Vhf/Uhf Transceiver Board; Frequency Generation Unit (See Figure 2); Antenna Switch; Receiver Front End (See Figure 3) - Motorola Handie-Talkie MTX Series Theory/Troubleshooting Manual

In the transceiver, SWB+ is routed directly to the 5v
regulator (U202). The regulated 5v supplies the IF IC
(U3), the reference oscillator (U203), the Fractional-N
synthesizer IC (U204), the D/A IC (U102), and the R/T
switch (Q108).
Internal to the synthesizer is a superfilter which sup-
plies the VCO module (U205) and the VCO buffer IC
(U201) with 4.6 volts, produced by the regulated 5V
supply.
In addition, two more 5-volt supplies exist, one for
transmit and one for receive: T5 and R5, respectively.
The regulated 5v is switched to either one or the other
by transistor Q108, under the control of the D/A IC. The
T5 voltage is used as a control line by the TX ALC IC
and provides bias for the RF PA input and the external
antenna connector. The R5 voltage is supplied to the
RF amplifier (U1) and the Mixer Buffer IC (U2).
III. VHF/UHF TRANSCEIVER BOARD
A. Frequency Generation Unit (See Figure 2)
The frequency generation unit (FGU) consists of
three major sections: the high stability reference oscilla-
tor(U203), fractional-N synthesizer (U204), and VCO
buffer IC(U201). The VCO provides the carrier frequen-
cy for the transmitter (TX OUT), and provides the local
oscillator (LO) injection signal for the receiver mixer
buffer (RX OUT).
The RX VCO uses an external active device, where-
as the TX VCO uses the internal device of the VCO
buffer IC. The phase lock loop (PLL) circuit is provided
by the fractional-N synthesizer IC.
The output of the VCO is amplified by the prescaler
buffer, routed through a low-pass filter, and applied to
the prescaler divider of the synthesizer. The divide ratios
are determined from information stored in memory that
was bussed to the synthesizer via the microcomputer.
The microcomputer extracts data for the division ratio as
determined by the channel select switch. The resultant
Negative Multiplier
Positive Multiplier
Reference
Divider
Counter for
Multiplier
A/D
U204
2.1/2.4 MHz
Reference
Clock to Modulating
ASFIC Signal
4
16.8 MHz
Ref Osc
Loop
Filter
Mod Out
Fractional
Pre-scaler
Divider
Divider
Fractional-N
Synthesizer
Low Pass Filter
Figure 2. VHF/UHF Frequency Generation Unit (FGU) Circuits
VCO buffer signal is applied to a comparator in the syn-
thesizer. The synthesizer comparator also receives a
reference frequency via a reference divider input from
the 16.8 MHz temperature-compensated reference
oscillator. If the two frequencies differ, the synthesizer
generates a control (error) voltage which causes the
VCO to change frequency.
Modulation of the carrier is achieved by using a 2-
port modulation technique. The deviation of the low
frequency tone, such as DPL/TPL, is achieved by inject-
ing the signal to an analog/digital circuit in the
synthesizer. The resultant digitized signal is then modu-
lated by the fractional N-divider, thus generating the
required deviation. The deviation of the high frequency
tone is achieved by modulating the modulation varactor
on the VCO. In order to cover a very wide bandwidth,
the VCO control voltage is stepped up by using a posi-
tive and negative multiplier circuit. A 13-volt supply
powers the phase detector circuitry. The VCO signal is
amplified by the integrated buffer amplifier of the VCO
buffer. The two output signals, receiver first LO injection
and transmitter carrier frequency, are filtered and then
routed to the mixer/buffer (U2) and the RF PA (U105),
respectively.
B. Antenna Switch
The function of the antenna switch is to route the
transmitter power to the antenna during the transmit
mode, or route RF from the antenna to the receiver front
end during the receive mode.
C. Receiver Front End (See Figure 3)
The RF signal from the antenna is coupled to the
first bandpass filter through the antenna switch. The out-
put of the bandpass filter is then applied to a wideband
RF amplifier IC (RF AMP). The bandpass filter is elec-
tronically tuned by the D/A IC, which is controlled by the
microcomputer. Wideband operation of the filter is
achieved by retuning the bandpass filter across the
band. After amplification, the RF signal is further filtered
by a second fixed-tuned filter to improve the spurious
rejection.
Switching CCTS
for VCO and Buffer
RX VCO
RX Buffer
TX VCO
TX Buffer
Prescaler
Buffer
U201
VCO Buffer IC
RX Out Low Pass
Mixer/Buffer
Filter
Matching
TX Out
RF PA Input
CCT
MAEPF-23266-O