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參數(shù)資料
| 型號(hào)： | LA7790M |
| 廠商： | Sanyo Electric Co.,Ltd. |
| 英文描述： | QPSK Transmitter for Cable TV |
| 中文描述： | 調(diào)制變送器有線電視 |
| 文件頁數(shù)： | 10/14頁 |
| 文件大小： | 174K |
| 代理商： | LA7790M |
2. Attenuator
The attenuator input signal comes from the modulator output via a high pass filter (f
C
= 2 MHz). The modulator
output is pin 19. Pin 19 is connected to the emitter of an NPN emitter follower through a 100
resistor. The emitter
sink current is about 1 mA, but can be increased by the addition of an external resistor between pin 19 and ground.
The minimum value for the external resistor is 1.5 k
.
The output level of pin 19 can be adjusted using pin 21. Pin 21 is connected to the base of a PNP emitter follower
through a 300
resistor. When pin 21 = 0 V, the base current is about 10 μA.
Equivalent circuit of attenuator block
3. RF Amplifier
The RF amplifier input signal comes from the attenuator output after the harmonics are removed by the LPF. The RF
amplifier is composed of a two-stage amplifier and includes mute and power save functions.
The gain of the RF amplifier can be adjusted using the external input resistor connected to pin 22 and the external
load resistor connected to pin 1. The input of pin 22 must be AC coupled. The output of pin 1 must be connected to
V
CC
via a choke coil. Pin 23 (mute control) and pin 20 (power save control) are both high impedance inputs (base
current less than 10 μA). For normal operation, the voltage at pins 20 and 23 must be less than 1 V. For Mute and
Power Save operation, the voltage at pins 20 are 23 must be between V
CC
–1 and V
CC
.
No. 5689-10/14
LA7790M
V
CC
Control
Circuit
ATT
HPF
300
1 k
Gain
control
0 to 10
μ
A
21
MOD
output
19
1 mA
From Modulator
1 k
1 k
100
1 mA
1 mA
Unit (resistance :
)
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