Af Amplifier Circuit (Main Unit); Noise Blanker Circuit; Agc Circuit (Main Unit) - Icom IC-821H Service Manual

(2) SSB and CW modes
The squelch circuit mutes audio output when the S-meter
signal is lower than the [SOL] control setting level.
A portion of the 10 MHz IF signal from the IF amplifier (062
[MainJ. 0101 [Sub]) is converted into DC voltage at the
AGC detector (044. 065 [Main]. 073. 0104 [Sub)) and
meter amplifier (IC15b [Main] or IC23a [Sub)). The ampli-
fied signal is applied to the CPU (DISPLAY unit. IC1) via the
ADMSM [Main]/ADSSM [Subjline. The CPU outputs con-
trol signals to the squelch control gate when the S-meter
signal is low level.
4·1·10 AF AMPLIFIER CIRCUIT (MAIN UNIT)
The AF amplifier circuit amplifies the demodulated signals
to drive a speaker. For the separate speaker function. a
stereo power amplifier is used.
AF signals from the squelch control gate are amplified at
the voltage controlled amplifier (VCA; IC30 [Main]. IC31
[Sub]) which functions as a volume control using the [AF]
control signal. The amplified AF Signals are applied to the
AF power amplifier circuit (IC32. pin 2 [Main]. pin 5 [Sub]).
The amplified audio signals of SUB band are output from
from pin 7
I
and are applied to the external speaker jack for
the SUB band (J17) via the [PHONE] jack (JACK board, Jl).
When no plug is connected to the jack. the signals are fed
back to the MAIN band audio input (IC32, pin 2) and
combined with the MAIN band audio. The mixed audio is
applied to the internal speaker via the [PHONE] jack and
external speaker jack for the MAIN band (J16).
4·1·11 NOISE BLANKER CIRCUIT
(MAIN UNIT)
The noise blanker circuit detects pulse-type noises, and
stops IF amplifier operation during detection.
• AGC and Noise Blanker circuits
FI2 [F16)
IF signal
013
(098) Fl1 [F15)
060
(077)
4-3
A portion of the 10 MHz IF signal from the bandpass filter
(F12 [Main], FI6 [Sub]) is amplified at the noise amplifier
circuit (07. IC6, 09 [Main], 0252, IC119, 0254 [Sub]). The
amplified signal is rectified at the noise detector circuit for
conversion Into DC voltage. The DC voltage is amplified at
the DC amplifier circuit (011, 012 [Main], 0256. 0257
[Sub)) and then applied to the noise blanker control circuit
(014 [Main]. 0212 [Sub]) to stop amplification of the IF
amplifier circuit (013 [Main]. 098 [Sub)).
Some DC voltage is fed back to the nOise amplifier circuit
(IC6 [Main]. IC116 [Sub)) via the DC amplifiers (08, 010
[Main]. 0253, 0255 [Sub]). The DC amplifiers function as
an AGC circuit to reduce averaged noise. Therefore. the
noise blanker function shuts off pulse-type noise only.
4-1-12 AGC CIRCUIT (MAIN UNIT)
The AGC (Auto Gain Control) circuit reduces IF amplifier
gain to keep the audio output at a constant level.
A portion of the 10 MHz IF signal from the IF amplifier (062
[MainJ. 0101 [Sub}) is applied to the AGC detector circuit
(044 [Main]. 073 [Sub}). The detected signal is then
amplified at the DC amplifier circuit (065 [Main]. 0104
[Sub]) and then applied to the IF amplifiers (013. 061. 062
[Main]. 098, 0100, 0101 [Sub]).
When strong signals are received, the detected voltage
increases and the output level of the DC amplifier, as AGC
voltage, decreases. The AGC voltage is used for the bias
voltage for the IF amplifiers. therefore. the IF amplifier gain
is decreased.
AGC response time is controlled by changing the time
constant at the AGC control line with a resistor (R276
[Main]. R432 [Sub]) and capacitors (C206. C210 [Main).
C381. C385 [Sub]). These are connected to the AGC
control line While AGC is set to slow. However, R276 and
C210 [Mainj/R432 and C385 are disconnected from the
AGC control line while AGC is set to fast. due to 067 and
068 [Main}/0105 and 0210 [Sub} being switched OFF.
Also, C206 [MainJ/C381 [Sub] is disconnected from the
AGC control line due to 054 and 055 [Main]/0206 and
0207 being switched OFF while scanning for faster re-
sponse than AGC fast mode .
061
(0100)
062 IC7
[0101::;»
----4~~®
demodulator
circuit
AGe
amplifier
065 044
[0104] [073]
. iAGC
~
..... J
detector
+---------0
-5 V
]: Sub band