Graphical representation of both AM and FM modulated signals.

The idea

Modulation superimposes information onto a carrier wave. In amplitude modulation the carrier's amplitude varies at the information (modulating) frequency while the carrier frequency stays constant, so the envelope of the displayed waveform rises and falls. In frequency modulation the carrier's amplitude stays constant while its frequency shifts above and below the unmodulated carrier value at the modulating rate, so the trace looks compressed and stretched. On an oscilloscope or waveform diagram, AM is recognised by a changing envelope with evenly spaced carrier cycles; FM by constant peak height with varying cycle spacing. A 1 kHz audio tone on a 100 kHz carrier gives an AM envelope repeating every 1 ms, whereas FM shows 100 kHz cycles bunching and spreading at that same 1 ms rate.

What earns the marks

• Identifies that in AM the amplitude of the carrier varies while its frequency remains constant.
• Identifies that in FM the frequency of the carrier varies while its amplitude remains constant.
• States that the rate of variation of amplitude or frequency equals the frequency of the modulating (information) signal.
• Describes the AM waveform as having a constant carrier period but an envelope that rises and falls.
• Describes the FM waveform as having constant peak amplitude but carrier cycles that bunch together and spread apart.
• Uses a labelled sketch or oscilloscope trace to distinguish the two, labelling axes as time and voltage or displacement.
• Relates the number of envelope cycles per second in AM, or the number of frequency excursions per second in FM, to the modulating frequency.

Common mistakes and how to put them right

• Thinking the carrier frequency changes in AM.

  Put it right by stating that only the amplitude envelope varies and the spacing of carrier cycles stays uniform.

• Believing FM changes the signal amplitude.

  Put it right by noting FM has constant peak amplitude and only the cycle spacing varies.

• Confusing the modulating frequency with the carrier frequency.

  Put it right by identifying the slow envelope or slow frequency wobble as the information rate and the fast cycles as the carrier.

Exam tips

• Sketch both waveforms side by side with the same time axis so the constant carrier spacing of AM and the varying spacing of FM are directly comparable.
• Annotate the carrier period and the envelope period on your diagram, then state which one equals the modulating period.
• If asked to interpret a trace, first check whether peak height changes (AM) or cycle spacing changes (FM) before writing your conclusion.