SECTION 4
CIRCUIT DESCRIPTION
4 - 1
4-1 RECEIVER CIRCUITS
4-1-1 ANTENNA SWITCHING CIRCUIT
The antenna switching circuit functions as a low-pass filter
while receiving and as resonator circuit while transmitting.
The circuit does not allow transmit signals to enter receiver
circuits.
Received signals enter the MAIN unit from the antenna con-
nector and pass through the low-pass filter (L23–L25, C134,
C136–C139). The signals are then applied to the RF circuit
via the antenna switching circuit (D17, L26, L27,
C141–C143).
4-1-2 RF CIRCUIT
The RF circuit amplifies signals within the range of frequen-
cy coverage and filters out-of-band signals.
The signals from the antenna switching circuit pass through
a tunable bandpass filter (D1, L1, C2–C4) where the object
signals are led to the RF amplifier circuit (Q2).
The amplified signals at Q2 are applied to the 3-stage tun-
able bandpass filter (D2–D4, L2–L4, C13, C14, C16–C18,
C20–C24) to suppress unwanted signals and improve the
selectivity. The signals are then applied to the 1st mixer cir-
cuit.
D1–D4 employ varactor diodes, that are controlled by the
PLL lock voltage, to track the band pass filters.
4-1-3 1ST MIXER AND 1ST IF CIRCUITS
The 1st mixer circuit converts the received signal to a fixed
frequency of the 1st IF signal with a 1st LO (VCO output) fre-
quency. By changing the 1st LO frequency, only the desired
frequency will pass through a pair of crystal filters at the next
stage of the mixer.
The signals from the RF circuit are mixed with the VCO sig-
nals at the 1st mixer circuit (Q3) to produce a 21.7 MHz 1st
IF signal.
The 1st IF signal is applied to a pair of crystal filters (FI1,
FI2) to suppress out-of-band signals and is then amplified at
the IF amplifier (Q4). The amplified signal is applied to the
2nd mixer circuit (IC1).
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double superheterodyne system (which converts
receive signals twice) improves the image rejection ratio and
obtains stable receiver gain.
The FM IF IC (IC1) contains the 2nd local oscillator, 2nd
mixer, limiter amplifier, quadrature detector, and noise
detector circuits, etc.
The 1st IF signal from Q4 is applied to the 2nd mixer section
of IC1 (pin 16), and is mixed with a 21.25 MHz 2nd LO sig-
nal generated at the PLL circuit using the reference fre-
quency (21.25 MHz) to produce a 450 kHz 2nd IF signal.
The 2nd IF signal from IC1 (pin 3) is passed through the
ceramic filter (FI3), where unwanted signals are sup-
pressed, and is then applied to the 2nd IF (limiter) amplifier
in IC1 (pin 5). The signal is applied to the FM detector sec-
tion in IC1 for demodulation into AF signals.
The FM detector circuit employs a quadrature detection
method (linear phase detection), which uses a ceramic dis-
criminator (X1) for phase delay to obtain a non-adjusting cir-
cuit. The detected signal from IC1 (pin 9) is applied to the AF
circuit.
• 2nd IF and demodulator circuits
Mixer
16
Limiter
amp.
2nd IF filter
450 kHz
PLL IC
IC2
X2
21.25 MHz
X1
(21.25 MHz)
RSSI
IC1 TA31136FN
14
1st IF (21.7 MHz) from Q4
"SQLI" signal to the DC amplifier (IC20, pin 1)
11
10
9
8
7
5
3
AF signal "DEMOD"
R8V
Squelch level
adjustment pot
2
17
16
Active
filter
FI3
Noise
detector
FM
detector
