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With the receive VCO, Q501 produces the first
injection signal in the range of 385-425 MHz which is
then amplified by Q502. With the transmit VCO,
Q531 produces the transmit frequency in the range of
430-470 MHz) which is then amplified by Q532.
In the receive mode, part of the Q502 output
signal is fed back through C227 and R210 to pin 10 of
U202, and then other part is applied to buffer Q503.
Filtering is provided by L506, C520, and C521, and
the signal is then applied to one of the gates of dual
gate GaAs FET mixer Q302. This is the first injection
signal, and it is 45.3 MHz below the receive
frequency.
In the transmit mode, part of the Q532 output
signal is fed back through buffer amplifier Q535 to pin
10 of U202, and the other part is applied to a buffer
amplifier formed by Q533 and Q534. This is a cascode
amplifier formed by common-emitter stage (Q533)
and common-base stage (Q534). A cascode amplifier
provides amplification also good isolation. The output
signal from Q534 is then applied to amplifier Q101 in
the transmitter.
4.2.3 VCO AND TCXO MODULATION
Both the VCO and TCXO are modulated in order
to achieve the required frequency response. If only the
VCO was modulated, the phase detector in U202
would sense the frequency change and increase or
decrease the VCO control voltage to counteract the
change (especially at the lower audio frequencies).
Conversely, if only the TCXO was modulated, the
VCO frequency would not change fast enough (espe-
cially at the higher audio frequency).
However, by modulating both the VCO and
TCXO, the two phase detector inputs remain in phase
and no frequency shift is sensed. This produces a rela-
tively flat audio response. Potentiometer 4 of U147 on
the logic board balances these signals, and potentio-
meters 1 and 3 set the data and audio deviation levels.
R203/R204 set the frequency of the TCXO by
adjusting the DC bias on pin 3.
4.2.4 SYNTHESIZER CHIP (U202)
Synthesizer chip U202 contains a prescaler, A, N,
and reference counters, and a phase detector, and
charge pump. Channels are selected by programming
the A, N, and reference counters to divide by a certain
number. This programming is performed by the micro-
processor via the serial data bus which consists of the
Clock, Data, and Latch Enable lines (pins 11-14 of
U202).
The divide numbers are chosen so that when the
VCO is operating at the correct frequency, the VCO-
derived input to the phase detector is the same
frequency as the TCXO-derived input. The TCXO-
derived is produced by the reference counter in U202.
This counter divides the 12.8 MHz TCXO frequency
by 2388 to produce a 6.25 kHz input to the phase
detector. Using a 6.25 kHz reference frequency allows
channels to be changed in 6.25 kHz steps.
The VCO-derived input is produced by dividing
the VCO frequency using the prescaler and N counter
in U202. The divide number of the prescaler is
controlled by the output signal from the A counter.
The A and N counters function as follows:
Both the A and N counters begin counting down
from the programmed number. When the A counter
reaches zero, it halts until the N counter reaches zero.
Both counters then reset and the cycle repeats. The
A counter is always programmed with a smaller
number than the N counter. When the A counter is
counting down, the control output to the prescaler
causes the prescaler to divide by 129. Then when the
A counter is halted, the control output changes states
and the prescaler divides by 128.
4.2.5 LOCK DETECT (Q201)
When the synthesizer is locked on frequency, the
LD (Lock Detect) output of U202 on pin 8 is a high
signal. This turns lock detect transistor Q201 off and
the Lock Detect signal fed to the microprocessor is
low. When an unlocked condition is indicated, low-
going pulses appear on pin 8 which turns Q201 on and
causes the Lock Detect signal to go high. The trans-
ceiver is then disabled by the control logic and an error
condition is indicated in the display.
4.2.6 DC-DC CONVERTER (U203), LOOP
FILTER
The supply voltage to an internal charge pump in
U202 on pin 4 is produced by DC-DC converter U203.
The DC-DC converter produces a 9.3-volt supply from
4-2
CIRCUIT DESCRIPTION
November 2001
Part No. 001-7240-001
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