Kirisun PT6800, Руководство по обслуживанию

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PT6800 Service Manual
3
Please refer to Chapter 7 Adjustment.
Chapter 4 Circuit Description
4.1 Preface
This portable radio is FM trunking portable equipment.
4.2 Frequency Configuration
Figure 4.1 Frequency Configuration
The receiver adopts double mixing. The first IF is
51.65MHz and the second IF is 450KHz.
The first local oscillator signal of the receiver is generated
by the frequency synthesizer. The second local oscillator signal
is generated by quadrupler X1.
The transmitter signal is directly produced by the frequency
synthesizer.
The reference frequency of the frequency synthesizer is
provided by TCXO.
4.3 Principle of Receiver (RX)
Figure 4.2 Principle of Receiver
Front End
The signal coming from the antenna passes through the
RX/TX switch circuit (D1, D39, D7, and D8), and passes
through a BPF consists of two LCs to remove unwanted
out-of-band signal, and then is routed to the low noise amplifier
(LNA) consists of Q18 and its peripheral components where it
is amplified.
Output signal from the LNA passes through a BPF consists
of three LCs to further remove unwanted out-of-band signal,
and then goes to the first mixer (Q19).
AGC Circuit
AGC circuit, which consists of Q17 and its peripheral
circuit, will work to reduce the gain of Q18 only when the input
signal is oversize.
First Mixer
The receiving signal from LNA is mixed with the first local
oscillator signal from the frequency synthesizer to produce the
first IF signal (51.65MHz).
IF Circuit
The first IF signal passes through crystal filter (XF1) to
remove the adjacent channel signal and signal outside the
adjacent channel. Then the filtered signal is amplified by the
first IF amplifier (Q20), and is routed to the IF processing IC
(IC4, TA31136).
IF IC consists of the second mixer, IF amplifier, limiter,
discriminator, noise amplifier, and audio low pass filter.
Signal (12.8MHz) from X1 is multiple-amplified by Q15
and its peripheral circuit to produce the second local oscillator
signal (51.2MHz). Then the second local oscillator signal
(51.2MHz) is mixed with the first IF signal (51.65MHz) in IC4
to generate the second IF (450KHz). And then the second IF
signal is amplified and limited in IC4, filtered in the ceramic
filter (CF1 or CF2, 450KHz), and demodulated in IC4. After
that, the demodulated signal is routed to the audio circuit to
output audio signal.
The selection circuit of the second IF filter consists of CF1,
CF2, D18, D19 and the peripheral circuit. When the radio is set
to wideband, CF2 is put through and takes effect, while CF1 is
cut off; when the radio is set to narrowband, CF1 is put through
and takes effect, while CF2 is cut off.
Squelch Circuit
The demodulated signal from IC4 is sent to the internal
noise amplifier in IC4. Then the resulting signal is further
amplified in Q21 and demodulated in D22, and then the
resulting DC level is routed to the MCU squelch control circuit.
This voltage is in inverse proportion to the input signal.
4.4 Principle of Transmitter (TX)
TX Power Amplification