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Determine AM Modulation with an Oscilloscope
Feb 2, 2003 12:00 PM
The table below allows you to easily determine the modulation level
of an AM transmitter. Connect an RF sample to the horizontal input of
an oscilloscope and set the peak-to-peak (pk-pk) unmodulated carrier
level to occupy two divisions. Then apply modulation and read the
peak-to-peak level. Find the nearest number in the scope division
column below and read the modulation percentage from the left-most
column of the same row.
|
Modulation (%)
|
Sideband Power
(dbc)1
|
Scope Divisions |
|
0
|
N/A
|
2.0
|
|
1
|
-46
|
2.02
|
|
2
|
-40
|
2.04
|
|
10
|
-26
|
2.2
|
|
20
|
-20
|
2.4
|
|
30
|
-16.5
|
2.6
|
|
40
|
-14
|
2.8
|
|
50
|
-12
|
3.0
|
|
60
|
-10.4
|
3.2
|
|
70
|
-9.1
|
3.4
|
|
80
|
-7.9
|
3.6
|
|
90
|
-6.9
|
3.8
|
|
95
|
-6.5
|
3.9
|
|
100
|
-6.0
|
4.0
|
|
105
|
-5.6
|
4.2
|
|
110
|
-5.2
|
4.4
|
|
115
|
-4.8
|
4.6
|
|
120
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-4.4
|
4.8
|
|
130
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-3.7
|
5.2
|
|
140
|
-3.1
|
5.6
|
|
150
|
-2.5
|
6.0
|
|
160
|
-1.9
|
6.4
|
|
170
|
-1.4
|
6.8
|
|
180
|
-0.9
|
7.2
|
|
190
|
-0.4
|
7.6
|
|
200
|
0.0
|
8.0
|
Notes:
1 - The power in each sideband relative to the carrier power (dBc), is
determined by
Pssb = 10log(Pssb/Pc), where Pssb is the power in a single sideband,
and Pc is the carrier power.
The power in a single sideband is equal to one-half the modulation
power.
Modulation power is determined by
Pm = m^2(Pc)/2, where Pm is the modulation power, m is the modulation
index (percentage expressed as a decimal), and Pc is the carrier
power.
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