Antenna Switch; Receiver Front End - Motorola Handie-Talkie MTX Series Theory/Troubleshooting Manual

diode CR203 sets the TX frequency while varactor
CR202 is the TX modulation varactor. The modulation of
the carrier is achieved by using a 2-port modulation
technique. The modulation of low frequency tones such
as DPL/TPL is achieved by injecting the tones into the
A/D section of the fractional-N synthesizer. The digitized
signal is modulated by the fractional-N divider, generat-
ing the required deviation. Modulation of the high
frequency audio signals is achieved by modulating the
varactor (CR203) through a frequency compensation
network. Resistors R207 and R208 form a potential
divider for the higher frequency audio signals.
In order to cover the very wide bandwidths, positive
and negative V-control voltages are used. High control
voltages are achieved using positive and negative multi-
pliers. The positive voltage multiplier circuit consists of
components CR204, C256, C257 and reservoir capaci-
tor C258.The negative multiplier circuit consists of
components CR205, CR206, C266, C267 and reservoir
capacitor C254 in VHF and UHF radios. Out-of-phase
clocks for the positive multiplier appear at U204 pins 9
and 10. Out-of-phase clocks for the negative multiplier
appear at U204 pins 7 and 8, and only when the nega-
tive V-control is required (i.e., when the VCO frequency
exceeds the crossover frequency). When the negative
V-control is not required, transistor Q201 is turned on,
and capacitor C259 discharges. The 13V supply gener-
ated by the positive multiplier is used to power up the
phase detector circuitry. The negative V-control is
applied to the anodes of the VCO varactors.
The TX VCO signal is amplified by an internal buffer
in U201, routed through a low pass filter and routed to
the TX PA module, U105 pin 1. The TX and RX VCOs
and buffers are activated via a control signal from U204
pin 38.
The reference oscillator supplies a 16.8MHz clock
to the synthesizer where it is divided down to either a
2.1MHz or a 2.4MHz clock. This divided-down clock is
fed to the controller board ASF IC (U701), where it is
further processed for internal use. Module U701 also
use this signal to synthesize the microcomputer clock.
The controller will program the synthesizer to provide
2.1MHz or 2.4MHz as required.
B. Antenna Switch
Two antenna switches are part of the radio circuitry.
One of the switches (S101) is mechanical. It switches
between the radio antenna and a remote antenna.
Switching of S101 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 (S101 pin 1 to pin 3) is broken; continuity is
made from the remote antenna to the radio RF line
(S101 pin 2 to pin 3). The second switch is a current
device. It is a pair 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 anten-
na. In the receive mode, RF is received from the
antenna, routed through receive switching diode CR109,
and applied to the RF amplifier, U1. 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), L131 (VHF) or L123
(UHF), L121, CR109, and L122. Sinking of the bias cur-
rent is through the 5-volt regulator, U103 pin 3.
C. Receiver Front End
The RF signal is received by the antenna and cou-
pled through RF switch S101 and applied to a low-pass
filter comprised of: VHF; L126, L127, L128, C130, C149,
C150, and C151, or UHF; L126, L127, L128, C149,
C150, and C151. The filtered RF signal is passed
through the antenna switch (CR109) and applied to a
bandpass filter comprised of: VHF; L11, L30 thru L35,
CR6 thru CR9, C1, C2, and C3, or UHF; L30, L31, L32,
L34, L35, CR6 thru CR9, C1, C2, and C3. The band-
pass 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 microcomput-
er.The D/A output range is extended through the use of
a current mirror, transistor Q108 and associated resis-
tors R115 and R116. When Q108 is turn 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 oper-
ation 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). Auto-
matic gain control (AGC) is applied to the RF amplifier
through an AGC network consisting of pin diode CR11,
resistor R72 (VHF) or R52 (UHF), and choke L32(VHF)
or L16 (UHF). The AGC control voltage is derived from
pin 4 of the IF IC, U3. Bypassing is provided by capaci-
tors C59 in UHF and C70 in VHF, while temperature
compensation is provided by Schottky diode CR12 and
R70 (VHF) or R51 (UHF). When a strong signal is
received, the voltage at U3 pin 4 drops, causing current
to flow from the receive 5-volt line (R5) through the pin
diode (CR11). When this happens, the RF signal is
shunted to ground via capacitor C16, in UHF and C71 in
VHF thus reducing the amplitude of the RF signal
applied to the 1st mixer. When the received RF signal is
very weak, the voltage at U3 pin 4 rises and no current
flows through CR11. The RF amplifier will then be at
maximum gain. 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,
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