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參數(shù)資料
| 型號： | LM1865N |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 接收器 |
| 英文描述： | Advanced FM IF System |
| 中文描述： | FM, AUDIO DEMODULATOR, PDIP20 |
| 封裝： | PLASTIC, DIP-20 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大小： | 309K |
| 代理商： | LM1865N |
Application Notes
(Continued)
WIDE BAND AGC THRESHOLD ADJUSTMENT
There are a number of criteria that determine where the
wide band threshold should be set. If the threshold is set too
high, protection against IM
3
will be lost. If the threshold is
set too low, the front end, under certain input conditions,
may be needlessly gain-reduced, sacrificing signal-to-noise
performance. Ideally, the wide band threshold should be set
to a level that will insure AGC operation whenever there are
out-of-band signals strong enough to generate an IM
3
prod-
uct of sufficient magnitude to exceed the narrow band
threshold. Ideally, this level should be high enough to allow
for a single in-band desired station to AGC the tuner, only
after the maximum signal-to-noise has been achieved.
In order to insure that the wide band loop is activated when-
ever the IM
3
exceeds the narrow band threshold, V
NB
, de-
termine the minimum signal levels for two out-of-band sig-
nals necessary to produce an IM
3
equal to V
NB
. Then, ar-
range for the wide band loop to be activated whenever the
tuner output exceeds the rms sum of these signals. There
are many combinations of two out-of-band signals that will
produce an IM
3
of a given level. However, there is only one
combination whose rms sum is a minimum at the tuner out-
put. IM
3
at the tuner output is given according to the
equation:
IM
3
e
aV
UD12
V
UD2
(assuming no gain reduction)
where a
e
constant dependent on the tuner;
V
UD1
e
out-of-band signal 400 kHz from center frequen-
cy, applied to tuner input;
(1)
V
UD2
e
out-of-band signal 800 kHz from center frequen-
cy and 400 kHz away from V
UD1
, applied to tun-
er input.
In general, due to tuned circuits within the tuner, the tuner
gain is not constant with frequency. Thus, if the tuner is kept
fixed at one frequency while the input frequency is changed,
the output level will not remain constant. Figure 12 illus-
trates this.
It can be shown that for a given IM
3
, the combination of
V
UD1
and V
UD2
that produces the smallest rms sum at the
tuner output is given by the equations:
V
UD1
e
1.12
A2
A1
IM
3
a
(/3
(2)
V
UD2
e
0.794
A1
2
A2
2
IM
3
a
(/3
(3)
Therefore, in order to guarantee that the AGC will be keyed
for an IM
3
e
V
NB
we need only satisfy the condition:
V
WB
s
V2
NB
a
(A1) (1.12)
A2
A1
V
NB
a
(/3
2
a
A2 (0.794)
A1
2
A2
2
V
NB
a
(/3
(
2
(4)
The right hand term of equation (4) defines an upper limit for
V
WB
called V
WBUL
. V
WBUL
is the rms sum of all the signals
at the tuner output for two out-of-band signals, V
UD1
and
V
UD2
[
as expressed in equations (2) and (3)
]
, applied to the
tuner input.
TL/H/7509–14
Define A
e
tuner gain at center frequency
A1
e
tuner gain at
f
o
a
400 kHz
A2
e
tuner gain at
f
o
a
800 kHz
FIGURE 12
12
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