Circuit Description; Receiver Circuits - Icom IC-2100H Service Manual

SECTION 3

3-1 RECEIVER CIRCUITS

3-1·1 ANTENNA SWITCHING CIRCUIT {MAIN unit)
The antenna switching circuit functions as a low-pass filter
while receiving and a resonator circuit while transmitting. The
circuit does not allow transmit signals to enter receiver cir­
cuits.
Received signals enter the antenna connector and pass
through the low-pass filter (L17-l20,
signals are passed through the A/ 4 type antenna switchfng
circuit (010 , 0 11, l22, l23) and are then applied to the RF
amplifier (020 ) .
3·1·2 SQUELCH ATTENUATOR
The attenuator circuit attenuates the signal strength to a
maximum of dB to protect the RF amplifier from distortion
10
When excessively strong signals are received.
The current flow of the antenna switching circuit (010 , 0 11)
is controlled by the [SOL] control via the attenuator controller
(IC?). When the [SOL] control is rotated clockwise deeper
than 12 o'clock, the current of 0 1 0 and 011 is Increased. In
this case, 010 and 011 act as an attenuator.
3-1-3 RF CIRCUIT (MAIN unit)
'The RF circuit amplifies signals within the range of frequen­
cy coverage and filters out-of-band signals.
The signals from the antenna switching circuit pass through
the tunable bandpass filter (0 13). The filtered signals are
ampli f ied at the RF amplifier (020 ) and then enter another
three-stage bandpass filters (014 -016) to suppress unwant­
ed signals. The filtered signals are applied to the 1st mixer
circuit (021).
The tunable bandpass filters
diodes to tune the center frequency of the RF passband for
wide bandwidth receiving and good image response rejec­
tion. These diodes are controlled by the
tne tune control circuit (IC2, 04).
3·1-41ST MIXER AND 1ST IF CIRCUITS
(MAIN unit)
The 1st
mixer circuit converts the received signals to a theed
frequency of the 1st IF signal with the PLL output frequency.
By changing the PLL frequency, only the desired frequency
will pass through a pair of crystal filters at the next stage of
the 1 st mixer.
The RF signals from the bandpass filter are applied to the 1 st
mixer circuit (021 ). The applied signals are mixed with the
1 st LO signal coming from the RX-VCO circuit (033, 023) to
produce a 15.6 5 MHz 1st IF signal. The 1st
through a pair crystal filters (F11a/b) to suppress out-of­
of
band signals. The filtered signal is amplified at the 1st
amplifier (022) and applied to the 2nd IF circuit.

CIRCUIT DESCRIPTION

C 55 -C6 4 ). The filtered
(013-016) employ varactor
PLL lock volt a ge via
IF signal passes
IF
3·1-5 2ND IF AND DEMODULATOR CIRCUITS
(MAIN unit)
The 2nd mixer circuit converts the 1st
signal. A double-conversion superheterodyne system
improves the Image rejection ratio and obtains stable receiv­
er gain.
The 1st IF signal from the IF amplifier (Q22) is applied to the
2nd mixer section of the
mixed with the 2nd LO signal for conversion to a 4 50 kHz 2nd
IF signal.
IC6 contains the 2nd mixer, limiter amplifier, quadrature
detector, $-meter detector, active fi l ter and noise amplifier
circuits, etc. A frequency from the PLL reference oscillator is
used for the 2nd LO signal ( 15.2 MHz).
The 2nd IF signal from the 2nd mixer (IC6, pin 3) passes
through the ceramic filter (Fl2) (during wide channel spacing
selection or passes through Fl3 during narrow channel spac­
ing selection; [EUR], [ITAJ only). It is then amplified at the lim­
iter amplifier section (IC6, pin 5) and applied to the quadra­
ture detector section (IC6, pins
the 2nd IF signal into AF signals.
The AF signals are output from pin 9 (IC6) and are then
applied to the AF amplifier circuit.
•2nd IF AND DEMODULATOR CIRCUITS
Squelch level
a<flustmeot A1 6 3
sigllaJ
3·1-6 AF CIRCUIT (MAIN unit)
The AF amplifier circuit amplifies the demodulated AF signals
to drive a speaker.
The AF signals from IC6 (pin 9 ) are amplified at the active fil­
ters (023 HPF, 024 LPF) and pass through the detector
mute switch (025), and are level adjusted with the volume
control on the LOGIC unit.
The AF amplifier ICB amplifies the signals to a sufficient level
to drive the speaker. The AF mute switch (028) turns ON to
cut the signal to be input to the AF amplifier (ICB) during
transmission.
3 -1
IF signal to a 2nd
IF IC (IC6, pin and is then
FM 16)
X2)
10, 11 and to demodulate
(15.2 MHz)
17
PLLIC
3 2
IC1
16
Xl
15.2 MHz
IC6
TA31136F
1st IF (15.65 M'°'z)
from022
IF