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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 filter. The filtered signals are passed
through the U4 type antenna switching circuit (D4-D6) and
apply the RF circuit.
4.1.2 RF AND 1ST MIXER CIRCUITS
The 1st mixer circuit converts the received signal to fixed
frequency of the 1st lF signal with the PLL output frequency.
By changing the PLL frequency, only the desired frequency will
be passed through a pair of crystal filters at the next stage of
the 1st mixer.
The signals from the antenna switching circuit are passed
through the tunable bandpass filter (D8, L9) and amplified at
the RF amplifier (Q1). The amplified signals are again passed
through the tunable bandpass filter (D9, D1 1, L1 1 , L13) and
applied to the 1st mixer.
The filtered signals are mixed at the 1st mixer (Q2) with the 1st
LO signal coming from the Rx VCO circuit to produce 1st lF
signal. The 1st lF signal is passed through the matching circuit
(L15, L52) and the pair of crystal filters (Fl1). The liltered
signal is amplified atthe 1st lF amplifier (Q4) and applied tothe
2nd lF circuit.
4-1.3 2ND IF AND DEMODULATOR CIRCUITS
The 2nd mixer circuit converts the 1st lF signal to a 2nd lF
signal. A double-conversion
superheterodyne system
improves the image rejection ratio and obtains stable receiver
gain.
The 1st lF signal from Q4 is applied to the 2nd mixer section of
lC1 (pin 16) and is then mixed with the 2nd LO signal for
conversion to 455 kHz 2nd lF signal.
lC1 contains the 2nd mixer, 2nd local oscillator, limiter
amplifier, quadrature detector and audio amplifier. The local
oscillator section generates 21.345 MHz using X1.
The 2nd lF signal from the 2nd mixer (lC1 pin 3) passes
through the ceramic filter (Fl2) to suppress unwanted
heterodyned frequency signals. lt is then amplified at the
limiter amplifier section (lC1 pin 5) and applied to the
quadrature detector section (lC1 pins 7, 8 and X2) to
demodulate the 2nd lF signal into AF signals.
The AF signals are output from lC1 (pin 9) and are then applied
to the AF circuit.
. 2ND lF AND DEMODULATOR
CIRCUITS
4-1-4 AF CIRCU|T
AF signals from lC1 (pin 9) are amplified at the AF amplifier
(lC2) and are then applied to lC3. lC3a/b are high-pass fitters
yrthose characteristics are controlled by the "HFSW" line.
When "HFSW" is "High," the cut off frequency is shifted higher
to remove CTCSS or DTCS signals.
The filtered signals from lC3 (pin 1) are passed through the de.
emphasis circuit (R68, C74) with frequency characteristics of
-6 dB/octave, and are then applied to the electronic volume
controller (lC7) via the AF mute switch (Q6).
Output signals from lC7 (pin 9) are applied to the AF power
amplifier (lC8) to drive the speaker.
4.1-5 RECEIVE MUTE CIRCUITS
o NOISE SOUELCH
A squelch circuit cuts out AF signals when no RF signal is
received. By detecting noise components in the AF signals,
the squelch circuit switches the AF mute switch.
A portion of the AF signals from lCl (pin g) are apptied to the
active filter in lC1 (pin 10). Noise components about 10 kHz
are amplified and output from pin 11.
The noise signals from lC1 (pin 1 1) are passed through a ievel
controller (f C5 pins2l,22), and are then converted to the pulse
signal (NOIS) at Q9 and Q10.
The "NOIS" signal from Q10 is applied to the CPU (1C20 pin
19). Then the CPU analyzes the noise condition and controls
the "AMUT" and "SP" ports to cut off the AF signal using AF
switches (Q6, O7).
2nd.lF signal
455 kHz
4 -1
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