How this calculator works
AM index compares message peak amplitude with carrier peak amplitude. FM index compares peak frequency deviation with message frequency. Inverse modes recover either member of that ratio. The chart illustrates a single sinusoidal message with a carrier ten times faster; it is not a prediction for an unspecified real carrier. FM bandwidth uses Carson's approximation and is not a strict spectral cutoff.
Formula
mAM = Am/Ac; betaFM = deltaf/fm. Carson bandwidth B approximately 2(deltaf+fm). AM waveform = (1+m cos(2 pi x)) cos(20 pi x). FM waveform = cos(20 pi x + beta sin(2 pi x)).
Worked example
A 2 V message on a 4 V carrier has AM index 0.5. FM deviation of 75 kHz with a 15 kHz message has index 5 and estimated bandwidth 180 kHz.
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Frequently asked questions
What does AM index above one mean?
The ideal envelope crosses zero. Conventional envelope detection then introduces distortion.
Does FM index have the same limit?
No. An FM index above one is common and does not imply AM-style envelope overmodulation.
Why is the FM chart sometimes empty?
For index above five the fixed illustrative sampling would not reliably show all oscillations, so the scalar results remain and the plot is omitted.