Circuit Description; Receiver Circuits - Icom IC-A200 Service Manual

SECTION 3

3-1 RECEIVER CIRCUITS

3-1-1 ANTENNA SWITCHING CIRCUIT (PA UNIT)
The antenna switching circuit functions as a low-pass filter
while transmitting and as a resonator circuit while receiving.
Received signals enter the PA unit from the antenna
connector and pass through the low-pass filter (C54–C57,
C66, L14–L16), the antenna switching circuit (D7–D9) and
the bandpass filter (C71–C75, L21–L23). The signals are
then applied to the MAIN unit.
3-1-2 RF CIRCUIT (MAIN UNIT)
The RF circuit amplifies signal within the range of frequency
coverage and filters out-of-band signal.
The signals from the PA unit pass through the attenuator
circuit (D6, D7). The attenuation level is controlled by AGC
voltages. The signals are passed through a bandpass
filter and are then amplified at the RF amplifier (Q1). The
amplified signals are passed through a bandpass filter again
and are then applied to the 1st mixer circuit (Q4).
The bandpass filters employ tuned-type filters using varactor
diodes (D1–D4) with PLL lock voltages to suppress the out-
of-band signals (especially FM broad band signals) and to
obtain high image response rejection.
AGC voltage is applied to Q1 as the bias voltage to obtain
wide range characteristics of signal strength.
RECEIVER CIRCUIT BLOCK DIAGRAM
��� ��
��������
������
������
�����
���� ����� �������

CIRCUIT DESCRIPTION

��
����� �����
���
��� �� ��� ��
��
3-1-3 1st MIXER AND 1st IF CIRCUITS (MAIN UNIT)
The 1st mixer circuit converts the received signals to a fixed
frequency of the 1st IF signal using a PLL output frequency.
By changing the PLL frequency, only the desired frequency
will be passed through a crystal filter located at the next
stage of the 1st mixer.
The signals from the bandpass filter are mixed with a 1st
LO signals from D25 at the 1st mixer (Q4) to produce a
32.455 MHz 1st IF signal. The 1st IF signal passes through
the crystal filter (FI1) and is then applied to the IF amplifier
(Q6).
3-1-4 2nd MIXER AND 2nd IF CIRCUITS
(MAIN UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 455 kHz
2nd IF signal. A double superheterodyne system (which
converts the received signal twice) improves the image
rejection ratio and obtains stable receiver gain.
The 1st IF signal from the 1st IF amplifier (Q6), is applied
to the 2nd mixer (IC14) and mixed with a 32.000 MHz
2nd LO signal from Q20 to be converted to a 455 kHz of
signals. The 2nd LO signal is commonly used with the PLL
reference oscillator.
The 2nd IF signal is applied to an IF filter (F12) to suppress
out-of-band signals. The 2nd IF signal is amplified at the IF
amplifiers (Q7–Q10) and is then applied to the AM detector
circuit (Q49).
The thermistor (R33) is used for Q9 bias voltage to obtain
stable amplifier gain during temperature fluctuations.
�������
������
�� �
��� ��� �� ���
����� ��� �����
�� �� ����
������ ���
��� ���
��� ���
���� ��� ���� ����
3 - 1
�������
������
�� �
��� ��
���
���
���
�����
���
��
���� ���
�� ���