Section 4 Circuit Description; Receiver Circuits - Icom IC-2710H Service Manual

SECTION
4

4-1 RECEIVER CIRCUITS

4-1-1 DUPLEXER CIRCUIT
The transceiver has a duplexer (low-pass and high-pass
fitters) on the first stage from the antenna connector to
separate the signals into VHF and UHF signals.
low-pass filter (L 1-L3, C30, C31) is for VHF signals and
high-pass filter (L100, L101, C34S-C347) is for UHF
signals. The separated signals are applied to each RF
circuit.
4-1-2 VHF ANTENNA SWITCHING CIRCUIT
The antenna switching circuit functions as a low-pass filter
while receiving. However, its impedance becomes very
high while transmitting by applying a current to 030 and
031. Thus, transmit signals are blocked from entering the
receiver circuits. The passed signals are then applied to
the RF amplifier circuit.
4-1-3 VHF RF CIRCUIT
The RF circuit amplifies signals within the range of fre­
quency coverage and filters·out-of-band signals.
The signals from the antenna switching circuit are applied to
the RF pre-amplifier (OSO) and are then applied to the
bandpass filter (L47, L1S6, C170, C761, C762) via the RF
attenuator circuit (033). The filtered signals are applied to
the RF amplifier (OS1) and are then applied to another
bandpass filter (036, 038, 040). The filtered signals are
then applied to the RF mixer circuit (OS2).
036, 038 and 040 track the bandpass filters and are
controlled by the PLL lock voltage. These diodes tune the
center frequency to obtain good image response rejection.
4-1-4 VHF RF ATTENUATOR CIRCUIT
The current flow of the RF attenuator circuit (033) is
controlled by the [SOL) control via the 0/A control IC (IC19).
When the [SOL) control rotated past the center, the current
of 033 is increased. In this case, 033 acts as an
attenuator.
4-1-5 1ST MIXER AND 1ST IF CIRCUITS
The mixer circuit converts the received signals to a fixed
frequency of the 1st IF signal with a 1st LO (VCO output)
frequency. By changing the PLL frequency, only the
desired frequency will be passed through a pair of crystal
filters at the next stage of the mixer.
2ND M IXER AND DEMODULATOR CIRCUl11S
CIRCUIT DESCRIPTION
The
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The received signals from the VHF RF circuit are mixed with
the 1st LO signal (VCO output signal) at the 1st mixer (OS2)
to produce a 4S.OS MHz 1st IF signal.
The 1st IF signal is applied to a pair of crystal filter (Fl2) to
suppress out-of-band signals. The filtered 1st IF signal is
amplified at the 1st IF amplifier (060) and is then applied to
the 2nd mixer circuit (IC20).
4-1-6 VHF RECEIVING VIA UHF DISPLAY
The transceiver has additional VHF RF and mixer circuits
for a VN para-watch function.
Some of the RF signals from the RF pre-amplifier (OSO) are
applied to the VN RF amplifier (07S) via the attenuator
circuit (06S) and bandpass filter (066).
signals are mixed with a 1st LO signal at the mixer circuit
(076) to produce an S7.6S MHz IF signal. The signal is
then applied to the UHF IF circuit (IC23 described in section
4-1-11).
The VHF VCO circuit (0113, 0114) in the UHF PLL circuit
generates the 1st LO signal for the VN para-watch function.
4-1-7 2ND IF AND DEMODULATOR CIRCUITS
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double superheterodyne system (which converts
received signals twice) improves the image rejection ratio
and obtains stable receiver gain.
The FM IF IC (IC20) contains the 2nd local oscillator, 2nd
mixer, limiter amplifier, noise amplifier, S-meter detector
and quadrature detector circuits, etc.
The 1st IF signal (4S.OS MHz) from the 1st IF amplifier
(060} is applied to the 2nd mixer section of IC20 (pin 16),
and is mixed with the 2nd LO signal (4S.SOS MHz) for
conversion to a 4SS kHz 2nd IF signal at the 2nd mixer
section.
The 2nd IF signal (4SS kHz) from the 2nd mixer section
(IC20 pin 3) passes through the ceramic filter (FIS} where
unwanted signals are suppressed. It is then amplified at the
limiter amplifier section (IC20 pin S) and applied to the
quadrature detector section (IC20 pin 8 and discriminator
XS} to demodulate the 2nd IF signal into AF signals.
AF signals output from IC20 (pin 9) are applied to the AF
amplifier circuit via the AF selector (IC22}.
3
13
Noise signal to noise detector circuit
4
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The amplified
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45.505 MHz
IC20 MC3372VM