Icom IC-R7100 Service Manual page 12

3·1·11 NOISE BLANKER CIRCUIT
(MAJN UNIT)
The noise blanker circ ui t effec tive ly reduces interference
trom pul se-type norse sucn
as car ignition systems and
functions in only SSB and AM modes.
A portion of the 2nd IF signal output (drain of 021) Is
applied 10 the noise amplifier (IC14) and is then detected
at th e noise detector circu it (046, 047). The detected
voltage is applied to a noise blanker swi tc h (027).
The threshold level of lhe noise blanke r switch (027)
is set at approximately 0.3
V
higher than thaI of 0 26 to
avoid malfunctions when the operating frequency or mode
is changed. When the detecled voltage exceeds the
tnresnold level, Q28 outputs a blanking siç nal to activate
the noise blanker gate c ircuit (0 30 - 0 32).
A
portion of
the detected voltage is applied la a nolse
bia nker AGC ci rcuit (026). The Ihreshold level of the
noise blanker AGC ci rc uit (026) is set at 0.6
V.
The
nol se components are led back la the norse amplifier
(IC14) through a OC amplifier (0 25). The time constant
of the naise blanker AGC circuit is determined by R206 .
R209 and C12 9. This AGC c irc uit does not detect putse-
type
norse.
The filtered signal is applied to the 3 rd IF ampl ifier circuit
(022. 023) and is then applied 10 the demodulator ci rcuits,
3·1·13 WFM DEMODULATOR CIRCUIT
(MAIN UNIT)
The 10.7 MHz 2nd IF signal output from FI2 is appl ied to
pin 1 of IC12 to demodulate
tne
2nd IF
signaI into an
AF
signa!.
IC1 2 contains a li miter amplifier, a filte r, an FM detector
and
a
signal meter circuit.
Input signa I from IC12 (pin 1) passes the limiter amplifier
and the filter sections of IC12.
Tha
fil tered signal is
demodulated into an AF signa! by using the FM detector
section of IC1 2 and the other components (Xl,
L2.
R128,
C72. C73. 0 26).
A va ractor diode (026) adj usts the distort ion of the center
frequency in a ce ramic disc riminator
(Xl)
by cha nging its
volta ge .
The
AF signal is output from IC1 2 (pin 6) and is then
applied to the buffer amplifier (01 6) for AF signaI switching
and AF signal level adjustment in other modes.
WFM9
• WFM DEMODULATOR CIRCUIT
046 turns Ihe noise blanker circuit ON
or
OFF. Whi le
pulse-type signals are received, Q46 turns Q28 ON.
Therefore. tne naise blanker gate circuit
(0 30- D32) is
reverse·bia sed to cut OFF the 2 nd IF signa!.
3-1 -12 3rd MIXER AND 3rd IF CIRCUITS
(MAIN UNIT)
The 3rd mixer circuit couverts the 2 nd IF signal to a 3 rd
IF signal with t he 3rd LO signal.
The signal output from the noise blanker gate circuit
(030 - 0 32) enters the 3rd mixer circuit (IC13). IC13 is
a OBM (Double Balanced Mixer).
2ncl 1 1"
""' "
'"
9
Vc< ,
~
"
FILTER
FM DE T.
•
- ,
AMP
«
• ONO
~l
,
, ,
L2
Xj ~
... ...,0
R128
~
A 10.245 MHz 3rd LO signa I is oscillated by (029, X2)
and is
th en
applied to the 3 rd mixer circuit to produce a
455 kHz 3 rd IF signal. C132 provides fr equency cont rol.
The 455 kHz 3rd IF signaI output from IC13 (pin 3) passes
through an impeda nc e converter
C L7.
C97) and is then
applied to three separate, high-quality ceramic filters
(FI4- FI6) depending on the mode.
(1) SSB MODE
The 455 kHz 3 rd IF signal is applied to FI4 cove ring
2.6 kHz of bandwidth in SSB mode.
(2) AM/FMN MODES
The 455 kHz 3rd IF signal is applied to FIS cove rin
6.0 kHz of bandwidth in AM
or
FMN mode.
(3) FM/AMW MODES
The 455 kHz 3rd IF signal is applied to FI6 covering 15.0
kHz of bandwidth in FM or AMW mode.
3 - 6
Ag. 6
3·1·14 WFM AGC CIRCUIT (MAIN UNIT)
The AGC (Automatic Gain Control) circuit reduces signal
fading and keeps the audio output level constant.
The strength of the 2nd IF signal is detected at the signal
meter circuit section of IC12. The de tected voltage is
output from pin 3 of IC1 2 depending on the signaI st renglh
of the input voltage (IC12. pin 1).
0 15 is a
oe
buffer amplifier. When the input voltage ol
IC12 (pin 1) becomes stronç, the base voltage of Qf 6
increases. Therefore, 0 15
turns
Q16 ON to cut olf the
AGC voltage.
01 9 and 0 20 are OC buffer amplifiers providing gain
control to pin 1 of IC11. 056 provides ga in conlrol to
0 55 and 01 4 by changing its voltage.