Wide/Narrow Switching Circuit
The Wide port (pin 65) and Narrow port (pin 64) of the CPU
is used to switch between ceramic filters. When the Wide
port is high, the ceramic filter SW diodes (D303, D302) cause
CF301 to turn on to receive a Wide signal.
When the Narrow port is high, the ceramic filter SW di-
odes (D303, D302) cause CF302 to turn on to receive a Nar-
row signal. (See Fig. 3.)
AF Signal System
The detection signal from IF IC (IC301) goes to D/A con-
verter (IC161) to adjust the gain and is output to AF filter
(IC251) for characterizing the signal. The AF signal output
from IC251 and the DTMF signal, BEEP signal are summed
and the resulting signal goes to the D/A converter (IC161).
The AFO output level is adjusted by the D/A converter. The
signal output from the D/A converter is input to the audio
power amplifier (IC252). The AF signal from IC252 switches
between the internal speaker and speaker jack (J1) output.
(See Fig. 4.)
The detection output from the FM IF IC (IC301) passes
through a noise amplifier (Q301) to detect noise. A voltage is
applied to the CPU (IC101). The CPU controls squelch ac-
cording to the voltage (SQIN) level. The signal from the RSSI
pin of IC301 is used for S-meter. The electric field strength of
the receive signal can be known before the SQIN voltage is
input to the CPU, and the scan stop speed is improved.
Wide/Narrow switching circuit
AF signal system
PLL Frequency Synthesizer
The PLL circuit generates the first local oscillator signal for
reception and the RF signal for transmission.
The frequency step of the PLL circuit is 5 or 6.25kHz. A
16.8MHz reference oscillator signal is divided at IC401 by a
fixed counter to produce the 5 or 6.25kHz reference fre-
quency. The voltage controlled oscillator (VCO) output signal
is buffer amplified by Q410, then divided in IC401 by a dual-
module programmable counter. The divided signal is com-
pared in phase with the 5 or 6.25kHz reference signal in the
phase comparator in IC401. The output signal from the
phase comparator is filtered through a low-pass filter and
passed to the VCO to control the oscillator frequency. (See
The operating frequency is generated by Q406 in transmit
mode and Q405 in receive mode. The oscillator frequency is
controlled by applying the VCO control voltage, obtained
from the phase comparator, to the varactor diodes (D405 and
D406 in transmit mode and D403 and D404 in receive mode).
The TX/RX pin is set high in receive mode causing Q408 and
Q407 to turn Q406 off, and turn Q405 on. The TX/RX pin is
set low in transmit mode. The outputs from Q405 and Q406
are amplified by Q410 and sent to the RF amplifiers. (See Fig.
IC401 : PLL IC