Receiver System Dr-435 - Alinco DR-135 Service Manual

3. Phase Comparator Circuit
4. PLL Loop Filter Circuit
5. VCO Circuit
6. VCO Shift Circuit

4) Receiver System DR-435

The receiver system is a double super-heterodyne system with a 30.85MHz first IF and a 455kHz second IF.
1. Front End
2. IF Circuit
6
The PLL (IC116) uses the reference frequency, 5 or 6.25 kHz. The phase
comparator in the IC116 compares the phase of the frequency from the
VCO with that of the comparison frequency, 5 or 6.25 kHz, which is obtained
by the internal divider in IC116.
If a phase difference is found in the phase comparison between the reference
frequency and the VCO output frequency, the charge pump output (pin 13) of
IC116 generates a pulse signal, which is converted DC voltage by the PLL
loop filter and input to the input to the variable capacitor of the VCO unit for
oscillation frequency control.
A Colpitts oscillation circuit driven by Q131 directly oscillates the desired frequency.
The frequency control voltage determine in the CPU (IC1) and PLL circuit is input
to the variable capacitor (D122 and D123). This change the oscillation frequency,
which is amplified by the VCO buffer (Q134) and output from the VCO area.
During transmission or the AIR band Reception (118 ~ 136 MHz), the VCO
shift circuit turns ON Q138, change control the capacitance of L123 and safely
oscillates the VCO by means of H signal from pin 16 of IC116.
The received signal at any frequency in the 430.000MHz to 449.995MHz 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
30.85MHz 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 super-heterodyne system.
The mixer mixes the received signal with the local signal to obtain the sum of
and difference between them. The crystal filter (XF101) selects 30.85 MHz
frequency from the results and eliminates the signal of the unwanted
frequencies. The first IF amplifier (Q105) then amplifies the signal of the selected
frequency.