Noise Blanker; Tuning Circuit - Icom IC-701 Instruction Manual

gate, and AGC voltage is fed to the second gate to increase
the dynamic range of the amplifier. The signal is then
passed through a Schottky Diode double balanced mixer
composed of D11 through D14 where it is mixed with the
local oscillator signal from the VCO. From the mixer, the
resultant 9.0115MHz signal passes through a monolythic
filter, having a bandwidth of 10KHz, and to the first gate
of amplifier, Q3. AGC voltage from pin 6 of J7 is fed to
the second gate of Q3 as it is to the gates of O15 through
Q20.
6-1-2 NOISE BLANKER CIRCUIT
The noise blanker circuit is composed of O08 through 012.
Noise amplified by Q8-Q10 is detected by Q20 and O21
and applied to the base of Q12. Since the collector of O12
is connected to the bias circuit of D1 and D2, when 012
conducts because of detected noise, it shuts off D1 and D2,
cutting off the noise. Therefore only clean signals are fed
through J1 to the IF circuit.
6-1-3 PASS BAND TUNING CIRCUIT
A VXO circuit composed of Q1, 02, X1 and D4 in the "A"'
unit. The bias voltage of D4 is changed by turning the Pass
Band Tune control, located on the front panel, which
varies the VXO frequency (19.7615MHz) by +1.5KHz.
Signals from Ji in the RF unit are fed through J1 in the
IF circuit to L5, then through FL1, a crystal filter. FL1's
center frequency is 9.0115MHz with a band width of
+1.2KHz. From the filter, the signal is fed to O10, where
it is amplified and fed to the balanced mixer, IC3, where
the 9.0115MHz signal is mixed with the 19.7615MHz VXO
signal. From the balanced mixer, the resultant 10.750MHz
signal is fed through FL2, a crystal filter having a center
frequency of 10.750MHz and a band width of +1.2KHz.
This 10.750MHz signal is then again mixed with the VXO
frequency, to result in the original 9.0115MHz signal to be
fed to the IF amplifier circuit.
The diagrams will explain the operation of the Pass Band
Tuning in more detail.
9.0115MHz +1.2KHz
Filter MHz
FL1 i
19.7615MHz
+1.5KHz
FL2. With the Pass Band Tune control in the center
position, signals between 9.0127MHz and 9.0103MHz
ine)
(9.0115+1.2KHz), when mixed with the VXO center
frequency (19.7615MHz) result in signals of 10.7512MHz
to 10.7488MHz (10.750MHz+1.2KHz), all signals will
pass through the filter.
: P.B.TUNE centered
® Pass band of
Output Signals Output Signals
of FL1 FL2 of 1C4
Output Signals
j of IC3
—j O +1 +2 —2 —1 0 +1 +2 —2 —1 0 +1 42
9 0115 (KHz) 10.75 (KHz) 9 0115 (KHz)
Gane aie ae
Turning the Pass Band Tune control fully clockwise, the
VXO frequency will become 19.7630MHz (19.7615+1.5
KHz). When mixed with the incomming 9.0115MHz
signals, the resultant frequencies become 10.7503MHz to
10.7527MHz. Since the upper frequency pass band limit
of the filter is 10.7512MHz, not all the signals will be
passed by the filter. Only the signals between 10.7503MHz
and 10.7512MHz, the upper limit, will be passed. As you
can see, the pass band has been narrowed to 900Hz to one
side of the center frequency of the filter. Therefore, all
signals outside the 10.7503MHz to 10.7512MHz range will
not pass through the filter.
B : P.B.TUNE fully clockwise
Output
Signals of '
F
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J
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By turning the pass band tuning control in the fully
counter-clockwise position, the VXO frequency becomes
19.760MHz. The 10.750MHz signals from the mixer will
be from 10.7497MHz to 10.7473MHz. Again with the
10.7512MHz to 10.7448MHz pass band width of the filter,
not all signals will be passed. Only those between 10.7497
MHz to 10.7488MHz will be passed. Again, the pass band
has been narrowed to 900Hz and offset to the opposite
side of the center frequency of the crystal filter.
C : P.B.TUNE fully counter-clockwise
Output
In the diagrams, ""A'' represents the 10.750MHz signals that ene
are the result of mixing of the 9.0115MHz input signals
with the 19.7615MHz VXO signal in 1C3. These 10.750
MHz signals are represented by the solid line. The dotted
line represents the pass band of the second crystal filter,
® Output Signal @® Pass band ©
of 1C3 1, of FL2 Output Signals
ee of IC4
1 4
ae
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Po 4
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1
—2—-1 0 +1 +42 —3 -2 —-1 0 +1 +2 —2 —-1 0 +1 +2
Gu (KHz) Ge (KHz) Gree (KHz)
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