Kenwood TK-3118 Service Manual page 14

TK-3118
2) VCO
The operating frequency is generated by Q5 in transmit
mode and Q4 in receive mode. The oscillator frequency is
controlled by applying the VCO control voltage, obtained from
the phase comparator, to the varactor diodes (D6 and D9 in
transmit mode and D7 and D11 in receive mode). The T/R pin
is set high in receive mode causing Q8 and Q9 to turn Q5 off,
and turn Q4 on . The T/R pin is set low in transmit mode. The
outputs from Q4 and Q5 are amplified by Q7 and sent to the
buffer amplifiers.
PLL DATA
12.8MHz
3) UNLOCK DETECTOR
If a pulse signal appears at the LD pin of IC1, an unlock
condition occurs, and the DC voltage obtained from D2, R4,
and C12 causes the voltage applied to the UL pin of the
microprocessor to go low. When the microprocessor detects
this condition, the transmitter is disabled, ignoring the push-to-
talk switch input signal. ( See Fig.5)
4. Transmitter
1) Transmit audio
The modulation signal from the microphone is amplified by
IC308 (A/4), passes through a preemphasis circuit, and
amplified by the other IC308 (B/4) to perform IDC operation.
The signal then passes through a low-pass filter (splatter fiIter)
IC308 (C/4 and D/4) and cuts 3kHz and higher frequencies.
The resulting signal goes to the VCO through the VCO
modulation terminal for direct FM modulation. (See Fig. 6)
2) QT/DQT encoder
A necessary signal for QT/DQT encoding is generated by
IC300 and FM-modulated to the PLL reference signal. Since
the reference OSC does not modulate the loop characteristic
frequency or higher, modulation is performed at the VCO side
TK-260 :K, K2
by adjusting the balance. (See Fig. 6)
14
CIRCUIT DESCRIPTION /
PLL IC IC1
5kHz/6.25kHz
I/N
PHASE
COMPARATOR
REF OSC
I/M
5kHz/6.25kHz
Fig. 4 PLL circuit / 图4 锁相环电路
IC1
LD
PLL IC
Fig. 5 Unlock detector circuit / 图5 失锁检测器电路
电路说明
2) 压控振荡器
在发射模式中通过Q 5 产生操作频率，在接收模式中通过Q 4
产生操作频率。通过相位比较器到变容二极管 ( 在发射模式中
为D6和D9，在接收模式中为D7和D11) 采用压控振荡器控制电
压来控制振荡频率。在接收模式中，由于Q 8 和Q 9 切断Q 5 并且
导通Q 4 ，所以发射/ 接收管脚设置为高电平。在发射模式中，
发射/接收管脚设置为低电平。Q4 和Q5 的输出通过Q7被放大并
被发送到缓冲放大器。
D6, 9
LPF
CHARGE
PUMP
D7, 11
3) 失锁检测器
如果IC1的LD管脚上出现高电平，则产生失锁状态，并从D2,
R4获得直流电压，且C12产生的提供给微处理器UL管脚的电压
降低。当微处理器检测到此情况时，不能进行发射，无视通话
转换开关输入信号。(参见图5)
5C
R4
IC300
D2
UL
C12
MPU
4. 发射部
1) 发射音频
来自于话筒的调制信号通过IC308 (A/4) 被放大，经过一个预
加重电路，并通过另一个IC308 (B/4) 放大后进行IDC处理。然
后信号通过一个低通滤波器 (分离滤波器) IC308 (C/4和D/4) 并滤
除比3kHz频率更高的部分。得到的信号进入压控振荡器直接进
行调频调制。(参见图6)
2) QT/DQT编码器
QT/DQT编码所需的信号通过IC300产生，被锁相环电路的基
准频率调整。由于基准振荡器不能对频率环路特性外的频率进
行调制，因此通过分配器在压控振荡器一侧进行调制。(参见图
6)
Q5
Q3
TX VCO
RF AMP
Q7
BUFF AMP
Q4
RX VCO
Q8, 9
T/R SW