input of a broadband mixer/buffer (U2). Mixer U2 uses
GaAs FETs, in a double-balanced Gilbert Cell configuration. The RF
signal is applied to the mixer at U2 pins 1 and 15. An injection signal
(1st LO) of about -10dBm, supplied by the FGU, is applied to U2 pin 8.
Mixing of the RF and the 1st LO results in an output signal which is
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