Main Unit; Circuit Description; Receiver Circuits - Icom IC-F510 Service Manual

SECTION 4

4-1 RECEIVER CIRCUITS

4-1-1 ANTENNA SWITCHING CIRCUIT (MAIN UNIT)
The antenna switching circuit functions as a low-pass filter
while receiving and as resonator circuit while transmitting.
The circuit does not allow transmit signals to enter receiver
circuits.
Received signals from the antenna connector (CHASSIS;
J1) are passed through the low-pass filter (L1–L3, C1, C2,
C6–C8, C526), and are then applied to the
switching circuit (D2, D3, D5).
While receiving, no voltage is applied to D2, D3 and D5.
Thus, the receive line and the ground are disconnected and
received signals are applied to the RF circuits.
4-1-2 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 two-stage tunable bandpass filters (D4, D8). The filtered
signals are amplified at the RF amplifier (Q2) and then
passed through another two-stage bandpass filters (D9,
D10) to suppress unwanted signals. The filtered signals are
applied to the 1st mixer circuit (Q3).
The tunable bandpass filters (D4, D8–D10) employ varactor
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 CPU (IC20) via the
D/A converter (IC7, pins 1–4).
• 2ND IF DEMODULATOR CIRCUIT
24
IC6
23
Quadrature
AF signals

CIRCUIT DESCRIPTION

λ
⁄ type antenna
4
8 7
Active
filter
Limiter
amp.
detector
9 10 11
X1 Discriminator
4-1-3 1ST MIXER AND 1ST IF CIRCUITS

(MAIN UNIT)

The 1st mixer circuit converts the received signals to a fixed
frequency of the 1st IF signal with the PLL output frequency.
By changing the PLL frequency, only the desired frequency
will pass through a MCF (Monolithic Crystal Filter; FI1) at
the next stage of the 1st mixer.
The RF signals from the bandpass filter are applied to the
1st mixer circuit (Q3). The applied signals are mixed with
the 1st LO signal coming from the RX VCO circuit (Q13) to
produce a 46.35 MHz 1st IF signal. The 1st IF signal passes
through a MCF (FI1) to suppress out-of-band signals. The
filtered signal is amplified at the 1st IF amplifier (Q4) and is
then applied to the 2nd IF circuit.
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double-conversion superheterodyne system im-
proves the image rejection ratio and obtains stable receiver
gain.
The 1st IF signal from the 1st 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 to be converted into a
450 kHz 2nd IF signal.
The FM IF IC (IC1) contains the 2nd mixer, limiter amplifier,
quadrature detector, active filter and noise amplifier etc.
A 2nd LO signal (45.9 MHz) is produced at PLL circuit by
tripling it's reference frequency (15.3 MHz).
2nd IF filter
450 kHz
FI2
5 3
Noise
amp.
Noise
detector
Noise
comparator
13
NOIS signal to the CPU (IC20)
5V
4 - 1
Q34
45.9 MHz
BPF 3
2
X2
15.3 MHz
Mixer
IC1
TA31136FN
16
1st IF from the IF amplifier (Q4)