4
control.
Speaker Impedance: 16
Q405: Receiving Audio Signal Switch
Any terminal of the speaker cannot be connected with the
ground!
The IC204 demodulated output audio signals may contain CTCSS
(continuous tone coded squelch system) and DCS (digital coded
squelch). The frequency spectrum of CTCSS/DCS is 2-250Hz. The
filtering circuit constructed by IC405 can filter out the signals out of
the CTCSS/DCS frequency spectrum to ensure MCU to decode
CTCSS/DCS more precisely.
Transmitter Power Amplifier
Ω
Notice:
CTCSS Signal Filtering
3.3 Transmitter (TX)
Figure 3.7 Power Amplifier and Antenna Switch Diagram
The modulated RF signals from VCO are amplified at Q101, Q102,
and Q103 and then are sent to Q104 for amplification. Q104 output
power: 4.5W.
The Q103 and Q104 gate offset is controlled by APC circuit.
Changing gate-offset voltage can control the transmitter output
power conveniently.
APC (Auto Power Control)
Figure 3.8 APC Circuit
R167, R168, and R169 are the amplifier current checker; IC101A is
the sample amplifier of the amplification current; IC101B is the
power comparison amplifier.
If the transmitter output power is too large, the amplifier current will
increase, IC101A output will mount, IC101B output voltage
decrease, the offset voltage added to Q103 and Q104 will decrease,
and then the transmitter output power will decrease. Vice versa,
such can ensure steady transmitter output power in different
working circumstances.
MCU changes the input power to IC101B to set the power.
R167, R168, and R169 are the amplifier current checker; IC3A is
the sample amplifier of the amplification current; IC3B is the power
comparison amplifier.
If the transmitter output power is too big, the amplifier current will
increase, IC3A output will mount, IC3B output voltage decrease, the
offset voltage added to Q11 and Q12 will decrease, and then the
Transmitter Audio Signal Processing
transmitter output power will decrease. Vice versa, such can ensure
steady transmitter output power in different working circumstances.
MCU changes the input power to IC3B to set the power.
Transmitter Audio Signal Processing
Figure 3.9 Transmitter Audio Circuit
IC402 and IC408 and their peripherals components construct the
transmitter audio processing circuit. After the audio signals from
MIC are amplified, one route is sent for demodulation (VOX signals)
and then sent to MCU; the other one route passes AGG circuit to
IC408, where the amplitude of signals is limited, and signals are
filtered and amplified, and then are sent together with CTCSS/DCS
to VCO for modulation.
J2 is the external MIC socket. When the external MIC is in use, the
inner MIC will be cut off automatically, but inner PTT will remain
activated.
3.4 Frequency Synthesizer
Figure 3.10 Frequency Synthesizer
The radio adopts PLL (Phase Locked Loop) frequency synthesizer.
The frequency synthesizer consists of standard oscillator, voltage
controlled oscillator (VCO), programmable frequency demultiplier,
phase comparator, and low pass filter.
Q302
Q307
L304
L310
D304
D302
D310
D308
C309
C310
C308
C307
C322
C323
and C321 constitute
VCO.
IC3 01 (M B15E03SL)
is P LL in tegrated ci rcuit, includ ing
programmable reference frequency demultiplier, programmable
swallowing frequency demultiplier, phase comparator, and charge
pump.
C326
R316
C328
C327
R317
R318
and R319 construct
the low pass filter.
The standard frequency is supplied by X301 (TCXO, 13MHz).
The
stan dard frequency from TCXO (Temperature Con trol
Transistor Oscillator) are demultiplied by the programmable
reference frequency demultiplier at IC301 to acquire 6kHz or
6.5kHz reference frequency (controlled by MCU according to the
preset channel frequency).
The oscillation frequency from VCO is sent to IC1, and demultiplied
by swallowing frequency demultiplier and compared with reference
frequency to acquire the error signals. Then pass the low pass filter
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PT5200 SERVICE MANUAL
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