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 M H z first IF and a 455kH z second IF.
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2. IF Circuit
3. Demodulator Circuit
4. Audio Circuit
The received signal at any frequency in the 4 3 0 .0 0 - to 4 4 9 .9 9 5 -M H z range is
passed through the low-pass filter (L2, L3, L8, C2, C9, C 1 0, C 1 1, and C 62)
and high-pass filter (C 56, C 57, C 6 1 , L25 and D 25), and amplified by the RF
amplifier (Q 9). The signal from Q 9 is then passed through the tuning circuit
(L19, L20, L21 and varicaps D12, D 13 and D 14) and converted into 4 5 .1 M H z
by the mixer (Q 10). The tuning circuit, which consists of L25, L19, L20, L 21 ,
D 15, D 12, D 13 and D 14 is controlled by the tracking voltage form the C P U so
that it is optimized for the reception frequency. T h e local signal from the V C O
is passed through the buffer (Q11), and supplied to the source of the mixer
(Q 10). 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 (X F 1) selects 4 5 .1 M H z 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 dem odulator IC (IC 4). The second local signal of 4 5 .5 5 5 M H z , 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 455kH z. T he second IF signal is
output from pin 3 of IC4 to the ceram ic filter (F L 1 ), where the unwanted fre­
quency band of that signal is eliminated, and the resulting signal is sent back
to the IC4 through pin 5.
T h e second IF signal input via pin 5 is dem odulated 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 com pensated to the audio frequency
characteristics in the de-emphasis circuit (R 106, R 107, C 128, C 127) and am ­
plified by the A F amplifier (Q 27). The signal is then input to pin 2 of the elec­
tronic volume (IC 6) for volume adjustment, and output from pin 1. T h e a d ­
justed signal is sent to the audio power amplifier (IC 5) through pin 2 to drive
the speaker.
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