Icom IC-F51 - Manual (Page 7)

4 - 1
SECTION 4 CIRCUIT DESCRIPTION
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 a resonator circuit while transmitting.
This circuit does not allow transmit signals to enter the
receiver circuits.
Received signals enter the antenna connector (CHASSIS;
J1) and pass through the low-pass filter (L1, L2, C1–C5).
The filtered signals are passed through the
λ
⁄ 4 type antenna
switching circuit (D5, D6, L5, L6) and then applied to the RF
circuit.
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, L7, L8). The
filtered signals are amplified at the RF amplifier (Q2) and
then passed through the another two-stage tunable band-
pass filters (D9, D10, L9, L11) to suppress unwanted sig-
nals. The filtered signals are applied to the 1st mixer circuit.
D4, D8–D10 employ varactor diodes, that are controlled by
the CPU via the D/A converter (IC6), 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 1ST IF CIRCUITS
(MAIN UNIT)
The 1st mixer circuit converts the received signal into fixed
frequency of the 1st IF signal with the PLL output frequency.
By changing the PLL frequency, only the desired frequency
passes through a crystal filter at the next stage of the 1st
mixer.
The RF signals from the bandpass filter are mixed with the
1st LO signals, where come from the RX VCO circuit via the
attenuator (R26–R28), at the 1st mixer circuit (Q3) to pro-
duce a 46.35 MHz 1st IF signal. The 1st IF signal is passed
through a monolithic filter (FI1) 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 amplifier (Q4).
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
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 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, 2nd local oscil-
lator, limiter amplifier, quadrature detector, active filter and
noise amplifier circuits. A 2nd LO signal (45.9 MHz) is pro-
duced at the PLL circuit by tripling it’s reference frequency
(15.3 MHz).
The 2nd IF signal from the 2nd mixer (IC1, pin 3) passes
through the ceramic filter (FI2) to remove unwanted hetero-
dyned frequencies. It is then amplified at the limiter amplifi-
er section (IC1, pin 5) and applied to the quadrature detec-
tor section (IC1, pins 10, 11) to demodulate the 2nd IF sig-
nal into AF signals.
The demodulated AF signals are output from pin 9 (IC1) and
applied to the AF circuit via the receiver mute circuit.
Mixer
16
Limiter
amp.
2nd IF filter
450 kHz
X2
15.3 MHz
45.9 MHz
IC1 TA31136FN
12
1st IF from the IF amplifier (Q4)
"RSSI" signal to the CPU (FRONT unit; IC401, pin 50)
11 10 9
8 7 5
AF signal "DET"
"SQIN" signal from the
D/A converter IC
(IC6, pin 2)
R5V
X2
2
Active
filter
Noise
detector
FM
detector
13
"NOIS" signal to the CPU (FRONT unit; IC401, pin 41)
RSSI
Noise
comp.
×3
PLL IC
IC4
Q19
1 2 FI2
3
• 2ND IF AND DEMODULATOR CIRCUITS