Amplitude Modulation, AM: Bandwidth Spectrum & Sidebands

The bandwidth of an amplitude modulated signal is of importance for many reasons.

The amplitude modulation, AM bandwidth is important when designing filters to receive the signals, determining the channel spacing, and for a number of other reasons.

The spectrum and bandwidth of a amplitude modulated signal are determined by the sidebands that are generated when amplitude modulation is applied to the carrier.

Amplitude modulation sidebands

When a carrier is modulated in any way, further signals are created either side of the steady carrier. These sidebands carry the actual modulation information.

The amplitude modulation sidebands are generated above and below the main carrier. To see how this happens, take the example of a carrier on a frequency of 1 MHz which is modulated by a steady tone of 1 kHz.

The process of modulating a carrier is exactly the same as mixing two signals together, and as a result both sum and difference frequencies are produced. Therefore when a tone of 1 kHz is mixed with a carrier of 1 MHz, a "sum" frequency is produced at 1 MHz + 1 kHz, and a difference frequency is produced at 1 MHz - 1 kHz, i.e. 1 kHz above and below the carrier.

If the steady state tones are replaced with audio like that encountered with speech of music, these comprise many different frequencies and an audio spectrum with frequencies over a band of frequencies is seen. When modulated onto the carrier, these spectra are seen above and below the carrier.

Amplitude modulation, AM bandwidth

It can be seen that if the top frequency that is modulated onto the carrier is 6 kHz, then the top spectra will extend to 6 kHz above and below the signal. In other words the bandwidth occupied by the AM signal is twice the maximum frequency of the signal that is used to modulated the carrier, i.e. it is twice the bandwidth of the audio signal to be carried.

It can be seen that limiting the top frequency of the modulating signal limits the overall bandwidth of the amplitude modulated signal. For audio transmissions, like broadcast transmissions, the overall bandwidth of the signal is dependent upon the highest frequencies to be transmitted.

Broadcast channel spacing varies according to the band used and also the area within the world where the transmission comes from. On the Long waveband broadcast in ITU region 1 (Europe, Africa, and northern and central Asia), the channel spacing is 9 kHz, for the medium wave broadcast band the channel spacing in ITU regions 1 and 3 is 9 kHz spacing, and in ITU region 2 (Americas) it is 10kHz.

To achieve these channel spacings, the audio spectrum must be limited. If no interference was to be caused to stations on adjacent channels this would severely limit the top audio frequency. On the short wave bands it would be half 5 kHz, i.e. 2.5 kHz. This would clearly not allow sufficient quality for broadcasting and therefore the bandwidth of the signals is a little higher than this and some interference is caused. Similarly on the medium and long wave bands the bandwidth of the AM signal is often higher than half the channel spacing. Interference is managed by not allowing stations in close proximity to occupy adjacent channels.

Typical AM channel and signal bandwidths

Some of the main uses of amplitude modulation these days are for broadcasting and for aeronautical communications. The channel spacing varies according to application, frequency and locations.

Typical AM channel and signal bandwidths
Applications / DescriptionChannel spacing (kHz)Theoretical maximum audio bandwidth (kHz)
Long / medium wave broadcasting outside Region 2 (Americas)94.5
Medium wave broadcasting in Region 2 (Americas)105