Determine AM Modulation with an Oscilloscope

August 15, 2013


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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