Circuit Description; Receiver System - Alinco DJ-493 Service Manual

CIRCUIT DESCRIPTION

1) Receiver System

The receiver system is a double superheterodyne system with a 45.1 MHz first IF and a 455kHz second IF.
1. Front End
2. IF Circuit
3. Demodulator Circuit
4. Audio Circuit
The received signal at any frequency in the 430.00- to 449.995-MHz range is
passed through the low-pass filter (L2, L3, L8, C2, C9, C10, C11, and C62)
and high-pass filter (C56, C57, C61, L25 and D25), and amplified by the RF
amplifier (Q9). The signal from Q9 is then passed through the tuning circuit
(L19, L20, L21 and varicaps D12, D13 and D14) and converted into 45.1MHz
by the mixer (Q10). The tuning circuit, which consists of L25, L19, L20, L21,
D15, D12, D13 and D14 is controlled by the tracking voltage form the CPU so
that it is optimized for the reception frequency. The local signal from the VCO
is passed through the buffer (Q11), and supplied to the source of the mixer
(Q10). The radio uses the lower side of the superheterodyne system.
The mixer mixes the received signal with the local signal to obtain the sum of
and difference between them. The crystal filter (XF1) selects 45.1MHz fre­
quency from the results and eliminates the signals of the unwanted frequen­
cies. The first IF amplifier (Q8) then amplifies the signal of the selected fre­
quency.
After the signal is amplified by the first IF amplifier (Q8), it is input to pin 16 of
the demodulator IC (IC4). The second local signal of 45.555MHz, which is
oscillated by the internal oscillation circuit in IC4 and crystal (X2). Then, these
two signals are mixed by the internal mixer in IC4 and the result is converted
into the second IF signal with a frequency of 455kHz. The second IF signal is
output from pin 3 of IC4 to the ceramic filter (FL1), where the unwanted fre­
quency band of that signal is eliminated, and the resulting signal is sent back
to the IC4 through pin 5.
The second IF signal input via pin 5 is demodulated by the internal limiter
amplifier and quadrature detection circuit in IC4, and output as an audio signal
through pin 10.
The audio signal from pin 10 of IC4 is compensated to the audio frequency
characteristics in the de-emphasis circuit (R106, R107, C128, C127) and am­
plified by the AF amplifier (Q27). The signal is then input to pin 2 of the elec­
tronic volume (IC6) for volume adjustment, and output from pin 1. The ad­
justed signal is sent to the audio power amplifier (IC5) through pin 2 to drive
the speaker.
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