Kenwood TK-380 Service Manual Download

Page: 22 / 66

TK-380

5. Frequency Synthesizer Unit

5-1. Frequency synthesizer

The frequency synthesizer consists of the VCXO (X1), VCO

(A1), PLL IC(IC14) and buffer amplifiers.

The VCXO generates 16.8MHz. The frequency stability is

1.5ppm within the temperature range of -30 to +60˚C. The
frequency tuning and modulation of the VCXO are done to
apply a voltage to pin 1 of the VCXO. The output of the VCXO
is applied to pin 8 of the PLL IC.

The TK-380’s VCO consists of 2VCO and covers a dual range

of the 395.15~425.15MHz (E), 361.15~405.15MHz (E3), and
the 440~470MHz. The VCO generates 395.15~425.15MHz (E),
361.15~405.15MHz (E3), for providing to the first local signal in
receive. In TX, the pin 3 of the VCO goes low and the VCO
generates 440~470MHz (E), 406~450MHz (E3).

The output of the VCO is amplified by the buffer amplifier

(Q16) and routed to the pin 5 of the PLL IC. Also the output of
the VCO is amplified by the buffer amplifier (Q18) and routed
to the next stage according to T/R switch (D9, D23).

The PLL IC consists of a prescaler, fractional divider,

reference divider, phase comparator, charge pump. This PLL
IC is fractional-N type synthesizer and performs in the 40.50
or 60kHz reference signal which is eighth of the channel step
(5, 6.25 or 7.5kHz). The input signal from the pins 5 and 8 of
the PLL IC is divided down to the 40, 50 or 60kHz and compared
at phase comparator. The pulsed output signal of the phase
comparator is applied to the charge pump and transformed
into DC signal in the loop filter (LPF). The DC signal is applied
to the pin 1 of the VCO and locked to keep the VCO frequency
constant.

PLL data is output from DT (pin 75). CP (pin 19) and EP

(pin 47) of the microprocessor (IC19). The data are input to
the PLL IC when the channel is changed or when transmission
is changed to reception and vice versa. A PLL lock condition is
always monitored by the pin 31 (UL) of the microprocessor.
When the PLL is unlocked, the UL goes low.

CIRCUIT DESCRIPTION

attached, MSW is connected to GND at inside of SP-MIC. For
this reason, Q300 is turned OFF, the internal microphone is
muted, and only the input of the external microphone is supplied
to the microphone amplifier of the TX-RX unit.

The signal from microphone passes through the limitter

circuit in D8, Mic mute switch (Q17 is off in TX) and through
the low-pass filter (IC25 : 1/2), the high-pass filter, the ALC
circuit, the low-pass filter, the high-pass filter, and pre-
emphasis/IDC circuit in IC13. When encoding DTMF, mute
switch (Q13) is turned OFF for muting the microphone input
signal.

The signal passes through the D/A converter (IC8) for the

maximum deviation adjustment, and enters the summing
amplifier consisting of IC7 (1/2), and is mixed with the low speed
data from the CPU (IC19) and 9600bps DATA from Optional
Board Terminal.

The output signal from the summing amplifier passes

through the D/A converter (IC8) again and goes to the VCO
modulation input.

The other output signal from the summing amplifier passes

through the D/A converter (IC8) again for the BAL adjustment,
and the buffer amplifier (IC1 (2/2)), and goes to the VCXO
modulation input.

4-2. Drive and Final amplifier

The signal from the T/R switch (D9 is on) is amplified by the

pre-drive (Q18) and drive amplifier (Q20) to 50mW.

The output of the drive amplifier is amplified by the RF power

amplifier (IC100) to 4.0W (1W when the power is low). The RF
power amplifier consists of two stages MOS FET transistor.
The output of the RF power amplifier is then passed through
the harmonic filter (LPF) and antenna switch (D12, D401 is
on) and applied to the antenna terminal.

4-3. APC circuit

The APC circuit always monitors the current flowing through

the RF power amplifier (IC100) and keeps a constant current.
The voltage drop at R245, R247 and R249 is caused by the
current flowing through the RF power amplifier and this voltage
is applied to the differential amplifier (IC23 1/2).

IC23(2/2) compares the output voltage of IC23(1/2) with the

reference voltage from IC8, and the output of IC23(2/2) controls
the VGG of the RF power amplifier to make the both voltages
to same voltage.

The change of power high/low is carried out by the change

of the reference voltage. Q22,23 and 25 are turned on in
transmit and the APC circuit is active.

Fig. 7 Microphone amplifier

Fig. 8 Drive and final amplifier and APC circuit

HPF

LPF

HPF

IDC

PRE

EMP