Frequency Synthesizer Circuits; Vcos; Uhf Vco; Pll - Icom IC-2730A Service Manual

4-3 FREQUENCY SYNTHESIZER CIRCUITS

(MAIN UNIT)

VCOs

The IC-2730A/E has total of four VCOs: two VCOs for the left
band and another two for the right band.
• Left band
VHF VCO
The VHF VCO (Q211, D203, D204, D206, D228 and D342)
generates the VHF TX signal. The output of buffer (Q215) is
amplifi ed by the LO AMP (Q240), and then applied to the
VHF TX AMP circuit, through the LO SW (D50).

UHF VCO

The UHF VCO (Q210, D201, D202 and D205) generates the
1st LO signal (for receiving a UHF signal on the left band).
The output of buffer (Q214) is amplifi ed by the LO AMP
(Q240), and then applied to the 1st IF mixer (Q146), through
the LO SW (D213) and LPF (L315, C988 and C995).
• Right band
VHF VCO
The VHF VCO (Q233, D225, D226 and D230) generates the
1st LO signal (for receiving a VHF signal on the right band).
The output of buffer (Q236) is amplifi ed by the LO AMP
(Q238), and then applied to the 1st IF mixer (Q143), through
the LO SW (D235) and LPF (L338, L339, C1105 and
C1106).
UHF VCO
The UHF VCO (Q232, D223, D224, D227, D229 and D236)
generates the UHF TX signal. The output of buffer (Q235) is
applied to the UHF TX AMP circuit.
• Left band frequency synthesizer circuit
Q210
D201
Left band VCOs Q214
D202
D205
BUFF
RX VCO (UHF)
Q211
D342
D203
D204
Q215
D206
D228
LOOP
BUFF
FIL
X200
TX VCO (VHF)
IC201 Q213
12.8MHz
Q200

PLL

BUFF
IC
X3
38.4MHz
IC121
IF IC
137-174MHz
Q240
156.85-212.85MHz
D50/D212/D213
TX/RX
AMP LPF
SW
Q145
LPF
336.15-511.15MHz
Q146
PLL
The PLL circuit controls the VCO oscillating frequency by
comparing the reference signal and the fed back VCO sig-
nal. When the oscillation frequency drifts, its phase changes
from that of the reference frequency, causing a lock voltage
change to compensate for the drift in the VCO oscillating fre-
quency.
• Left band
A portion of VHF and UHF VCOs output signal is ampli-
fi ed by the buffer (Q213), and then fed back to the PLL IC
(IC201, pin 17).
The PLL IC (IC201) phase-compares the output of reference
frequency oscillator (TCXO: X200) and VCO, and the phase
difference is output as the charge pump current. The current
is passed though the loop fi lter (R715, R717, R719, R722,
C924, C926 to C928 and C930) to be converted into the lock
voltage, which controls the oscillating frequency of VCO.
• Right band
A portion of VHF and UHF VCOs output signal is ampli-
fi ed by the buffer (Q234), and then fed back to the PLL IC
(IC206, pin 17).
The PLL IC (IC206) phase-compares the output of reference
frequency oscillator (TCXO: X201) and VCO, and the phase
difference is output as the charge pump current. The current
is passed though the loop fi lter (R797, R799, R802, R805,
C1041, C1043 to C1045 and C1048) to be converted into the
lock voltage, which controls the oscillating frequency of VCO.
• Right band frequency synthesizer circuit
Right band VCOs
LOOP
FIL
RX VCO (VHF)
X201
IC206
15.3MHz PLL
Q220 IC
X3
45.9MHz
IC120
IF IC
4-7
Q232
D223
D224
D227
Q235
D229
D236
BUFF
Q238
D234/D235
TX VCO (UHF)
AMP SW
Q233
D225
Q236
D226
D230
BUFF
LPF LPF
328.65-
Q234
503.65 MHz
BUFF
400-470MHz
164.35-
220.35 MHz
Q143
Q144