Alinco DR-435 - DR-435 Receiver System

9
5. VCO Circuit
A Colpitts oscillation circuit driven by Q503 directly oscillates the desired frequency.
The frequency control voltage determined in the CPU (IC1) and PLL circuit is
input to the varicaps (D503 and D504). This change the oscillation frequency,
which is amplified by the VCO buffer (Q504) and output from the VCO area.
7) Receiver System (DR-435)
The receiver system is a double superheterodyne system with a 30.85 MHz first IF and a 455 kHz second IF.
1. Front End
The received signal at any frequency in the 430.00MHz to 439.995MHz range
is passed through the low-pass filter (L115, L114, L116, C204, C203, C202,
C216 and C215) and amplified by the RF amplifier (Q107). The signal from
Q107 is then passed through the BPF circuit (L103, L102) and converted into
30.85 MHz by the mixer (Q106). The local signal from the VCO is passed
through the buffer (Q503,Q504), and supplied to the source of the mixer (Q106).
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 (XF101) selects 30.85MHz frequency
from the results and eliminates the signals of the unwanted frequencies. The first IF
amplifier (Q105) then amplifies the signal of the selected frequency.
3. Demodulator Circuit
After the signal is amplified by the first IF amplifier (Q105), it is input to pin 20 of
the demodulator IC (IC108). The second local signal of 30.85MHz (Crystal
oscillator) is input pin 1 of IC108. Then, these two signals are mixed by the
internal mixer in IC108 and the result is converted into the second IF signal
with a frequency of 455 kHz. The second IF signal is output from pin 4 of IC108
to the ceramic filter (FL101 or FL102), where the unwanted frequency band of
that signal is eliminated, and the resulting signal is sent back to the IC108
through pins 6.
The second IF signal input via pin 6 is demodulated by the internal limiter
amplifier and quadrature detection circuit in IC108, and output as an audio
signal through pin 11.
4. Audio Circuit
The audio signal from pin 11 of IC108 is amplified by the audio amplifier
(IC104:A), and switched by the signal switch IC (IC111) and then input it to the
de-emphasis circuit.
and is compensated to the audio frequency characteristics in the de-emphasis
circuit (R203, R207, R213, R209, C191, C218, C217) and amplified by the AF
amplifier (IC104:D). The signal is then input to volume (VR1) . The adjusted
signal is sent to the audio power amplifier (IC117) through pin 1 to drive the
speaker.