How Does Amplitude Modulation (AM) Work and What are its Applications?

AM… Amplitude Modulation. Derisively called “ancient modulation” by some, AM was the first method used to send information via radio waves. The first radio systems did not send voice signals but instead were used to send text using Morse code, similar to wired telegraph systems.

This was accomplished by the crudest method of AM possible: on-off keying or OOK. When the sender pressed down on the telegraph key, it turned on the transmitter, and when released, it turned off. Crude but effective—so effective, in fact, that OOK is still used in many simple systems for data transmission, such as garage door openers.

It didn’t take long before it was discovered that changing the amplitude of the radio wave could also convey sound. Conventional AM, such as that used in AM broadcasting, is the most well-known example of this. Here’s how it works: The instantaneous voltage of the audio waveform determines the amplitude of the radio signal. At the lowest voltage of the audio, the output from the transmitter is essentially zero. At the highest voltage, the transmitter produces twice as much power as the average power, which is the power produced during silence. Want to learn the basics of modulation? Check out the first post in our Methods of Transmission series.

I’ve heard about things called sidebands. What are those, and how do they relate to AM?

That’s where things get a bit complicated. When the audio signal modulates the amplitude of the radio wave (called the carrier), the modulation process creates signals that are on either side of the carrier frequency, known as sidebands. These sidebands are a result of the modulation process, and are mirror images of each other, and represent the audio that is modulated onto the carrier. As it turns out, each sideband contains all the information needed to recover the audio. The other sideband and the carrier are unnecessary; the receiver can supply the required carrier, but the other sideband is simply not used. This is how single-sideband (SSB) radios work.

What are the primary uses of Single Sideband Radio?

SSB is a mainstay for shortwave communications, used by Amateur Radio (Ham) operators as well as various government and military users. In terms of power versus range, SSB gives the most “bang for the buck” of all analog modulation systems. It’s also the most bandwidth-efficient analog voice modulation form.

However, there is one major drawback of single-sideband modulation. Without a carrier reference, the receiver has no frequency or amplitude reference. This causes three problems:

1) With a frequency reference, accurate tuning of the receiver becomes easier. A mistuned receiver shifts all the audio frequencies being transmitted, resulting in odd-sounding, difficult-to-understand audio. This is commonly referred to as the “Donald Duck” syndrome.

2) Without the carrier as a reference, it becomes difficult to adjust the receiver’s gain accurately. For example, when there is no speech, the receiver sees no signal and increases its gain accordingly. This creates noise during speech pauses, which, while acceptable to skilled operators, may be distracting for users not accustomed to this behavior.

3) Related to number two, it becomes easier to make an effective squelch circuit with a constant carrier to indicate the presence of a signal during silence. Squelch mutes the receiver during periods of no-receive signal to mute the noise that comes from the receiver when no signal is present. This becomes difficult when silence produces a situation where there is no signal on a regular basis.

AM isn’t the only way to send information over the airwaves, you can learn more about FM transmission on our blog!

Is there a solution to the issues in SSB Systems?

Fortunately, there are ways to mitigate the issues in single-sideband systems. A system known as amplitude compandored single-sideband (ACSSB or ACSB) overcomes these problems using a pilot tone, an audio signal mixed in with the speech at the transmitter. The receiver uses this tone to adjust the frequency for accurate tuning and as a reference to set the receiver’s gain. The presence of tone also provides a signal for the squelch circuit to work. There is much more to ACSSB than just those simple things, but that will be discussed in a later installment.

So, while AM and its derivatives are frequently disparaged as outdated, “ancient” technology, there are still many applications where AM not only makes sense but is the best candidate for the job! Read more about Methods of Transmission here (first blog post) and sign up to be notified when our next installment drops!