Determine AM Modulation with an Oscilloscope


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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

-4.4

4.8

130

-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.

Source: The AM Window




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