Chapter 3 Electrocircuit - Kirisun PT4200 Service Manual

Chapter 3 Electrocircuit

3.1 Frequency Configuration
Figure3.1 Frequency Configuration
This radio adopts the 2nd Mixer, the 1st IF 49.95MHz, the 2nd
IF 450kHz.
The receiver's first local oscillation is generated by the
frequency synthesizer. The second local oscillation is generated by
the crystal oscillator.
The transmitter signals are generated by frequency synthesizer.
The reference frequency of frequency synthesizer is generated
by TCXO.
3.3 Receiver (RX)
Figure 3.2 Receiver Illustration
The Receiver Front Terminal
Signals from the antenna pass through the RX/TX switch (D1,
D2, D4, D5); and then undesirable
filtered out at the band pass filter (BPF) consisting of C227, L8, L15,
C70, C126, C218, L9, C217, L10, C127; then signals are amplified
at the low noise amplifier (LNA) consisting of Q20 and its peripheral
components.
The output from the LNA passes the BPF consisting of L5, L6,
L7, C228, C124, and C132 for filtering and then is sent to the first
grade frequency mixer (Q19).
MCU produces output voltages and alters the capacitance of
the variable capacitor diodes D21, D22, D23, D24, D26, D30 to
control the center frequency of the band-pass filter.
The First Frequency Mixer
After mixing the receiving signals and the first local oscillation
signals from the frequency synthesizer, the 1st IF signals
49.95MHz are generated. The first IF signals pass the crystal
filter (XF1, XF2) , which will filter the signals of adjacent channel
and those out of band.
IF Circuit
The 1st IF signals from the crystal filter are amplified at the first
IF amplifier (Q21), and then are sent to the IF processing IC (IC5,
MC3361). The IF IC consists of the 2nd frequency mixer, the 2nd
local oscillator, IF amplifier, limiter, phase frequency detector, and
noise amplifier.
out-of-band signals will be
PT4200
IC5 inner circuit and X3 (49.5MHz) compose the 2nd local
oscillator. The 2nd local oscillation (49.5MHz) and the 1st IF signal
(49.95MHz) are mixed at IC5 to generate the 2nd IF 450MHz .
After the 2nd IF signal is amplified and its amplitude is limited at
IC5, and then filtered at porcelain filter (CF1,450kHz), IC5
demodulates and sends out audio signals.
Figure 3.3IF System
Receiver Audio Signal Processing
IC6 and its peripheral circuit compose the receiver audio signal
processing circuit. IC5 sends audio signals to IC6-C for
amplification, and then to IC4 (CTCSS signaling filtering circuit) and
IC6-D. After the signals are amplified, de-emphasized, filtered at
IC6 and other cells, the HF (high frequency) and LF (low frequency)
will be eliminated, and the remaining 300-3000Hz audio is sent to
Q9 for amplification and then to volume potentiometer for
adjustment and finally sent to the audio amplifier (Ic8).
Figure 3.4 Receiver Audio Processing
Squelch Circuit
Demodulation output from IC5 is sent to the selection frequency
noise amplifier, which consists of IC5 inner noise amplifier and
C107, R124, R152, C33, and C35, to be filtered off the noise. After
being amplified at Q7, noise is sent to D17 and D18 for wave
checking and then sent to MCU, which determines the noise
volume to control the squelch.
Audio Amplifier
IC8 and peripheral components construct the audio amplifier.
Receiving audio signals, voice indication signals, indication tone
signals and warning tone signals are collected for audio
amplification to drive the speaker. Warning tone has no volume
control.
When AFCO is at the high level, Q37, Q8, Q33, and Q38 will be
connected, and IC8 starts working and voice comes out from the
speaker.
Q36: Receiving audio signal switch
Q35: Warning tone switch
Q25: Indication tone switch
SERVICE MANUAL
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