Icom IC-R7000 Service Manual page 15

4·1·21 AGC CIRCUIT FOR FM (narrow), SSB AND
AM MODES (IF UNIT)
In FM (narrow) mode, SSB mode and AM mode, a part
of 3rd IF signals from 019 are applied to AGC detector
circuit D28 to rectify 3rd IF signals. The DC voltage
from D28 is DC-amplified at 018.
In AM mode and SSB mode, 016 turns 017 ON and C89
is applied to C91, resulting in a longer time constant
than in FM (narrow) mode. The rise (attack) time
constant for AGC is determined by C89, C91 and
R114, and the fall (delay) time constant is determined
by R113.
The AGC voltage (when the IC-R7000 receives no
signal) is offset by approximately 4 V by R117 and
R116.
018 supplies IF, RF and MAIN UNITS with the AGC
signal.
4·1·22 5kHz NOTCH CIRCUIT (IF UNIT)
AF signals in each mode are fed to a 5kHz notch
filter consisting of IC6(A), R167-R172, and C135-
C140. This circuit attenuates the 5kHz AF signal
generated by leakage of the PLL reference frequency.
In this circuit, the 0-factor can be adjusted by R171.
IC6 (A) sends the AF signals to the MAIN UNIT.
o---------
to
:s!.
Qj
-20
>
~
"S
-40
.e
:l
0
u.
-60
<(
I
~
1 5 10
Fig.
4
Audio frequency [kHz]
4·1·23 POWER SUPPLY CIRCUIT (IF UNIT)
014 and 015 switch 9V in FM (wide) mode and other
modes. In FM (wide) mode, 015 is switched ON,
while 014 is switched ON in other modes.
Mode signals from the LOGIC UNIT switch the AF
amplifier of each mode and IF filters by the diode
matrix composed of D22-D26, 029, D45 and D46.
4·1·24 NOISE SQUELCH CIRCUIT
(IF UNIT)
Noise squelch operates in FM (narrow) mode and AM
mode.
A part of AF signals from X3 is fed to active filter
IC6(B). The noise components of AF signals pass
through the active filter, and are double voltage
rectified by D35 and D36. The voltage from 035 and
036 is then charged to C30. The voltage of C30 is
fed to the squelch circuit in the MAIN UNIT as squelch
voltage (SQLV).
(MAIN UNIT)
Noise voltage from the IF UNIT is DC-amplified at
IC7(A). The output of IC7(A) passes through a voltage
limiter circuit consisting of D28, D29 and R116, and
then enters comparator IC7(B). The reference voltage
of IC7(B) is varied by the [SQUELCH] CONTROL, and
controls the squelch threshold level.
4·1·25 METER SQUELCH CIRCUIT (MAIN UNIT)
AGC signal from the IF UNIT is applied to IC4(A),
and is inversion-amplified. Reference voltage of the
meter squelch is varied by the [SQUELCH] CONTROL,
and is applied to IC4(B) via Q9 and D19.
The meter squelch is switched ON and OFF depend-
ing on the output of the above comparator.
Fig. 5
4·1·26 CENTER DETECTOR CIRCUIT
FM(wide)
DET
R16 R17
9V
2nd IF IC2
>------_____,,,VV--.--'W'v---.----M._
(IF UNIT)
9V
'" "'
a:
N
"'
a:
Fig. 6
-7V
CENT
Output voltage of the FM (wide) and FM (narrow)
detector circuits differ from each other in the center
frequency of IF signals. Therefore, in FM (narrow)
mode the offset voltage is applied to the output of
the detector circuit by R92-R94, and is adjusted at
R92 to the same output voltage of FM (wide) mode.
Q3 and Q8 are DC voltage buffers.
(MAIN UNIT)
CENT signal is applied to a window comparator
consisting of IC5(A) and IC5(B). The offset voltage
is adjusted at R66 to make the window comparator
output OV when the IC-R7000 receives the center fre-
quency of the signal. IC5(A) and IC5(B) output the
STOP signal to the LOGIC UNIT.
4-5