Circuit Description; Receiver Circuits - Icom IC-F2610 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT
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
circuits.
Received signals enter the antenna connector (J1) and pass
through the low-pass filters (L1–L3, C2, C3, C8–C10, C415).
The filtered signals are passed through the
switching circuit (D5, D6, L5) and then applied to the RF cir-
cuit.
4-1-2 RF CIRCUIT
The RF circuit amplifies signals within the range of frequen-
cy coverage and filters out-of-band signals.
The signals from the antenna swiching circuit pass through
the two- stage tunable bandpass filters (D7, D8, L8, L9). The
filtered signals are amplified at the RF amplifier (Q1) and
then enter the another three-stage bandpass filters
(D9–D11, D514, L12, L13, L507) to suppress unwanted sig-
nals. The filtered signals are applied to the 1st mixer circuit.
D7–D11 and D514 employ varactor diodes, that are con-
trolled by the PLL lock voltage, to track the bandpass filter.
These varactor diodes tune the center frequency of an RF
pass band for wide bandwidth receiving and good image
response rejection.
4-1-3 1ST MIXER AND IST IF CIRCUITS
The 1st mixer circuit converts the received signal to fixed
frequency of the 1st IF signal with the PLL output frequency.
By changing the PLL frequency, only desired frequency will
pass through a pair of crystal filters at the next stage of the
1st mixer.
The RF signals from the bandpass filter are mixed with the
1st LO signals, which come from the Rx VCO circuit via the
buffer amplifier (Q3), at the 1st mixer circuit (Q2) to produce
a 30.875 MHz 1st IF signal. The 1st IF signal is passed
• 2nd IF and demodulator circuits
AF signals
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CIRCUIT DESCRIPTION

λ
⁄ type antenna
4
X2
Discriminator
+8V
8 7
Limiter
amp.
Quad.
detector
Active
filter
9 10
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through the matching circuit (L14, L15) and a pair of crystal
filters (FI1a/b) in order to obtain selection capability and to
pass only the desired signals. The filtered signal is applied
to the 2nd IF circuit after being amplified at the 1st IF ampli-
fier (Q4).
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
The 2nd mixer circuit converts the 1st IF signal into a 2nd IF
signal. The double-conversion superheterodyne system
(which convert receive signals twice) improves the image
rejection ratio and obtains stable receiver gain.
The 1st IF signal from the IF amplifier (Q4) is applied to the
2nd mixer section of the FM IF IC (IC1, pin 16) and is then
mixed with the 2nd LO signal for conversion into 455 kHz
2nd IF signal.
IC1 contains the 2nd mixer, 2nd local oscillator, limiter ampli-
fier, quadrature detector, active filter and noise amplifier cir-
cuit. The local oscillator section and X1 generates the
30.420 MHz 2nd LO signal.
The 2nd IF signal from the 2nd mixer (IC1, pin 3) passes
through the ceramic filters (FI3 and FI4) during narrow chan-
nel spacing selection or passes through FI4 (bypassing FI3)
only during wide channel spacing selection to suppress
unwanted heterodyne frequencies signals via the N/W
switches (D501, D502).
The filtered signal is applied to the quadrature detector sec-
tion in the FM IF IC to demodulate the 2nd IF signal into AF
signals using the ceramic discriminator (X2) after being
amplified at the limiter amplifier section (pin 5). The demod-
ulated AF signals are output from pin 9 of the IC and applied
to the AF circuit via the receiver mute citcuit.
The N/W switches (D501, D502) select a ceramic filter (FI3
or bypass), and the other N/W switch (Q61) adjusts the input
level of the FM IF IC (IC1, pin 8) to switch the bandwidth
depending on the NWC signal from the CPU (IC20, pin 57).
When NWC signal is high level , bandwidth setting is wide.
2nd IF filter
FI3 FI4
5 3 1, 2
Noise
amp.
Mixer
11 13 16
RSSI signal to the CPU (IC20)
4 - 1
X1
30.875 MHz
IC1
MC3372
1st IF from the IF amplifier (Q4)