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 4.1
 110 -5.2 4.2
 115 -4.8 4.3
 120 -4.4 4.4
 130 -3.7 4.6
 140 -3.1  4.8
 150 -2.5  5.0
 160 -1.9  5.2
 170 -1.4  5.4
 180 -0.9 5.6
 190 -0.4  5.8
 200 0.0 6.0

 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 = m2(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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