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參數(shù)資料
| 型號(hào)： | LM1865M |
| 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, PDSO20 |
| 封裝： | 0.300 INCH, PLASTIC, SOP-20 |
| 文件頁(yè)數(shù)： | 13/16頁(yè) |
| 文件大小： | 309K |
| 代理商： | LM1865M |
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Application Notes
(Continued)
In order to make the calculation in equation (4), the con-
stants a, A1, A2 must first be determined. This is done by
the following procedure:
1. Connect together two RF generators and apply them to
the tuner input. Since the generators will terminate each
other, remove the 50
X
termination at the tuner input.
2. Connect a spectrum analyzer to the tuner output. Most
spectrum analyzers have 50
X
input impedances. To
make sure that this impedance does not load the tuner
output use a FET probe connected to the spectrum ana-
lyzer. The tuner output should be terminated with a ce-
ramic filter.
3. Disconnect the AGC line to the tuner. Make sure that the
tuner is not gain-reduced.
4. Adjust the two RF generators for about 1 mV input and to
frequencies 400 kHz and 800 kHz away from center fre-
quency (Figure 13).
5. Note the three output levels in volts.
6. Knowing the tuner input levels for V
UD1
and V
UD2
and
the resulting IM
3
just measured, ‘‘a(chǎn)’’ is calculated from
the formula:
a
e
IM
3
V
UD12
V
UD2
(5)
where all levels are in volts rms. A typical value for ‘‘a(chǎn)’’
might be 2
c
10
6
.
7. A1 and A2 are calculated according to the following for-
mulas
V
IN
à
f
o
a
800 kHz
A1
e
V1
V
IN
f
o
a
400 kHz
(6)
A2
e
V2
(7)
If the wide band threshold was set to V
WBUL
, then when a
single in-band station reached the level V
WBUL
at the tuner
output, AGC action would start to take place. For this rea-
son it is hoped that V
WBUL
is above the level that will allow
for maximum signal-to-noise. If, however, this is not the
case, consideration might be given to improving the inter-
modulation performance of the tuner.
The lower limit for V
WB
is the minimum tuner output that
achieves the best possible signal-to-noise ratio in the recov-
ered audio. In general, it is desirable to set V
WB
closer to
the upper limit rather than the lower limit. This is done to
prevent AGC action within the narrow band loop except
when there is a possibility of an IM
3
greater than V
NB
.
The wide band threshold at the pin 20 input to the LM1865
is fixed at 12 mVrms. Generally speaking, if pin 20 were
driven directly from the tuner output. V
WB
would be too low.
Therefore, in general, pin 20 is not connected directly to the
tuner output. Instead the tuner output is attenuated and then
applied to pin 20. Increasing attenuation increases the wide
band threshold, V
WB
.
Pin 20 has an input impedance at 10.7 MHz that can be
modeled as a 500
X
resistor in series with a 19 pF capacitor,
giving a total impedance of 940
X
K
b
58
§
. Thus an easy way
to attenuate the input to pin 20 is with the arrangement
shown in Figure 14.
Notice that pin 20 must be AC coupled to the tuner output
and that C1 is a bypass capacitor. R1 adjusts the amount of
attenuation to pin 20. The wide band threshold will roughly
increase by a factor of (R1
a
940
X
)/940
X
.
AGC CIRCUIT USED AS A CONVENTIONAL AGC
If for some reason the dual AGC thresholds are not desired,
it is easy to use the LM1865 as a more conventional
LM3189 type of AGC. This is accomplished by AC coupling
the pin 20 input after the ceramic filters rather than before
the filters. Thus, as with the LM3189, only in-band signals
will be able to activate the AGC.
TL/H/7509–15
TL/H/7509–16
FIGURE 13. Spectrum Analyzer Display of Tuner Output
FIGURE 14. Wide Band Threshold Adjustment
13
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