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Amplitude Modulation

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Amplitude Modulation

AM transmitters vary the amplitude of the carrier wave.

The amplitude of the carrier wave is proportional to the amplitude of the signal being modulated.
If the modulation signal frequency increases, the amplitude of the carrier changes at a greater rate.
If the modulation signal frequency increases, the sidebands move further from the carrier.

Example: A microphone converts sound waves (energy) into an electrical signal (energy) proportional to the sound wave pressure. These frequencies are too low to transmit. A high frequency carrier wave is needed. This can be transmitted. The carrier wave is modulated (varied) by the signal from the microphone.

A radio frequency carrier is needed.
As the audio waveform rises and falls, the carrier amplitude increases and decreases in direct proportion.
As the audio amplitude increases, the modulation of the carrier increases.
If the audio frequency rises, the carrier amplitude varies faster, in step with the audio signal.
The carrier frequency does not change.

This diagram shows about 80% modulation. The carrier amplitude rises to 180% and drops to 20%. The percentage modulation depth can rise to 100% without problems.

Modulation Depth

A is the peak to peak audio amplitude (after modulation).
B is the unmodulated peak to peak carrier amplitude.
The percentage modulation depth is 100 x A / B %

Over Modulation

If the modulation depth rises above 100%, the transmission looks like this.

The carrier has completely gone for part of the audio cycle. This results in severe distortion. The transmission also splatters outside its usual bandwidth causing interference to stations on adjacent channels. This situation must be avoided.

AM Demodulation or Detection

A diode is used to pass the radio frequency signal in one direction only.
A low pass RC filter is used to pass the audio frequencies and decouple or smooth out the radio frequencies.
Silicon diodes are not ideal because 0.7 volts is needed before the diode starts to conduct.
Germanium diodes are better because only 0.2 volts is needed.
Active circuits are even better because these can mimic an ideal diode.

AM Bandwidth and Spectrum Diagrams

Bandwidth

BANDWIDTH = 2 x F_max
The bandwidth of an AM signal is double the highest audio frequency modulated onto the carrier.
AM radio stations are placed very close together on the long, medium and short wave bands. To make this possible, the bandwidth must be limited. This prevents the transmissions from different stations overlapping each other. Good receiver tuning is needed (selectivity) to pick up only one station at a time.
At the transmitter, the audio signal is filtered so the highest frequency present is 4kHz. This gives an AM bandwidth of 8 kHz. This filtering gives the characteristic rather poor sound quality of AM radio stations.
This allows 9 kHz channel spacing with a small safety margin.

Sidebands

It is possible to plot the frequency spectrum of an AM transmission using a spectrum analyser.
If a 1 kHz sine wave tone is modulated onto a 1 MHz carrier, two sidebands will be produced 1 kHz above and 1 kHz below the 1 MHz carrier signal. This is what the diagram shows.

When music and speech are transmitted, many frequencies are modulated onto the carrier and the sidebands look more like this (diagram below), varying rapidly as the content of the broadcast changes. The image below shows the spectrum of an AM transmission with a carrier on 700kHz and audio signals with frequencies ranging between 300 and 4000 Hz. AQA have set exam questions where you need to sketch this diagram with all the labels and frequencies correctly positioned.

Here is a screen shot showing the spectrum of three shortwave AM broadcasting stations. The three carriers can easily be seen. The bandwidth is also clearly visible. The sidebands for each station look different because they are changing rapidly depending on the speech or music being transmitted. The green rectangle shows the band pass filtering used in the receiver so only one station comes out of the loudspeaker. Digital signal processing allows the spectrum on each side of the selected station to be seen. This advanced receiver can be re-tuned with a single mouse click but it also relies on the attached computer to do a lot of signal processing work. The colourful area below the spectrum plot shows how much energy is present at each frequency over a period of time. Blue is the background noise (sometimes called static). Yellow is a strong signal and red is a very strong signal.

AM Bandwidth Calculation

Bandwidth = 2 x f_max

f_max is the highest frequency transmitted (music or voice - about 4 kHz on AM radio)

Single Sideband (SSB)

All the information in an AM transmission is in the sidebands. The carrier could be removed and the transmission would still work. This is called Double Sideband (suppressed carrier). Removing the carrier makes the transmitter more efficient but the bandwidth is not reduced.

By using high performance filters, it is possible to remove the carrier and also one sideband. This leaves a Single Sideband (SSB). These transmissions occupy less than half the bandwidth or spectrum space of an AM transmission. This is useful because the transmitter is more efficient and more transmissions can be crammed into the limited spectrum space available. To demodulate DSB or SSB, the carrier must be artificially replaced on the exact correct frequency. This makes DSB and SSB transmissions tricky to tune in. A mistuned receiver makes the person sound like Donald Duck. SSB is not used for commercial broadcasting because it is too hard to tune in (especially for music). It's efficiency and narrower bandwidth make it ideal for military, shipping, air traffic, industrial and amateur radio voice communication.

Note the missing carrier and only one sideband.

Amplitude Modulation of Light

This circuit uses a source follower to drive an LED.
It is capable, with a heat-sink, of driving a higher power LED.
Using the labelled component values, a standard 10mA LED should be used.
Using lenses or fibre optics, a range of several meters is attainable with ease.
The world record for communication using techniques like this is over 100 Miles!
If the LED is replaced with a radio transmitter, AM Radio will be produced.