2Nd Mixer Circuit (Main Unit); Noise Blanker Circuit (Main Unit); 2Nd If Circuit (Main Unit); If Shift Circuit (Main Unit) - Icom IC-703 Service Manual

The amplified signals are then applied to the 2nd mixer cir-
cuit (D901).

3-1-5 2ND MIXER CIRCUIT (MAIN UNIT)

The 2nd mixer circuit mixes the 1st IF signals and 2nd LO
signal (64 MHz) to convert the 1st IF to a 2nd IF.
The 1st IF signals from the 1st amplifier (Q881) are convert-
ed to 455 kHz 2nd IF signals at the 2nd mixer circuit (D901).
The 2nd IF signals are applied to the bandpass filter
(FI1501) to suppress undesired signals, such as the 2nd LO
signal, and are then applied to the noise amplifier (Q1053).

3-1-6 NOISE BLANKER CIRCUIT (MAIN UNIT)

The noise blanker circuit detects pulse type noise, and turns
OFF the signal line when noise appears.
A portion of the signals from FI1501 are amplified at the
noise amplifiers (Q1051–Q1053), then detected at the noise
detector (D1002) to convert the noise components to DC
voltages.
The converted voltages are then applied to the noise
blanker gate (Q1002, Q1003, Q1402) by applying reverse-
biased voltage.
The detected voltage from Q1002 is also applied to the
noise blanker AGC circuit (Q1004, Q1005) and is then fed
back to the noise amplifier (Q1051–Q1053) as a bias volt-
age. The noise AGC circuit prevents closure of the noise
blanker gate for long periods by non-pulse-type noise.
The 2nd IF signals from the noise blanker gate are then
applied to the 2nd IF circuit.

3-1-7 2ND IF CIRCUIT (MAIN UNIT)

The 2nd IF circuit amplifies and filters the 2nd IF signals.
The 2nd IF signals from the ceramic bandpass filter (FI1501)
are amplified at the IF amplifier (IC1401) and applied to a
2nd IF filter as shown right column via the Tx/Rx switch
(D1402).
• NOISE BLANKER CIRCUIT
to the detector circuits
• Used 2nd IF filter
The filtered or bypassed signals are applied to the 2 ampli-
fiers (Q1101, Q1201) and buffer amplifier (Q1251) to obtain
a detectable level at the demodulator circuit
The amplified signals from the buffer amplifier (Q1251) are
shared between the SSB/CW/RTTY detector (IC1901), AM
detector (D2001), FM detector (IC2101) and AGC detector
(D2501).

3-1-8 IF SHIFT CIRCUIT (MAIN UNIT)

The IF shift circuit shifts the center frequency of IF signals to
electronically shift the center frequency.
The IF shift circuit shifts the 1st LO and BFO within ±1.2 kHz
in SSB/CW/RTTY modes or ±250 Hz in CW-N/RTTY-N
modes. As a result, the 2nd IF (also 1st IF) is shifted from
the center frequency of the 2nd IF filter (FI1301, FI1341 or
optional IF filters). This means 2nd IF signals do not pass
through the center of the 2nd IF filter. Therefore, the higher
or lower frequency components of the IF are cut out. Since
the BFO frequency is also shifted the same value as the 1st
IF, frequency is corrected at the detector.
In the IC-703, the 1st LO frequency is shifted to change the
2nd IF because a fixed 2nd LO frequency (64 MHz) is used.
The 1st IF filter (FI1301) and 2nd IF filter (FI1501) have 15
kHz pass-band widths, and do not affect IF shift operation.
Q1401
IF
Q1002
Q1003
Q1402
NB GATE
"NBLV" signal
Q1051
D1002
NB DET.
Amp.
Q1004
Q1005
AGC
3 - 2
Mode Used filter
SSB, CW, RTTY
FL-65 (FI1301)
AM nar.
AM, FM nar. FL-94 (FI1341)
FM Bypassed
SSB nar. Optional FL-222
Optional FL-52A,
CW nar., RTTY nar.
SSB wide Optional FL-257
D1403
IC1401
"ALCL" signal
"AGC" signal
"T8V" signal
Q1052 Q1053
Amp. Amp.
Q1001
Q1006
NB SWITCH
Control signal
2F23
2F80
2FTH
2FOP
2FOP
FL-53A
2FOP
2nd IF signal
"NBS" signal