Icom IC-706MKIIG Service Manual page 12

3-3-3 2ND LO AND REFERENCE OSCILLATOR
CIRCUITS
The reference oscillator (X621, Q621) generates a 30.0
MHz frequency used for the 1st LO and BFO circuits as a
system clock and for the 2nd LO signal.
The oscillated signal is amplified at the buffer amplifier
(Q661), and is doubled at Q681 and the 60 MHz frequency
is picked up at the bandpass filter (L681, L682). The 60 MHz
signal is applied to the MAIN board as a 2nd LO signal.
• FREQUENCY CONSTRUCTION
[ANT1] [ANT2]
0.03– 60–
1st mixer
60 MHz 470 MHz
MAIN BOARD
PLL UNIT
69.0415–
129.0115 MHz
Q301
Q331
Q361
1
⁄
N
Phase
detector
Q1
Phase
detector
69.0115 MHz
D271
(WFM: 70.7 MHz)
1st IF
BPF
69.0415–
539.0115 MHz
(1st LO)
129.0115–
539.0115 MHz
2
Main loop PLL
IC461
divider
1
⁄
86
10.665–
10.683 MHz
Ref. loop PLL
IC101
DDS
3-3-4 BFO CIRCUIT
The DDS IC (IC901) generates a 10-bit digital signal. The
signal is converted into an analog wave signal at the D/A
converter (R951–R970). The analog wave is passed
through the high-pass filter and low-pass filter. The 9 MHz
BFO signal is then applied to the MAIN board via the "BFO"
signal line.
While transmitting in RTTY mode, the RTTY keying signal is
applied to IC901 pin 3 to shift the generated frequency and
to obtain 2 frequencies for FSK operation.
While receiving in FM or FM narrow mode, the BFO circuit
generates a 9.4665 MHz frequency as the 3rd LO signal.
While transmitting in FM or FM narrow mode, the BFO cir-
cuit generates a 9.0115 MHz or 9.0100 MHz frequency as
the TX FM PLL reference frequency, respectively.
to WFM detector
(IC631)
2nd mixer
D551
2nd IF
60.0 MHz
(2nd LO)
BPF
2 Q681
Ref. Osc.
30.0 MHz
X621 Q621
3 - 7
Product
detector
IC841
AF signals
to AF selector switch
9.01 MHz
(BFO)
to TX FM
PLL IC
(IC1011)
BPF
D/A
DDS IC901
to FM IF IC
(IC791)