Amplitude Modulation, AM Spectrum & Bandwidth

- notes and details about the spectrum and bandwidth of amplitude modulation or amplitude modulated, AM signals.

The amplitude modulation spectrum usage and bandwidth are critical for most applications.

The bandwidth that an amplitude modulated space occupies determines the number of transmissions that can be accommodated within a certain band, and also the levels of interference caused to other users.

With pressure on the radio spectrum increasing, and the number users increasing in many areas, the bandwidth of an amplitude modulated signal is important.

Amplitude modulation bandwidth basics

In order to determine the bandwidth of an amplitude modulated signal it is necessary to understand the make-up of the signal

The amplitude modulated signal consists of a carrier with two sidebands that extend out from the main carrier. This results from the modulation process. It is found that if the carrier is modulated with a 1 kHz tone, for example, two sidebands each 1 kHz away from the carrier will appear. A 5 kHz tone would produce sidebands 5 kHz away from the carrier.

Amplitude modulated carrier showing sidebands
Effect of audio bandwidth on overall
amplitude modulation signal bandwidth

The sidebands produced by the modulation of a carrier by a typical audio signal will extend out from the carrier as shown - the highest audio frequencies in the audio bandwidth will be furthest away from the carrier.

Accordingly the bandwidth of the signal can be seen to be twice that of the highest audio transmitted:

Signal bandwidth B   =   2 . Audio bandwidth

As a result the audio bandwidth of many amplitude modulation transmissions is limited.

Typical 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.

Applications / Description Channel spacing (kHz) Theoretical maximum audio bandwidth (kHz)
Long / medium wave broadcasting outside Region 2 (Americas) 9 4.5
Medium wave broadcasting in Region 2 (Americas) 10 5
Short wave broadcasting 5  

The audio bandwidth theoretical limits for amplitude modulated broadcast stations appears far more limited than occurs in reality. Audio bandwidth figures of up to 6 kHz are not uncommon - generally adjacent channels are not allocated so that signals spreading into adjacent channels can be accommodated. This is fine for broadcasting on bands such as the medium wave band during the day, but at night when signals travel further as a result of ionospheric propagation, more interference is experienced.

For the short wave bands interference levels are often high - some broadcast stations have experimented and sued single sideband with full carrier. This effectively reduces (halves) the bandwidth of the overall signal for a given audio bandwidth. For this to provide gains within a channelized band plan, all stations need to adopt the same plan.

Aeronautical communications often use amplitude modulation. Channel bandwidths of 25kHz and 8.33 kHz are standard dependent upon the aircraft and location. Audio can be tailored to suit the channel spacing as a typical communications audio bandwidth of 300 Hz to around 300 kHz can be adopted.

Effect of over-modulation on AM bandwidth

To ensure that an amplitude modulated signal does not create spurious emissions outside the normal bandwidth it is necessary to ensure that the signal does not become overmodulated - this is a conditions that occurs when the modulation exceeds 100%. At this point the carrier breaks up and intermodulation distortion occurs leading to large levels of unwanted noise spreading out either side of the carrier and beyond the normal bandwidth. This can cause interference to other users.

If over-modulation occurs, the carrier becomes phase inverted and this leads to sidebands spreading out either side of the carrier.

By Ian Poole

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