Icom IC-Q7A Service Manual page 7

SECTION 4
CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUITS
4-1-1 ANTENNA SWITCHING CIRCUIT (RF UNIT)
The RF signals from the antenna connector pass through the
antenna switching circuit (D3, D6, D9). The passed signals
are then applied to either Duplexer or RX band switching cir-
cuit.
4-1-2 DUPLEXER CIRCUIT (RF UNIT)
The transceiver has a duplexer (low-pass and bandpass fil-
ters) on the frist stage from the antenna switcing diode to
separate the signals into VHF and UHF signals.
* RF signals below 175.0 MHz
The RF signals below 175.0 MHz are passed through the
low-pass filter (L57-L59, С8-С11, C67) and are applied to
the VHF RF circuit.
• RF signals 330.0 МН2--469.995 MHz
The 330 MHz-469.995 MHz RF signals are passed through
the bandpass filter (13-15, C21-C24, C218) and are applied
to the UHF RF circuit.
4-1-3 VHF/UHF RF CIRCUIT (RF UNIT)
The VHF/UHF RF circuit amplifies the received signals with-
in the range of frequency coverage and filters out-of-band
signals.
(1) VHF RF CIRCUIT
The filtered signals from the low-pass filter circuit are ampli-
fied at the VHF RF amplifier (Q14) through the Tx/Rx swich-
ing diode (D15), and are then passed through the two stages
of tunable bandpass filters (D1, D2, L28, L1). The filtered sig-
nals are applied to the 1st mixer circuit (IC1, pin 1) via the
band switching diode (D25).
(2) UHF RF CIRCUIT
The filtered signals from the bandpass filter circuit are ampli-
fied at the UHF RF amplifier (Q35) via the Tx/Rx switching
diode (D27) and are passed through the two stage of tunable
bandpass filters (D4, D5, L61, L60). The filtered signals are
applied to the 1st mixer circuit via the band switching diode
(D29). The filtered signals are applied to the 1st mixer circuit
(IC1, pin 1).
The tunable bandpass filters employ varactor diodes (D1, D2,
D4, D5) 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 CPU (LOGIC unit;
IC11, pin 9).
4-1-4 RX BAND SWITCHING CIRCUIT (RF UNIT)
The signals from the antenna connector pass through the
antenna switching circuit (D6, D9). The signals are then
applied to the RX RF circuit via RX the band switching circuit
(D11, D13, D31) which suppress out-of-band signals.
4-1-5 RX RF CIRCUIT (RF UNIT)
The RX RF circuit amplifies the received signals within the
range of frequency coverage and filters out-of-band signals.
(1) 470.0 MHz-1026.995 MHz signals
RF signals (470 MHz-1026.995 MHz) from a band switching
circuit (011) pass through a bandpass filter (L7, L8, L42,
C32, C33, C35-C39, C145) and are amplified at an RF
amplifier (Q24). The amplified signals are then applied to the
1st mixer circuit (IC1) through the band switching diode
(D32).
(2) 30.0 MHz-117.995 MHz, 175 MHz-329.995 MHz
The 30.0 MHz – 117.995 MHz, 175 MHz – 329.995 MHz sig-
nals pass through a low-pass filter (L9, L10, C40-C43) via
the band switching diode (D13), and are then amplified at the
RF amplifier (Q36). The amplified signals are applied to the
1st mixer circuit (IC1) via the band switching diode (D34).
(3) 1027.0 MHz-1309.995 MHz
The 1027.0 MHz-1309.995 MHz signals pass through a
bandpass filter (L11, L12, L43, C4, C5, C45 – C51) via the
band switching diode (D13), and are then amplified at the RF
amplifier (Q36). The amplified signals are applied to the 1st
mixer circuit (IC1) via the band switching diode (D36).
4-1-6 1ST MIXER CIRCUIT (RF UNIT)
The 1st mixer circuit converts the received RF signals to a
fixed frequency of the 1st IF signal with a PLL output fre-
quency. By changing the PLL frequency, only the desired fre-
quency will pass through the bandpass filter at the next stage
of the 1st mixer.
The filtered RF signals are mixed with 1st LO signals at the
1st mixer (IC1) to produce a 266.7 MHz 1st IF signal. The 1st
IF signal is output from pin 6, and passed through the band-
pass filter (ҒІ1) to suppress unwanted harmonic compo-
nents. The filtered 1st IF signal is applied to the 2nd mixer
circuit.
The 1st LO signals are generated at the V VCO (Q32, D45)
or U VCO (Q28, Q30, D54) circuit (according to the receiving
frequency), and are applied to the 1st mixer (IC1, pin 3)
directly or passing through the doubler circuit (Q31) after
being amplified at the buffer amplifier (IC4, Q40).