Frequency Synthesizer - Icom ID-880H Service Manual

4-3 FREQUENCY SYNTHESIZER

VCOs
VHF VCO
The VHF VCO is composed by the oscillator circuit (Q111,
D145, 146, 147) and buffer AMP (Q113), and generates
both of 1st LO signals and TX signal for VHF band.
The VCO oscillating signals are buffer-amplified by Q113,
amplifi ed by the LO AMP (IC45) to obtain adequate LO level,
then applied to the TX amplifi ers or 1st IF mixer via the LO
SW, ATT and fi lter.
While transmitting;
The VCO output signals from the LO AMP (IC45) are applied
to the TX amplifi ers via the LO SW (D155) as the TX signal.
While receiving;
The VCO output signals from the LO AMP (IC45) are passed
through the LO SW (D170), ATT and the LPF, then applied to
the 1st IF mixer (Q124), as the 1st LO signals for 140 MHz
band.
The VCO output signals from the LO AMP (IC45) are passed
through the LO SW (D104), ATT, RF SW and LPF, then
applied to the 1st IF mixer (Q125), as the 1st LO signals for
300 MHz band.
UHF VCO
The UHF VCO is composed by the oscillator circuit (Q73,
D87, 91, 92) and buffer AMP (Q76), and generates both of
1st LO signals and TX signal for UHF band.
The VCO oscillating signals are buffer-amplified by Q76,
amplifi ed by the LO AMP (IC45) to obtain adequate LO level,
then applied to the TX amplifi ers or 1st IF mixer via the LO
SW, ATT and fi lter.
While transmitting;
The VCO output signals from the LO AMP (IC45) are applied
to the TX amplifi ers via the LO SW (D103) as the TX signal.
While receiving;
The VCO output signals from the LO AMP (IC45) are passed
through the LO SW (D101), ATT, doubler (HPF) and LPF,
then applied to the 1st IF mixer (IC64), as the 1st LO signals
for 800 MHz band.
The VCO output signals from the LO AMP (IC45) are passed
through the LO SW (D179), ATT and the LPF, then applied to
the 1st IF mixer (IC64), as the 1st LO signals for 440 MHz band.
• FREQUENCY SYNTHESIZER CIRCUITS
LOOP
FIL
IC14
IC1018
PLLCK
PLLDATA PLL VCO
PLLSTB IC SELECT
X5
15.3MHz
TCXO
Q52
45.9 MHz
X3
IF IC
IC15
D160,D175
VHF VCO
Q113
VCO
BUFF
SW
Q110,111
D145-148
Q61,62
LOOP
FIL
Q76
UHF VCO
BUFF
Q73
D87,91,92
Q 1015
BUFF
PLL
The PLL circuit provides stable oscillation of the transmit
frequency and receive 1st LO frequency. The PLL output
frequency is controlled by control signals including divide
ratio ("PLLCK,""PLLDATA"and "PLLSTB") from the CPU
(IC25).
PLL (VHF VCO)
A portion of VHF VCO output signals from the buffer
(Q113) are applied to the PLL IC (IC14) via the VCO switch
(D175). The applied signals are divided at the prescaler and
programmable counter.
The divided signal is phase-compared with the 15.3 MHz
reference frequency signal from the reference frequency
oscillator (X5), at the phase detector.
The phase difference is output from pin 5 as a pulse type
signal after being passed through the internal charge pump.
The output signal is converted into the DC voltage (lock
voltage) by passing through the loop fi lter (R694-698, C759-
763). The lock voltage is applied to the variable capacitors
(D145, 146), and locked to keep the VCO frequency
constant.
PLL (UHF VCO)
A portion of VHF VCO output signals from the buffer (Q76)
are applied to the PLL IC (IC14) via the VCO switch (Q1015).
The applied signals are divided at the prescaler and
programmable counter.
The divided signal is phase-compared with the 15.3 MHz
reference frequency signal from the reference frequency
oscillator (X5), at the phase detector.
The phase difference is output from pins 15 and 16, and
passed through the external charge pump (Q61, 62) to be
converted into the pulse type signal.
The output signal is converted into the DC voltage (lock
voltage) by passing through the loop fi lter (R355, 362, 365,
369, 378, C451, 456, 473). The lock voltage is applied to the
variable capacitors (D91, 92), and locked to keep the VCO
frequency constant.
D155
LO SW
To VHF TX Aamplifier
D103
LO SW To UHF TX Aamplifier
D 170
LO SW ATT
IC45
LO -> RX LO signals for 140 MHz band (118-174 MHz) ->
AMP
D 179
LO SW ATT
-> RX LO signals for 440 MHz band (420-550 MHz) ->
D102
D 101
VCO
SW
LO SW ATT
-> RX LO signals for 800MHz band (810-999 MHz) ->
D104
LO SW ATT
-> RX LO signals for 300 MHz band (230-420 MHz) ->
4 - 6
Q124
LPF
140 MHz band
1st IF MIXER
Q125
LPF
440 MHz band
1st IF MIXER
D181
IC64
X2
LPF LO SW
(HPF)
800 MHz and
300 MHz bands
1st IF MIXER
Q79 D180
RF
LPF
LO SW
SW