Icom IC-7600 Service Manual page 15

3-3 FREQUENCY SYNTHESIZER (PLL UNIT)
REFERENCE FREQUENCY OSCILLATOR CIRCUIT
The crystal oscillator (X51) oscillates the 32 MHz reference
frequency signal. This reference signal is applied to the
Local Oscillator (LO) circuits via the buffer (Q51) and LPF
(L53, C55–57).
2ND TX LO CIRCUIT
The 32 MHz reference signal from the reference frequency
oscillator circuit is doubled by the doubler (Q61) and passed
through the double-tuned fi lter (L61, 62) to extract the 64 MHz
of 2nd harmonic component. The extracted 64 MHz signal is
applied to the RF UNIT via the buffer (Q71) and the BPF as
the 2nd TX LO signal.
3RD TX LO CIRCUIT
Using the 32 MHz reference clock signal from the reference
frequency oscillator circuit, the 491 kHz 3rd TX LO signal is
directly generated by 10-bit DDS (IC701) and D/A converter
(R701–720). The generated 491 kHz signal is passed
through the BPF and amplifi ed by Q702, then applied to the
RF UNIT.
The generated 491 kHz signal from Q702 is also used to
generate the 2nd RX LO signal, by being applied to the 2nd
RX LO circuit.
2ND RX LO CIRCUIT
The 64 MHz reference signal from the double-tuned filter
(L61, 62) and the 491 kHz signal from the 3rd TX LO circuit
is mixed by the 2nd RX LO mixer (D821, L821, 822), to
generate the 2nd RX LO signal.
The generated 2nd RX LO signal is filtered by the crystal
fi lter (FI831) and amplifi ed by Q831, then applied to the RF
UNIT as the 2nd RX LO signal.
1ST RX (Main readout)/TX LO CIRCUITS
The 32 MHz reference signal from the reference frequency
oscillator circuit is amplified by Q152, and applied to the
DDS (IC101) as the reference clock signal. The DDS (IC101)
generates the PLL reference frequency signal, using applied
reference clock signal.
The generated PLL reference frequency signal is fi ltered by
the ceramic BPF (FI101) and LPF, amplifi ed by Q151, then
applied to the PLL IC (IC381) vian LPF as the main loop
reference frequency signal.
The reference frequency signal is used for phase comparison
of oscillating signals from the VCO-A (Q201, 221, 251, 271).
The resulting signal of phase comparison is converted into
the lock voltage which controls the oscillation frequency of
VCO-A (Q201, 221, 251, 271), by being passed through the
passive loop fi lter.
The output signals of the VCO-A (Q201, 221, 251, 271) are
buffer-amplifi ed by Q301, fi ltered by the BPF, and amplifi ed
by IC320, then fi ltered by the LPF for harmonic removal.
The fi ltered VCO output signals are passed through the BPF
and LO mute switch (Q361, D361, 362), then applied to the
RF UNIT as the 1st RX LO (for Main readout)/TX LO signals.
A portion of VCO output signals from the LPF are fed back
to the PLL (IC381) for the phase-locked loop.
VCOs (VCO-A)
Q201, Q221, Q251 and Q271 compose four oscillators,
and each VCO generates the 1st RX/TX LO signals. These
VCO employ high "Q" components as resonator (L202, 222,
252, 272) for high C/N characteristic and wide oscillation
frequency range.
The table below shows the oscillation frequency range of
VCO-A and the RX frequency range.
Composed
RX Frequency range
by
Q271 0.03–7.999999 MHz
Q251 8–19.999999 MHz
Q221 20–44.999999 MHz
Q201 45–60 MHz
3 - 6
Oscillation frequency range
(1st LO)
64.485–72.454999 MHz
72.455–84.454999 MHz
84.455–109.454999 MHz
109.455–124.455 MHz