SECTION 4
CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUIT
S
4-1-1 ANTENNA SWITCH (MAIN UNIT)
The received signals from the antenna connector are passed
through the antenna switch which toggles the receive (RX)
line and transmit (TX) line.
The received signals from the antenna connector are passed
through the low-pass filter (LPF; L21, L22, C126−C129) and
the antenna switch (D14 and D21 are OFF).
While transmitting, voltage on the T5V line is applied to
D14 and D21, and these are turned ON. Thus the TX line is
connected to the antenna, and RX line is connected to the
GND to prevent transmit signal entering.
While receiving, no voltage is applied to D14 and D21, and
these are turned OFF. Thus the TX line and the antenna is
disconnected to prevent received signals entering, and the RX
line is disconnected from the GND, and the received signals
are passed through the LPF (L31, C142, C143). The filtered
signals are then applied to the RF circuits.
4-1-2 RF CIRCUITS (MAIN UNIT)
The RF circuit amplifies received signals within the frequency
coverage. The received signals are filtered at the bandpass filter
(BPF) and amplified at the RF amplifier.
The received signals from the antenna switch (D14 and D21
are OFF) are divided into the signals for voice communications
and the signals for DSC channnel at the divider (L35, C151,
C152). The divided signals (voice communications) are
applied to the tuned RF amplifier (Q21, D26−D29, L36, L37,
C161, C162). The amplified signals are then applied to the
double balanced 1st mixer (D32, L40, L41) via a two-staged
tunable BPF (D30, D31, L38, L39, C173−C178, C196).
4-1-3 1st IF CIRCUITS (MAIN UNIT)
The received signals are converted into the 1st IF signal, and
filtered and amplified in the 1st IF circuits.
The amplified received signals from the RF circuits are applied to
the 1st mixer (D32, L40, L41), and converted into the 21.7 MHz
1st IF signal by being mixed with the 1st local oscillator (LO)
signals from the VCO (Q72, D51, D52).
The converted 1st IF signal is amplified by the 1st IF amplifier
(Q22, Q23), and passed through two 1st IF filters (FI1, FI2) to
filter out adjacent signals. The filtered 1st IF signal is amplified
by the 1st IF amplifier (Q24), then applied to the FM IF IC (IC2,
pin 16).
4-1-4 2nd IF AND DEMODULATOR CIRCUITS (MAIN UNIT)
The 1st IF signal is converted into the 2nd IF signal and
demodulated in the FM IF IC.
The 1st IF signal from the 1st IF circuits is applied to the 450 kHz
2nd IF mixer in the FM IF IC (IC2, pin 16), and converted into
the 2nd IF signal by being mixed with the 21.25 MHz 2nd LO
signal from the reference oscillator (IC2, pin 1, X3).
The converted 2nd IF signal is output from pin 3, and
passed through the 2nd IF filter (FI3) to suppress sideband
noise. The filtered 2nd IF signal is applied to the limiter
amplifier (IC2, pin 5). The amplified 2nd IF signal is then
FM-demodulated at the quadrature detector (IC2, pins 10,
11, X2) and output from pin 9. The demodulated AF signals
are applied to the AF circuits.
4-1-5 AF AMPLIFIER CIRCUITS (AF UNIT)
The demodulated AF signals from the FM IF IC are amplifi ed
and fi ltered in the AF circuits.
The AF signals from the FM IF IC (MAIN UNIT; IC2, pin 9) are
passed through the RX selector (IC26, pins 1, 7), AF mute switch
(IC3, pins 10, 11), BPF (Q31, Q32) and another AF mute switch
(IC4, pins 8, 9) in sequence.
The AF signals from the AF mute switch (IC4, pins 8, 9) are
then applied to the volume control pot (VR UNIT; R1) to be ad-
justed its signal level (=audio output level). The level-adjusted
AF signals are applied to the AF power amplifi er (IC9, pin1) via
the AF mute switch (Q65). The AF signals are obtained 4.5 W
(typ.) of AF output power at the AF power amplifi er (IC9), and
output from pin 4, then applied to the internal speaker (FRONT
UNIT; SP1), HM-126B/G or connected external speaker.
4 - 1
Mixer
Quadrature
detector
R214
1st IF signal from the IF amplifier (Q24)
16
Noise
detector
R10V
X2
11
10
FM IF IC (IC2)
Filter
amp.
Limiter
amp.
Demodulated signals
to the AF circuits
• 2nd IF AND DEMODULATOR CIRCUITS
9
“SQL” signal to the CPU (IC1: pin 91)
14
X3
21.25 MHz
2
8
7
3
5
FI3