4 - 2
4-1-4 2ND IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 2nd mixer circuit converts the 1st IF signal into a 2nd IF
signal. A double superheterodyne system (which converts
receive signals twice) improves the image rejection ratio and
obtains stable receiver gain.
The FM IF IC (IC6 for channel 70, IC1 for other channels)
contains the 2nd local oscillator, 2nd mixer, limiter amplifier,
quadrature detector, and noise detector circuits, etc.
• CHANNEL 70 CIRCUIT
The 1st IF signal from the 2nd IF amplifier (Q10) is applied
to the 2nd mixer section of FM IF IC (IC6, pin 16), and is
mixed with a 21.25 MHz 2nd LO signal, which is generated
at the 2nd oscillator section in IC6 and X3, to produce a 450
kHz 2nd IF signal.
The 2nd IF signal from IC6 (pin 3) is passed through the
ceramic filter (FI6), which unwanted signals are suppressed,
and is then applied to the 2nd IF (limiter) amplifier in IC6 (pin
5). The signal is applied to the FM detector section in IC6 for
demodulating into AF signals.
The FM detector circuit employs a quadrature detection
method (linear phase detection), which uses a ceramic dis-
criminator (X2) for phase delay to obtain a non-adjusting cir-
cuit. The detected signal from IC6 (pin 9) is applied to the AF
circuit.
• OTHER CHANNELS CIRCUIT
The 1st IF signal from the 2nd IF amplifier (Q2) is applied to
the 2nd mixer section of FM IF IC (IC1, pin 16), and is mixed
with a 30.6 MHz 2nd LO signal, which is generated at the
PLL circuit using the reference frequency (15.3 MHz), to pro-
duce a 450 kHz 2nd IF signal.
The 2nd IF signal from IC1 (pin 3) is passed through the
ceramic filter (FI3), which unwanted signals are suppressed,
and is then applied to the 2nd IF (limiter) amplifier in IC1 (pin
5). The signal is applied to the FM detector section in IC1 for
demodulating into AF signals.
The FM detector circuit employs a quadrature detection
method (linear phase detection), which uses a ceramic dis-
criminator (X1) for phase delay to obtain a non-adjusting cir-
cuit. The detected signal from IC1 (pin 9) is applied to the AF
circuit.
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