Receiver Back End; Transmitter - Motorola Handie-Talkie MTX Series Theory/Troubleshooting Manual

C80, C86, C87, C88, C97, C99, L3, L4, L5, and L30
(VHF); or C4 thru C7, C88 thru C94, C99, and L11 thru
L15 (UHF) to improve the spurious rejection.
Via a broadband 50-ohm transformer, T1, the fil-
tered RF signal is routed to the input of a broadband
mixer/buffer (U2). Mixer U2 uses GaAs FETs, in a dou-
ble-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 the first IF frequency.
The first IF frequency of VHF and UHF bands are 44.85
MHz and 73.35 MHz respectively. The 1st LO signal for
VHF is 44.85 MHz higher than the carrier frequency
while that for the UHF is 73.35 MHz lower than the carri-
er frequency. The 1st IF signal output, at U2 pins 4 and
6, is routed through transformer T2 and impedance
matching components, and applied to a 2-pole crystal fil-
ter (FL1), which is the final stage of the receiver front
end. The 2-pole crystal filter removes unwanted mixer
products as the filtered IF signals being routed to the IF
module, U3. Impedance matching between the output of
the transformer (T2) and the input of the filter (FL1) is
accomplished by capacitors C35 and C36 and inductor
L20.
D. Receiver Back End
The output of crystal filter FL1 is matched to the
input of IF buffer amplifier transistor Q4 by components
C39, L22 and C38. Transistor Q4 is biased by the volt-
age level on U2 pin 3. The IF frequency on the collector
of Q4 is applied to U3 pin 2, where it is down converted,
amplified, filtered, and demodulated, to produce the
recovered audio at U3 pin 28. This IF IC is electronically
programmable, and the amount of filtering (which is
dependent on the radio channel spacing) is controlled
by the microcomputer. Additional filtering, which used to
be provided externally by conventional ceramic filters, is
replaced by internal filters in the IF module. The IF IC
uses a type of direct conversion process where the sec-
ond LO frequency is very close to the first IF frequency.
The IF IC synthesizes the second LO and phase locks
the VCO to track the first IF frequency.
In the absence of an IF signal, the VCO will "hunt,"
or its frequency will vary about a frequency close to the
IF frequency. When an IF signal is received, the VCO
will lock onto the IF signal. The 2nd LO/VCO is a Col-
pitts oscillator built around transistor Q1. The VCO has a
varactor diode, CR5, to adjust the VCO frequency. The
control signal for the varactor is derived from a loop filter
consisting of C52, C53, and R16.
The IF IC (U3) performs several other functions. It
provides a received signal strength indicator (RSSI) and
a squelch output. The RSSI is a dc voltage monitored by
the microcomputer and used as a peak indicator during
bench tuning of the receiver front-end varactor filter. The
RSSI dc voltage is sent from U3 pin 9 to connector jack
14
J301 pin 11, where it is routed through the jumper flex
(P301 pin 11 to P704 pin 11) and applied to the con-
troller board. In the controller board the RSSI is routed
through the ASF IC (U701 pin 11 to U701 pin 41), and
applied to the front cover flex at J701 pin 21.Via the
front cover flex, the RSSI voltage reaches its destina-
tion, at the universal connector at P403 pin 7 as RTS.
The squelch output of U3, on pin 29, is a high-frequency
audio signal. The squelch signal is routed to shaping
and detection circuits within U701 on the controller
board, for use in other parts of the radio. The IF module
(U3) also monitors the strength of the received signal, to
provide an AGC voltage at pin 4, which is then fed to the
RF amplifier AGC circuit. Inductor L23 and capacitor
C70 prevent any IF signal from leaking back to the front-
end circuits.
E. Transmitter
The transmitter consists of three major sections:
• Harmonic Filter
• RF Power Amplifier Module
• ALC Circuits
1. Harmonic Filter
RF from the Power Amplifier (PA) module, U105 is
routed through the coupler (U104), passed through
the transmit antenna switch (CR108), and applied to
a harmonic filtering network. The harmonic filtering
circuit is comprised of the following components:
L126, L127, L128, C149, C150, and C151 (for VHF
models); or L126, L127, L128, C129, C130 C149,
C150, and C151 (for UHF models). Resistor R128
(UHF) or R117 (VHF) provides a current limited 5V
to P402 for mobile vehicular adapter (MTVA) appli-
cations.
2. RF Power Amplifier Module
The RF power amplifier module (U105) is a wide-
band multi-stage amplifier (3 stages for the VHF
models and 4 stages for the UHF models). Nominal
input and output impedance of U105 is 50 ohms.
The dc bias for U105 is on pins 2, 4, 5. In the trans-
mit mode, the voltage on U105 pins 2 and 4 (close
to the B+ level) is obtained via switching transistor
Q101. Transistor Q101 receives its control base sig-
nal as follows:
• the microcomputer keys the D/A IC to produce
a ready signal at U102 pin 3
• the ready signal at U102 pin 3 is applied to the
TX ALC IC at U101 pin 14 (5V)
• the synthesizer sends a LOC signal to the TX
ALC IC (U204 pin 40 to U101 pin 16
When the LOC signal and the ready signal are both
received, the TX ALC IC (pin 13) sends a control
signal to turn on transistor Q101.