Circuit Description; Receiver System - Alinco DJ-175 Service Manual

CIRCUIT DESCRIPTION

1) Receiver System

The receiver system is a double superheterodyne system with a 21.7MHz first IF and
a 450kHz second IF.
1.Front End
The received signal at frequency in the 136~173.995MHz (E version:
144~145.995MHz) range is passed through the low-pass filter (L101, L102,
L104, L118, C101, 108, C109, C110, C117, C118, C119, C123, C124, C125,
C126, C127, and C188) and tuning circuit (C255, C256, C257, C258, D120,
L138, and L139) and amplified by the RF amplifier (Q119). The signal from
Q119 is then passed through the tuning circuit (C242, C243, C244, C245,
C269, C270, C271, D117, D118, D119, L135, L136, and L137) and converted
into 21.7MHz by the mixer (Q118).
The tuning circuit, which consists of L139, L135, L136, varicaps D117, D118,
D119, and D120 is controlled by the tracking voltage from the CPU so that it is
optimized for the reception frequency. The local signal from the VCO is
passed through the buffer (Q115), and supplied to the source of the mixer
(Q118). The radio uses the lower side of the superheterodyne system.
2. IF Circuit
The mixer mixes the received signal with the local signal to obtain the sum of
and difference between them. The crystal filter (FL101, and FL102) selects
21.7MHz frequency from the results and eliminates the signal of the unwanted
frequencies. The first IF amplifier (Q121) then amplifies the signal of the
selected frequency.
3. Demodulator Circuit
After the signal is amplified by the first IF amplifier (Q121), it is input to pin 16
of the demodulator IC (IC103). The second local signal of 21.25MHz (shared
with PLL IC reference oscillation), which is oscillated by the TCXO (X102).
Then, these two signals are mixed by the internal mixer in IC103 and the result
is converted into the second IF signal with a frequency of 450kHz. The second
IF signal is output from pin 3 of IC103 to the ceramic filter (FL103), where the
unwanted frequency band of that signal is eliminated, and the resulting signal
is sent back to the IC103 through pin 5.
The second IF signal input via pin 5 is demodulated by the internal limiter
amplifier and quadrature detection circuit in IC103, and output as an audio
signal through pin 9.