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3 - 2

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).

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.

• Used 2nd IF filter

Mode

Used filter

Control signal

SSB, CW, RTTY

AM nar.

AM, FM nar.

SSB nar.

CW nar., RTTY nar.

SSB wide

FL-65 (FI1301)

FL-94 (FI1341)