Abell TH308G, Руководство по обслуживанию

Страница: 9 / 58

ABELL TH-308G VHF Service Manual
- 8 -
CIRCUIT DESCRIPTION
1. Frequency configuration
The receiver utilizes double conversion. The first IF is 21.70MHz and the second IF is
450 KHz. The first local oscillator signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the necessary frequencies.
2. Receiver
The receiver is double conversion super heterodyne, designed to operate in the
frequency range of 136 to 150MHz, 150 to 174MHz and 136-174MHz.
1
）
Front-end RF amplifier
An incoming signal from the antenna is applied to an RF amplifier (Q500) after
passing through a transmit/receive switch circuit (D500 and D501 are off) and a band
pass LC filter. After the signal is amplified, the signal is filtered through a band pass
filter to eliminate unwanted signals before it is passed to the first mixer.
The voltages of these diodes are controlled by to track the CPU (IC302) center
frequency of the band pass filter.
2
）
First Mixer
The signal from the RF amplifier is heterodyned with the first local oscillator signal
from the PLL frequency synthesizer circuit at the first mixer (Q501) to create a
21.70MHz first intermediate frequency (1
st
IF) signal. The first If signal is then fed
through two monolithic crystal filters (X500 and X501) to further remove spurious
signals.
3
）
IF AMP
The first IF signal is amplified by Q502, and then enters IC500 (FM processing IC).
The signal is heterodyned again with a second local oscillator signal within IC500 to
create a 450 KHz second IF signal. The second IF signal is then fed through a 450 KHz
ceramic filter (CF1) to further eliminate unwanted signals before it is amplified and FM
detected in IC200.
4
）
AF amplifier
The AF signal from the IF IC500 is amplified by IC101C and passes through the
high-pass filter IC101B and de-emphasis filter Q102 and though R135, C112 remove
embitter. The signal passes through AF volume control circuit and enters the IC102
audio power amplifier to drive the speaker.
5
）
Squelch
Part of the AF signal from IC500 enters IC500 again, and the noise component is
amplified and rectified by a filter and an amplifier to produce a DC voltage
corresponding to the noise level. The DC signal goes to the analog port of the
microprocessor(IC302).IC302 determines whether to output sounds from the speaker