Amateur Extra: Vestigial Sideband Modulation

Ew, this sounds kind of gross, but this aspect of sideband modulation is on the test! [E2B06]

What is vestigial sideband modulation?

A. Amplitude modulation in which one complete sideband and a portion of the other are transmitted
B. A type of modulation in which one sideband is inverted
C. Narrow-band FM transmission achieved by filtering one sideband from the audio before frequency modulating the carrier
D. Spread spectrum modulation achieved by applying FM modulation following single sideband amplitude modulation

Vestigial sideband modulation? Sounds like something you’d pick up in the jungle, and have to take a lot of pills to get rid of.

It’s nothing that evil, and you’ll probably not run into it very much.  But hey, its on the test!

The definition of “vestigial” is: forming a very small remnant of something that was once much larger or more noticeable.

What this means for us, is that if you were to look at a sideband radio signal with a vestigial sideband on a scope, you would see something like the image above, where there is a full USB signal, and then part of the lower sideband.

That signal in the picture is what the old analog television signals looked like.  The audio was transmitted in a sideband, and the video in the other.  This is why it was possible, for example, to pick up the audio from old analog channel 6 on your car radio at 87.7 MHz.  Current ATSC digital television uses something called 8VSB.  The VSB stands for, you guessed it, vestigial sideband.

So, lets look at our answers.  D? Nope. No spread spectrum action going on here. C? Again, no. There is no requirement that the signal be narrow FM at all.  B? Getting warmer, but still wrong.

The correct answer is A. Amplitude modulation in which one complete sideband and a portion of the other are transmitted.

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