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參數(shù)資料
| 型號： | LM1865 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Advanced FM IF System |
| 中文描述： | 先進(jìn)的調(diào)頻中頻系統(tǒng) |
| 文件頁數(shù)： | 11/16頁 |
| 文件大?。?/td> | 309K |
| 代理商： | LM1865 |
Application Notes
(Continued)
voltage, while V
D
is allowed to increase. V
D
will increase
until it reaches the level of the wide band threshold at the
tuner output. When this occurs V
UD
is no longer needed to
keep V
o
l
V
WB
as V
D
takes over the job. Thus V
UD
will
drop as the amount of AGC increases, while V
D
is held con-
stant by the AGC.
When compared to the simple case of a single in-band sig-
nal, we see that because of the presence of a strong out-of-
band signal, AGC action has occurred earlier. For the simple
case, AGC started when V
D
t
V
WB
. For the two signal case
above, AGC started when V
D
t
V
NB
. Thus, the LM1865
achieves an early AGC when there are strong adjacent
channels that might cause IM
3
, and a later AGC when these
signals aren’t present.
For the range of signal levels that the tuner was gain-re-
duced and V
D
k
V
WB
there was loss in signal-to-noise in
the recovered audio as compared to the case where there
was no gain reduction in this interval. Note, however, that
the tuner is not desensitized by the AGC to weak desired
stations below the narrow band threshold.
NARROW BAND AGC THRESHOLD ADJUSTMENT
Both the narrow band and wide band AGC thresholds are
user adjustable. This allows the user to optimize the AGC
response to a given tuner. Referring to Figure 5, when the
meter output exceeds 1V a comparator closes SW1. A sim-
plified circuit diagram of this comparator is shown in Figure
10.
The 1K resistor in series with pin 8 allows for an upward
adjustment of the narrow band threshold. This is accom-
plished by externally loading pin 8 with a resistor.Figure 11
illustrates how this adjustment takes place.
From Figure 11 it is apparent that loading the meter output
not only moves the narrow band threshold, but also de-
creases the meter output for a given input.
In general one chooses the narrow band threshold
based on what signal-to-noise compromise is considered
acceptable.
TL/H/7509–12
FIGURE 10. Narrow Band Threshold Circuit
TL/H/7509–13
FIGURE 11. Affect of Meter Load on Narrow Band Threshold
11
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LM1865M | Advanced FM IF System |
| LM1865N | Advanced FM IF System |
| LM1868 | Radio System |
| LM1868N | Radio System |
| LM1870 | STEREO DEMODULATOR WITH BLEND |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM1865M | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Advanced FM IF System |
| LM1865N | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Advanced FM IF System |
| LM1865N/A+ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:FM Receiver Circuit |
| LM1865N/B+ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:FM Receiver Circuit |
| LM1866 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LOW VOLTAGE AM/FM RECEIVER |
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