Which Modulation Is Better for AM?

Modulation refers to waveform or carrier signal variations of the modulation signal.

Using modulation can lessen mixed signals and possibly extend signal distance. Bearing that in mind, what is the best modulation for AM?

Double-sideband-suppressed carrier modulation, or DSB, is an optimal choice for AM modulation. It omits the carrier and has an efficiency rating of 50 percent. DSB also uses less power. Modulation of the vestigial sideband or VSB and single-sideband or SSB modulation are other options for AM.

This article will cover DSB, VSB, and SSB AM modulation so you can select the right one for your needs.

AM Modulation Options – The Pros and Cons

Double-Sideband-Suppressed Carrier Modulation

Let’s begin by discussing double-sideband-suppressed carrier modulation, one of three forms of AM modulation and among the best for modulating AM radio.

DSB modulation takes AM frequencies and places them symmetrically and strategically near the carrier frequency.

Some AM frequencies go underneath the carrier frequency, and others over it.

The “suppressed” part of double-sideband-suppressed carrier modulation refers to what happens to the carrier level. It decreases by a significant margin and is considered suppressed.

AM modulation usually transmits the carrier wave, but that doesn’t happen when modulating DSB. This allows the power levels to remain between either sideband.

The result is more coverage when using DSB modulation without increasing power.

Mixers produce DSB modulation, with the resulting signal combining a carrier and message signal. Demodulating the signal requires multiplying a carrier signal with the DSB signal.

From there, the signal goes through a low pass filter until scaled. This reduces the higher frequencies and restores the message to its original status.

However, attenuation and distortion risks are higher when demodulating if the phase and frequency of the oscillator differ.

Pros

• The efficiency rating of DSB modulation is 50 percent. AM transmission has an efficiency of only just over 33 percent due to the required carrier power failing to transmit valuable data. Also, the sidebands don’t differentiate from one another in the information they carry.
• It’s a lower-cost AM modulation solution than some alternatives.
• The dual sidebands increase the bandwidth of modulation, subsequently elevating the output frequency to at least twice what a modulated signal usually contains.
• DSB signals require less power since they skip the carrier.

Cons

• Information received is sometimes more complicated than it needs to be.
• The dual sidebands, as valuable as they are, aren’t always used as well as they could be. This causes a lot of power to be wasted, so DSB modulation isn’t the greenest solution.
• If using a carrier, DSB’s efficiency begins to drop drastically.

Vestigial Sideband Modulation

The next type of modulation band for AM is the vestigial sideband or VSB.

Rather than modulating the entire signal, VSB only modulates part of it. That part of the signal is referred to as a vestige.

This vestige creates a data-encoded signal and changes the carrier frequency’s amplitude.

The carrier does not change amplitude, but the AM signal does.

The modulated data in the sidebands changes frequencies compared to the carrier. The sidebands might have higher frequencies than the carrier in some instances and lower in others.

The vestige in one sideband is modulated, usually the upper sideband. However, sometimes the lower sideband also receives modulation, so it’s more like one and a half sidebands being used.

VSB modulation requires a guard band in between the vestige to prevent interference.

Although VSB modulation is frequently used for television broadcasting, it also has its place in modulation.

Pros

• The phase characteristics of VSB are optimal.
• This form of modulation can transmit lower frequencies very easily.
• VSB does not require a high degree of accuracy.
• Its efficiency rating is relatively high, partly because it doesn’t require as much bandwidth.

Cons

• Demodulating VSB modulation is much more difficult than DSB.
• This form of modulation has a more complex bandwidth than single-sideband modulation.

Single-Sideband Modulation

The last form of modulation we’ll examine for AM is single-sideband modulation or SSB, sometimes called single-sideband suppressed-carrier modulation or SSB-SC.

As the name implies, this modulation relies on only one sideband rather than two, as with DSB modulation.

SSB modulation sends a radio signal or other information across radio waves.

Traditional AM modulation leads to an output signal with a bandwidth of at least twice as much as the original baseband signal’s greatest frequency.

However, with SSB modulation, the bandwidth does not increase. Carrier power isn’t needlessly wasted, either.

This does come at a cost: more complex receiver tuning.

Eliminating a carrier and second sideband from the equation is achieved by increasing the final amplifier stage to a high degree for AM modulation. The linear-amplified signal doesn’t use much power but has a lower efficiency.

Pros

• You’re less likely to deal with signal fading with SSB modulation.
• You can also enjoy less noise with this form of AM modulation.
• Since SSB modulation only ones one sideband, it saves power.
• You can transmit more signals using this form of modulation.
• SSB modulation doesn’t require as much spectrum space as DSB and AM signal modulation.

Cons

• If you’re using a cheap AM receiver, SSB modulation doesn’t work since it needs stable frequencies.
• It’s a complex system, more so than other forms of modulation.
• If the frequency strength is too high, the signal quality lessens, which can detract from the radio experience.

How Do You Avoid Overmodulation?

You risk overmodulating the signal when modulating AM signals using SSB, VSB, or DSB modulation.

Modulation has a minimum index value, which is always zero.

That value can rise over 1. In that case, the signal is overmodulated.

Overmodulation is off the scale. In other words, it’s a fault condition. Data loss is likelier during overmodulation, as are distortions.

How do you prevent overmodulation?

The carrier and message signals’ max amplitude must match or be similar. The message signal amplitude can be slightly below the other but shouldn’t exceed the carrier signal’s max amplitude.

When both values are the same, that’s perfect modulation.

However, you can also sometimes experience under-modulation.

This is the opposite of overmodulation. The carrier signal’s max amplitude does not match the message signal’s max amplitude, and the latter is lower by a significant margin.

Under-modulation is not ideal, but it’s better than overmodulation, as the former doesn’t lead to any audial distortions or disturbances.

Make sure you’re using signal limiters if you aren’t already, as they’re your best tool for combating overmodulation.

A limiter keeps the output at a specific value so it can’t overmodulate the signal.

You can still enjoy a dynamic range with a limiter but less distortion to the signals coming in and any signal outputs.

What are the best AM radios? We put together a list here!

Wrapping Up

AM modulation includes a double-sideband-suppressed carrier or DSS, vestigial sideband or VSS, and single-sideband or SSB modulation.

While each is advantageous, DSS modulation is renowned for its efficiency, bandwidth, and reduced power needs.

It’s also less expensive to use DSS modulation and less complex than some modulation types, such as SSB.

When modulating AM radio, watch the message signal and carrier signal. The frequencies should ideally match perfectly or be very close to matching.

Beware when one signal outpaces the other, as this is overmodulation.

Using a limiter will prevent the distortions that come with overmodulation, as will frequency management.