Icom IC-R7000 Service Manual page 14

4·1-13 AGC CIRCUIT FOR FM (wide) MODE (IF UNIT)
IC2 (pin 3) generates a voltage corresponding to the
signal strength of IF signals. 030 is a buffer for DC
voltage. When IF signals are strong, 030 turns 026
ON to cut off the AGC voltage.
When IF signals become stronger the AGC maintains
the voltage of IC2 (pin 3) at a constant value.
2nd IF
(FROM Fl1)
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Fig. 2
Signal strength
Signal strength
Fig. 3
4·1-14 2No IF CIRCUIT FOR FM (narrow), SSB AND
AM MODES (IF UNIT)
In FM (narrow) mode, SSB mode, or AM mode, 2nd IF
signals (10.7 MHz) are applied to Fl3. Fl3 sufficently
attenuates unrequired signals. The 2nd IF signals
are then amplified at 05. The gain from L4 to L5 is
approximately 28dB.
4·1-15 NOISE BLANKER CIRCUIT (IF UNIT)
Part of the receive signals from 05 is amplified by
IC4 and detected by D32 and D33. AGC detector 09
controls the gain of IC4 via DC amplifier 029. The time
constant of the noise blanker circuit is determined by
R156, R157 and C65. 010 turns ON or OFF according
to the detected pulses. While pulse-type noise is
received, 010 turns 011 ON. The noise blanker
switch consisting of D7 and D9 is reverse-biased,
and the 2nd IF signals are cut OFF.
4-4
4·1-16 3Ro MIXER AND 3Ro LO CIRCUIT (IF UNIT)
2nd IF signals from the noise blanker switch are
applied to double-balanced mixer IC3. IC3 converts
2nd IF signals (10.7MHz) to 3rd IF (455kHz) signals
utilizing 3rd LO signals (10.245 MHz) which are
oscillated by X2 and 012.
Output signals from IC3 is impedance-converted by
L 13 and C25, and is fed to the 3rd IF circuit.
4·1-17 3Ro IF CIRCUIT (IF UNIT)
3rd IF (455kHz) signals from the noise blanker switch
are then applied to filters corresponding to each
mode.
Fl4 (pass-bandwidth 15kHz) is a filter for FM (narrow)
and AM modes. Fl5 (pass-bandwidth 6kHz) is a
filter for AM mode. Fl6 is a filter for SSB mode. (pass-
bandwidth 2.8kHz.) Cermaic filters are used for
all of these.
In AM mode, Fl4 and Fl5 can be selected by JS on
the IF UNIT.
06 and 07 amplify 3rd IF signals. R165 is a ther-
mistor for temperature compensation.
4-1-18 BFO (BEAT FREQUENCY OSCILLATOR) CIRCUIT
AND SSB DETECTOR CIRCUIT (IF UNIT)
A BFO (Beat Frequency Oscillator) consists of 021
and 022 and supplies to the SSB detector circuit a beat
frequency signal.
In LSB mode, 031 is turned ON by the LSB mode (LSB)
signal from the EF UNIT, and the capacitance of
C133 and C134 is added to L15, C130, and C119 to
shift the oscillation frequency 3kHz up.
Output signal from the 3rd IF circuit and a beat
frequency signal from the BFO circuit are applied
to the SSB detector circuit consisting of D37-
D40. This circuit detects SSB signals.
AF amplifier 024 switches AF signals from the SSB
detector circuit and 024 matches the level of AF
signals with other modes.
4·1·19 FM (narrow) DETECTOR CIRCUIT (IF UNIT)
In FM (narrow) mode, 3rd IF signals from 07 are
applied to 019, and then are limiter-amplified by IC5 to
remove AM components in 3rd IF signals. Ceramic
discriminator X3 is used for FM detection to obtain
AF signals.
013 and 023 form a deemphasis circuit and a HPF to
attenuate low frequency (below 200 Hz) components
in AF signals.
4·1·20 AM DETECTOR CIRCUIT (IF UNIT)
In AM mode, 3rd IF signals from 019 are detected at
D30. AF signals from D30 are amplified by 020.