Kenwood TK-7150 Service Manual page 14

TK-7150
3-4. Automatic Power Control, Circuit and Transmitter
The automatic power control (APC) circuit stabilizes the
transmitter output power at a predetermined level, and con-
sists of forward/reflected power detector circuits. The for-
ward/reflected power detector circuits detects forward RF
power and reflected RF power to DC voltage, and consists of
a C coupling type detection circuit, RF detector D5/D6, and
DC amplifier IC2 (A/2).
The voltage comparator (IC2 B/2) compares the voltage
obtained by the above detected voltage with a reference volt-
age, set using the microprocessor and IC718 and IC715.
An APC voltage proportional to the difference between
the sensed voltage and the reference voltage appears at the
output of IC2. This output voltage controls the gate voltage
for the drive amplifier Q4 and final amplifier Q5, which keeps
the transmitter output power constant.
4. PLL Frequency Synthesizer
The transmit signal and the receive first L.O. signal are
generated by the PLL digital frequency synthesizer. The fre-
quency synthesizer consists of a transmitter voltage con-
trolled oscillator (TX VCO; Q507), a receiver voltage con-
trolled oscillator (RX VCO; Q509), a buffer amplifier (Q513
and Q515), an RF amplifier (Q516), a low-pass filter (Q501,
Q503 and Q504), a PLL IC (IC501), and TX VCO/RX VCO
switches (Q510 and Q512).
In the transmit signal mode, an operating frequency pro-
gramming data is sent to IC501, from the CPU (IC701), to set
the programmable counter within IC501. Q512 is turned on
to activate the TX VCO and the output signal of the TX VCO is
amplified by Q513 and Q516.
The signal is then divided down in frequency, at the pro-
grammable counter in IC501, to 5.0kHz or 6.25kHz, 7.5kHz
which is compared in phase with a 5.0kHz or 6.25kHz, 7.5kHz
reference signal, derived from 16.8MHz VCXO (X501) and a
8CL
MO
Q507
TX VCO
STR SW Q512
SW SW
Q514 Q510
8CL
Q509
RX VCO
12CL 8CL
IC501
LPF
PLL IC
Q501
Q503
Q504
IC701
CPU
Fig. 4 PLL circuit
14
CIRCUIT DESCRIPTION
8CL 8C 8T
D513
Q513 Q515
D514
BUFF BUFF
SW
AMP AMP
RX
8R
Q516
AMP 5C
X501
16.8MHz
LPF VCXO
MB
Q518 Q519
SW SW
1/3360 or a 1/2688, 1/2240 fixed counter in IC501, at the
phase comparator in IC501. The VCXO operates at 16.8MHz
and its frequency stability is maintained within 2.5ppm (tem-
perature range of –30 to +60 degrees).
The phase comparator output signal is fed into a low-pass
filter (Q501, Q503 and Q504) before being applied tot the TX
vco as a frequency control voltage. This low-pass filter's
power is supplied by the DC/DC converter (Q502, Q505,
Q506, Q508, and Q511). If an unlock condition occurs in the
phase locked loop, this condition is detected by Q518 and
Q519. This cause the transmitter 8V supply cut off, resulting
in the prevention of an unauthorized transmission.
The transmitter modulation signals (processed Mic. audio
and sub-audible signalling) are applied to the TX VCO for fre-
quency modulation.
In the receive mode, the VCO is substituted with Q509
(RX VCO) and it generates the receiver first local oscillator
signal according to the data sent from the CPU (IC701). The
basic operation of the synthesizer remains the same.
5. Control Circuit
The control circuit mainly consists of CPU, memory cir-
cuit, DSP circuit, and power supply circuit.
5-1. CPU
The CPU (IC701) controls the flash ROM (IC705), the DSP
(IC710), the receiver circuit, the transmitter circuit, the con-
trol circuit, and the display circuit and transfers data to or
from an external device.
5-2. Memory Circuit
IC705 has a flash ROM with a capacity of 4M bits that
contains the control program for the CPU, the signal process-
ing program for DSP and data such as channels and operating
features.
This program can be easily written from an external de-
vices. Data such as the operating status are programmed
into the EEPROM (IC704).
TX
IC704
EEPROM
IC103
Reset IC
X700
12.288MHz
5C
Q700 SW
BEAT
SHIFT
IC711
INV
IC705
Flash ROM
IC703
232C driver
IC701
IC718
CPU
D/A IC
IC700/702
Shift register IC
Interface IC IC706~709
DSP IC710
CODEC IC IC713
Fig. 5 Control circuit