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Frequency Configuration
The receiver utilizes double conversion. The first IF is
49.95MHz and the second IF is 450kHz. The first local oscil-
lator signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the neces-
sary frequencies. Figure 1 shows the frequencies.
Receiver System
The receiver is double conversion superheterodyne. The
frequency configuration is shown in Figure 1.
■ Front-end RF Amplifier
An incoming signal from the antenna is applied to an RF
amplifier (Q26) after passing through a transmit/receive
switch circuit (D31 is off) and a band pass filter (L36, L38
and varactor diodes: D25, D26). After the signal is amplified
(Q26), the signal is filtered through a band pass filter (L30,
L32 and varactor diodes: D21, D22) to eliminate unwanted
signals before it is passed to the first mixer.
The voltage of these diodes are controlled by tracking the
CPU (IC6) center frequency of the band pass filter. (See Fig. 2)
■ First Mixer
The signal from the RF amplifier is heterodyned with the
first local oscillator signal from the PLL frequency synthe-
sizer circuit at the first mixer (Q21) to create a 49.95MHz
first intermediate frequency (1st IF) signal. The first IF signal
is then fed through two monolithic crystal filters (MCFs :
XF1) to further remove spurious signals.
ANT
D30,31
ANT
SW
IC3
D/A
CIRCUIT DESCRIPTION / 电路说明
ANT
1st
MIX
ANT
RF
SW
AMP
RX
POWER
RF
AMP
AMP
TX
Fig. 1 Frequency configuration / 图1 频率构成
L36,38
L30,32
Q26
D25,26
D21,22
BPF
RF AMP
BPF
TV
1/2 divider
CPU
1st local
OSC (VCO/PLL)
Fig. 2 Receiver system / 图2 接收部系统
频率构成
接收部采用二次变频超外差方式。第一中频为49.95MHz，
第二中频为450kHz。第一本振频率信号由锁相环电路提供。
发射部由锁相环电路直接产生所需要的频率。 图 1 显示各种
频率。
CF 450kHz
MCF
49.95MHz
AF
IF SYSTEM
PA
50.4MHz
X3
multiply
TCXO
16.8MHz
MIC
1/2
PLL/VCO
AMP
接收部系统
接收部为二次变频超外差方式。频率构成如图 1 所示。
■ 前级射频放大器
从天线进入的信号经过收发转换电路 (D31断开) 和带通滤波
器 (L36, L38 和变容二极管: D25, D26) 进入射频放大器 (Q26)。
信号被放大 (Q26) 之后，信号被带通滤波器 (L30, L32 和变容
二极管: D21, D22) 滤波，在进入第一混频器之前消除不需要的
信号。
通过跟踪带通滤波器的CPU (IC6) 中心频率来控制这些二极
管的电压。 (参见图 2)
■ 第一混频器
来自于射频放大器的信号与来自于第一混频器 (Q21) 的锁相
环频率合成器电路的第一本振信号混合后产生49.95MHz的第一
中频 (1st IF) 信号。 然后， 第一中频信号进入两个单片晶体滤波
器 (MCFs∶XF1) 进一步消除寄生信号。
CF1 (Wide)
CF2 (Narrow)
Q21
XF1
Q19
MIX
MCF
IF AMP
IF system
IC23
X1
TCXO
X3 multiply
TK-7108
SP
MIC
IC3
D/A CONVERTER
IC5
DEO
Q6
15
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