SECTION 4CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUITS
4-1 -1 DUPLEXER CIRCUIT (1 FUNIT)
The transceiver has aduplexer (low-pass and high-pass
filters) on the first stage from the antenna connector to
separate the signals into VHF and UHF signals. The
low-pass filter (L10-L12, C16-C22) for VHF signals and
high-pass fiiter (L1-L3, C1-C5) for UHF signals. The
separated signals are applied to each RF circuit.
4-1-2 VHF ANTENNA SWITCHING CIRCUIT
{IF UNIT)
The antenna switching circuit functions as alow-pass filter
while receiving. However, its impedance becomes very
high while transmitting by applying acurrent to D51 and
D52. Thus, transmit signals are blocked from entering the
receiver circuits. The antenna switching circuit employs a
1/4 Atype 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 fre-
quency coverage and filters out-of-band signals.
The signals from the antenna switching circuit are applied to
the bandpass filter (D52, L53), and are then applied to the
RF amplifiers (Q51, Q52). The amplifier consists of a
cascade circuit. The amplified signals are passed through
the next stage bandpass 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 bandpass filters and are controlled
by the PLL lock voltage. These diodes tune the center
frequency to obtain good image response rejection.
4-1-4 UHF RF CIRCUIT (2F UNIT)
The signals from the antenna switching circuit (IF unit
D551, D552) are ampiified at the the RF amplifier (Q201)
and are then passed though the bandpass filter (FI201).
The filtered signals are applied to another RF amplifier
(Q202) and are then 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 afixed
frequency of the 1st IF signal with a1st LO (VCO output)
frequency. By changing th PLL frequency, only the desired
frequency will be passed though acrystal filter at the next
stage of the mixer.
The receive signals from the VHF or UHF RF circuit are
mixed with the 1st LO signal (VCO output signal) at the 1st
mixer (0401) to produce a45.15 MHz 1st IF signal.
The 1st IF signal is applied to acrystal filter (FI401) to
suppress out-of-band signals. The filtered 1st IF signal is
amplified at the IF amplifier (01 01) and is then applied to
the 2nd mixer circuit (IC101).
4-1-6 2ND IF AND DEMODULATOR CIRCUITS
(2F UNIT)
The 2nd mixer circuit converts the 1st IF signal to a2nd IF
signal. Adouble superheterodyne system (which converts
receive signals twice) improves the image rejection ratio
and obtains stable receiver gain.
The FM IF 1C (IC101) contains the 2nd mixer, 2nd local
oscillator, limiter amplifier, S-meter detector and quadrature
detector circuits.
The 1st IF signal (45.15 MHz) from the IF amplifier (01 01)
is applied to the 2nd mixer section of IC101 (pin 16), and is
mixed with the 2nd LO signal (45.6 MHz) for conversion to a
450 kHz 2nd IF signal at the 2nd mixer section.
The 2nd IF signal (450 kHz) from the 2nd mixer section
(IC101 pin 3) passes though the ceramic filter (F1 101)
where unwanted signals are suppressed. It is then
amplified at the limiter amplifier section (IC101 pin 5) and
applied to the quadrature detector section to demodulate
the 2nd IF signal into AF signals.
AF signals output from IC101 (pin 9) are applied to the AF
drive amplifier (012) on the LOGIC unit. The S-meter
output “SD” signal from IC101 (pin 12) is applied to the CPU
(LOGIC unit pin 3).
2nd IF AND DEMODULATOR CIRCUIT
2nd IF filter 450 kHz
4-1
