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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： | 先進的調頻中頻系統(tǒng) |
| 文件頁數(shù)： | 9/16頁 |
| 文件大?。?/td> | 309K |
| 代理商： | LM1865 |
Application Notes
(Continued)
The LM1865 has been carefully designed to insure low AFT
offset current at the point of minimum THD. AFT offset cur-
rent will cause a non-symmetric deviation mute/stop win-
dow about the point of minimum THD. No external AFT off-
set adjustment should be necessary with the LM1865. The
amount of resistance in series with the 18
m
H quadrature
coil drive inductor, L1, has a significant effect on the mini-
mum THD. This series resistance is contributed not only by
R13 but also by the Q of L1. The Q of L1 should be as high
as possible (ie: Q
l
50) in order to avoid production prob-
lems with the Q variation of L1. Once R13 has been opti-
mized for minimum THD, adjustment on a radio by radio
basis should be un-necessary.
DUAL THRESHOLD AGC
(AUTOMATIC LOCAL/DISTANCE SWITCH)
There is a well recognized need in the field for gain reducing
(AGCing) the front end (tuner) of an FM receiver. This gain
reduction is important in preventing overload of the front
end which might occur for large signal inputs. Overloading
the front end with two out-of-band signals, one channel
spacing apart and one channel spacing from center fre-
quency, or, two channel spacings apart and two channel
spacings from center frequency, will produce a third order
intermodulation product (IM
3
) which falls inband. This IM
3
product can completely block out a weaker desired station.
The AGC in the LM1865 has been specially designed to
deal with the problem of IM
3
.
With the LM1865 system, a low AGC threshold is achieved
whenever there are strong out-of-band signals that might
generate an interfering IM
3
product, and a high AGC thresh-
old is achieved if there are no strong out-of-band signals.
The high AGC threshold allows the receiver to obtain its
best signal-to-noise performance when there is no possibili-
ty of an IM
3
product. The low AGC threshold allows for
weaker desired stations to be received without gain-reduc-
ing the tuner. It should be noted that when the AGC thresh-
old is set low, there will be a signal-to-noise compromise,
but is assumed that it is more desirable to listen to a slightly
noisy station than to listen to an undesired IM
3
product. The
simplified circuit diagram (Figure 5) of the AGC system
shows how the dual AGC thresholds are achieved.
V
m
e
1V corresponds to a fixed in-band signal level (de-
fined as V
NB
) at the tuner output. V
NB
will be referred to as
the ‘‘narrow band threshold’’. V
WB
also corresponds to a
fixed tuner output which can either be an in-band or out-of-
band signal. This fixed tuner output will be called the ‘‘wide
band threshold’’. Always V
WB
l
V
NB
. R11 and C13 define
the AGC time constant. A reverse AGC system is shown.
This means that V
AGC
decreases to gain-reduce the tuner.
The LM1865 AGC output is an open-collector current
source capable of sinking at least 1 mA.
TL/H/7509–9
FIGURE 5. Dual Threshold AGC
I
1
e
GM
1
V
m
only if V
m
l
1V
otherwise I
1
e
0
Gm
1
, V
WB
e
constants
I
AGC
e
Gm
2
V
o
where Gm
2
e
I
1
/26 mV and
V
o
l
V
WB
otherwise I
AGC
e
0
9
相關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 |
相關代理商/技術參數(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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