Pll Circuit; Power Supply Circuit; Overheating Protection Circuit; Reference Oscillator Circuit - Kenwood TM-V71A Service Manual

3-4. Overheating Protection Circuit

To protect thermal destruction of the power modules
(IC131, IC161), the voltage of the thermistors (TH131,
TH161) near the power modules (IC131, IC161) is monitored
by the MPU (IC918). If their temperature becomes high,
this circuit reduces the APC voltage to cool down the
temperature.

4. PLL Circuit

4-1. Reference Oscillator Circuit

Band A divides 12.8MHz inside the PLL IC (IC1) or band
B divides 16.8MHz inside the PLL IC (IC66) to generate a
5kHz or 6.25kHz reference frequency. The 16.8MHz signal
on IC66 side is tripled by Q547 to obtain the second local
oscillator for band B. The reference oscillation circuit is also
modulated to improve the modulation characteristics of the
DCS and 9600bps packet signal when it is transmitted.
4-2. Phase Comparator
Part of the band A VCO output is amplifi ed by Q5 and
goes to PLL IC (IC1). Also, a part of the band B VCO output
Band A
TCXO A
Mod Signal
Band B
TCXO B
Mod Signal

5. Power Supply Circuit

5-1. Microcomputers and Peripheral Circuits
■ Reset and Backup Circuits
The MPU reset signal detects a rising edge of the M5
line voltage with the reset IC (IC920). When the voltage
supplied to the transceiver decreases and the B line voltage
falls below the detection voltage of the voltage detection
IC (IC919), the MPU (IC918) detects it through the interrupt
terminal, stores the data in the EEPROM (IC916), and shuts
the power off.
CIRCUIT DESCRIPTION
X1
LPF
12.8MHz PLL IC
TCXO
IC1
X66 LPF
16.8MHz PLL IC
TCXO IC66
Tripler
Q547
x3
Fig. 8 PLL circuit
TM-V71A/V71E
is amplified by Q70 and goes to the PLL IC (IC66). The
pulse-swallow type PLL IC divides the input VCO oscillator
frequency using the data from the MPU (IC918). It compares
its phase with that of the reference frequency to make the
PLL synthesizer to generate the desired step.
4-3. Lock Voltage (VCO Control Voltage)
The phase differential comparator, output from the PLL
IC, results in a phase difference pulse. As for the result of
phase comparison with the PLL IC, a phase difference is
output as a pulse. This pulse is amplifi ed by a charge pump
(Q2, Q3 or Q67, Q68) and the ripples are removed by a
low-pass fi lter, then the signal is supplied as the oscillator
frequency control voltage for each VCO.
4-4. Unlock Detection Circuit
When the PLL is unlocked, a low state voltage is output
from the LD terminal (pin 8) of the PLL IC. This signal is
monitored by the MPU (IC918) to control transmission/
reception switching timing.
VCO A
Charge Pump
Mod Signal
LPF
Q2
VCO
A
Q3
VCO B
Charge Pump
Mod Signal
LPF
Q67
VCO
B
Q68
5-2. Voltage Detection Processing
The MPU (IC918) monitors and process various voltage
statuses at A/D ports of IC918.
The squelch voltage is input from the FM IC and a
change in the noise voltage is detected to control squelch.
The S-meter voltage is input from the FM IC to control
the S-meter display.
The thermistor voltage (temperature) is also detected
through the A/D port.
Q7 LOCAL
Q5
Q72 LOCAL
Q70
7