Icom IC-701 Instruction Manual page 24

This action, and what this circuit really can do for you is
better explained with an example.
If you are receiving a signal, whose frequency at the input
of the mixer is 9.0115MHz and another signal whose
frequency is 9.0125MHz is present, after being mixed with
the 19.7615MHz VXO frequency (center frequency) the
resultant frequencies are 10.750MHz, the signal you are
listening to, and 10.7490MHz, the other signal. Both are
within the pass band of the filter, and therefore, both will .
be heard. However, by turning the pass band tune control
until the VXO frequency reaches 19.7605MHz, the 9.0125
MHz interfering signal when mixed with the VXO frequency
will have a resultant frequency of 10.7480MHz, which is
out of the pass band of the filter. The signal you are listen-
ing to, however, when mixed with the VXO frequency
becomes 10.749MHz. This is within the pass band of the
filter, and when mixed again with the VXO frequency in
the next stage, becomes it's original 9.0115MHz signal and
is sent to the IF amplifier. You have pushed the interfering
signal out of the pass band of the filter, and therefore
eliminated it and the interference it caused.
In the CW mode (CW or CW-N position of the MODE
switch), a fixed bias voltage is applied to D4 of the VXO
circuit, so the VXO frequency shifts to 19.7634MHz
(+1.9KHz). The bandwidth then becomes +250Hz for
clear CW reception.
The signal which has passed through the pass band tuning
circuits are amplified by O11 and Q12, dual gate MOS
FET's. AGC voltage is applied to the second gate of both.
The amplified IF signal is then sent to the detector circuit.
6-1-4 DETECTOR CIRCUIT
The detector circuit is composed of IC2, a differential
amplifier that consists of a pair of similar input circuits
and a constant-current source. The IF signals are fed to
the both input circuits of IC2 180 degrees out of phase,
and the BFO signal is fed to the constant-current source.
The detected audio signal is sent to the CW AF filter circuit
and the AF amplifier circuits through the MODE switch.
6-1-5 BFO CIRCUIT
The BFO circuit is composed of 018 to Q20 and crystals
X1 and X2, all located on the "'B" unit. O19 is the crystal
oscillator, Q18 is the buffer, and Q20 is the FSK
(Frequency Shift Keying) keyer for RTTY.
The crystals are switched by D12 and D13, and the
oscillation frequencies are shifted by D14 to D18, according
to the mode used. Crystal X1 (9.013MHz) is used for USB
transmitting and receiving, CW receiving, and RTTY receiv-
ing. Crystal X2 (9.010MHz) is used for LSB transmitting
and receiving, CW transmitting, and RTTY transmitting.
In the RTTY transmitting mode, the bias voltage to D19 (a
varicap) is changed according to the MARK and SPACE
RTTY signals, by Q20. S3 (RTTY: shift switch) changes
the shift width for Narrow (170Hz) shift, and Wide (850Hz)
shift.
BFO FREQUENCY CHART
P.B.TUNE FULLY CLOCKWISE
P.B.TUNE FULLY
COUNTER-CLOCKWISE
CW PASS BAND
RTTY MARK] CW (T)
{WIDE)); RTTY SPACE
cw (R) RTTY (R)
9.01155MHz | 9.0124MHz ae
R (
USB 32MHz 9.014525MHz
9.010MHz
RTTY MARK
(NARROW)
9.01223MHz
USB
9.013MHz
RECEIVE LSB
FREQUENCY
9.010 MHz
9.013 MHz
9.0132 MHz
9.014525MHz
9.010 MHz
9.013 MHz
9.0124 MHz
9.0124 MHz (SPACE)
9.01223 MHz (170MARK)
9.01155 MHz (850MARK)
TRANSMIT
6-1-6 CW AF FILTER CIRCUIT
After detection, the audio signal is applied to IC6. Next,
the signal is fed to a CW AF filter composed of high and
low pass filters. The filters have a pass band of about
800Hz +100Hz. If the mode switch on the front panel is
turned to CW-N, the signal passes through this band pass
filter and goes to the audio amplifying section in the next
stage. During other modes, the detected output is directly
applied to the audio amplifier circuit.
6-1-7 AUDIO AMPLIFIER CIRCUIT
The audio amplifier circuit is composed of 044 to O46.
The amplified signal passes through an AF gain control on
the front panel, and then goes into Q27, an audio low pass
filter, where signal components above 3KHz are removed.
After that, the signal is amplified in the audio power
amplifier circuit, 1C1, which then drives the speaker.
6-1-8 AGC CIRCUIT
The AGC circuit is composed of 028 to Q36 on the "A"
unit. After detection, for CW-N after passing through the
CW AF filter, the AF signal is amplified by Q28. It then
undergoes peak hold detection by detector O32. The
detected DC voltage charges C101 and C102. The DC
voltage is then amplified further by Q33 and Q34. Thus,
the AGC voltage for RF and IF amplifier circuits is
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