Amplitude of an AM wave

amelectromagneticwave

A message signal is modulated on a carrier wave of amplitude \$50V\$ using amplitude modulation. The modulation index is given as \$50\%\$. Find the amplitude of the AM wave.

I was able to calculate the amplitude of the message signal as \$25V\$. However I'm stuck at the amplitude of AM wave.

The maximum displacement of the AM wave will be \$50+25=75V\$ and the minimum displacement will be \$50-25=25V\$.

So is the amplitude of the AM wave \$75V\$ or is it \$75-25=50V\$ i.e. is it the maximum displacement or the difference between the maximum and minimum displacements?

I know amplitude is the maximum displacement from the mean position however I can't seem to figure the mean position of this AM wave.

Best Answer

The amplitude of the AM wave depends on the modulating signal, that's why it is called amplitude modulation. Taking your values, we have:

  • If the modulating signal is at its mininum (negative peak), the AM wave amplitude is 25 V (swings between 25 and -25 V).
  • If the modulating signal is at its maximum (positive peak), the AM wave amplitude is 75 V (swings between 75 and -75 V).
  • If the modulating signal is 0, the AM wave amplitude is 50 V (swings between 50 and -50 V).

One thing to note is that the modulating signal is a very slow time-varying signal compared to the carrier frequency.

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