Receiving Circuit; Noise Blanker - Icom IC-502A Instruction Manual

CIRCUITS |
. Section IX shows a block diagram of the IC-502A.
The receiving section is a single conversion superheterodyne with a 36MHz high-stability VFO as
the local oscillator. The transmitting section is a single conversion system which employs a filter-
type SSB generator using а 13.9985MHz crystal filter and the same local oscillator as the receiving
section. A double-balanced mixer is used for the transmitting mixer to minimize spurious radia-
tion. Although a portable unit, the IC-5024 also features built-in circuits such as RIT, AGC, ALC,
and an IF noise blanker.
The transceiver can be used with ease outdoors, in the car, or as a fixed station since it may be
powered either with it's self-contained batteries (C size x 9) or with a 13.8VDC external source.
RECEIVING CIRCUIT
The signal from the whip antenna, or antenna terminal, passes through the harmonic filter, through
the T/R switching diode D23 (M1301), amplified by RF amplifier 02 (3SK74), and is then fed to
the mixer Q3(2SK19) gate.
The switching diode is turned on by T/R control Q1 (2SA750), and D23 is turned on with for-
ward voltage bias, thus directing the input signal to Q2. During transmission, the Receiver section
+9V goes to zero turning off 01, and forward voltage bias is not applied to D23, while at the
same time, the transmit output is switched around Q2 to the antenna system. D23, is turned off
as reverse bias is generated when the transmit signal is present.
The 36MHz local oscillator output from the VFO unit is injected to mixer Q3's source. The result-
ant conversion is an IF Frequency of 13.9985MHz. The IF signal passes through the diode switch
D1(1SS53) which serves as both a transmit-receive switch and noise blanker gate. The IF selectivity
is obtained by the 13.9985MHz crystal filter, then passes the switching diode D4(1SS53) and is
amplified up to a suitable level by the IF amplifiers consisting of Q4(3SK74), 05(35К74) and |
IC1(BA401). The output of IC1 is applied to the detector and AGC circuits.
The detector circuit is a ring demodulator composed of D6 to D9(1N60's) which uses the 13.997
MHz from the BFO to generate the resultant audio signal. Higher audio frequencies of the de-
modulated signal are cut off by a low-pass filter consisting of C46, L11 and C47.
The volume control (R3) adjusts this output level which is fed to AF amplifier IC2 (uPC575C2)
providing 1 watt of audio. The network RS and D1(1SS53) provides position bias to IC2 for
muting audio during transmit and silent transmit-receive switching.
NOISE BLANKER |
A sample of the IF signal is picked up at the drain of mixer Q3, amplified by IC2 and IC3 (BA
401's), and detected by D11(1N60). This detected output is separated into signal audio compo-
nents, and pulse components (noise).
The signal component is amplified by Q6(2SC945) and provides AGC control of IC2. The noise
pulse component turns on Q7(2SC945), and, as long as noise exists, turns off D1, by gounding
the anode of the noise blanker gate diode D1, thus the noise component is not transferred to
the crystal filter.
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