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SECTION 4

CIRCUIT DESCRIPTION

4 - 1

4-1

RECEIVER CIRCUITS

4-1-1

DUPLEXER CIRCUIT (1F UNIT)

The transceiver has a duplexer (low-pass and high-pass fil-
ters) on the first stage from the antenna connector to sepa-
rate the signals into VHF and UHF signals. The low-pass fil-
ter (L10–L12, C16–C22) for VHF signals and high-pass filter
(L1–L3, C1–C5) for UHF signals. The separated signals are
applied to each RF circuit.

4-1-2

VHF ANTENNA SWITCHING CIRCUIT
(1F UNIT)

The antenna switching circuit functions as a low-pass filter
while receiving. However, Its impedance becomes very high
while transmitting by applying a current to D51 and D52.
Thus, transmit signals are blocked from entering the receiv-
er circuits. The antenna switching circuit employs a 1/4

type diode switching system. The passed signals are then
applied to the RF amplifier circuit on the 2F unit.

4-1-3

VHF RF CIRCUIT (2F UNIT)

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 are applied to
the limitter (D55), and are then passed through the band-
pass filter (D52, L53). The filtered signal is applied to the RF
amplifiers (Q51, Q52). The amplifier consists of a cascade
circuit. The amplified signals are passed through the next
stage band-pass filter (D53, D54, L54, L55) to suppress
unwanted signals. The filtered signals are then applied to
the mixer circuit (Q401).

D53 and D54 track the band-pass filters and are controlled
by the PLL lock voltage. These diodes tune the center fre-
quency to obtain good image response rejection.

4-1-4

UHF RF CIRCUIT (2F UNIT)

The signals from the antenna switching circuit (1F unit;
D551, D552, D722–D724 and Q204) are applied to the limit-
ter (D201), and are then amplified at the the RF amplifier
(Q201). The amplified signals are passed through the band-
pass filter (FI201), and are then applied to another RF ampli-
fier (Q202). The amplified signals are applied to the 1st
mixer circuit (Q401).

Common circuits with VHF band are used later stage from
the 1st mixer.

4-1-5

1ST MIXER AND 1ST IF CIRCUITS
(2F UNIT)

The mixer circuit converts the received signal to a fixed fre-
quency of the 1st IF signal with a 1st LO (VCO output) fre-
quency. By changing the PLL frequency, only the desired
frequency will be passed through a crystal filter at the next
stage of the mixer.

The received signals from the VHF or UHF RF circuit are
mixed with the 1st LO signal (VCO output signal) at the 1st
mixer (Q401) to produce a 45.15 MHz 1st IF signal.

The 1st IF signal is applied to a crystal filter (FI401) to sup-
press out-of-band signals. The filtered 1st IF signal is ampli-
fied at the IF amplifier (Q101) and is then applied to the 2nd
mixer circuit (IC101, pin 16).

4-1-6

2ND IF AND DEMODULATOR CIRCUITS
(2F UNIT)

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 (IC101) contains the 2nd mixer, 2nd local
oscillator, limiter amplifier, S-meter detector and quadrature
detector circuits.

• 2nd IF AND DEMODULATOR CIRCUITS

Mixer

Limiter
amp.

2nd IF filter
450 kHz

PLL IC

IC851

15.2 MHz

RSSI

IC2 TA31136FN

2nd IF (45.15 MHz)
from Q101 (1F unit)

"SD" signal to the CPU
pin 3 (Logic unit: IC1)