Does turning a volume knob create sidebands?

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Please note that that the "original numbers" had a ~ in front of them , a footnote saying that "'~' means "approximately" and a footnote saying that I was purposely fudging the exact frequencies (and only having two of them instead of three) in an attempt to make the thought experiment easier for Dan to follow. My stated goal was to get him to understand how the signals cancel and reinforce in the simplified case, then to expand it to three frequencies once he got the basic concept. Remember, I was relying to a series of posts that claimed that wiggling a pot creates no new frequencies at all, not trying to explain modulation to a bunch of experts who have forgotten more about RF than I ever learned.

change

From a systems engineering standpoint, clear from biochemistry to psychology, humans are awful kluges. You've just got to accept that, and balance the many silly forces, or get all neurotic.

Which musing balances the manual I'm supposed to be writing.

John

(snip)

The FCC says the emission designator for "Amplitude modulated single sideband voice" has changed from 3A3J to 3K00J3E, the point being that they consider single sideband as a form of AM.

And that kind of makes sense since it is the amplitude of what is emitted that conveys information. Either that, or I don't understand any of this.

Cheers to you, too. John

"John KD5YI"

• name * of something with its technical definition.

Names merely distinguish one entity from another - like company names do - but are not definitions of the entity.

Think of all all the possible " SSB " signals that are NOT related to an AM signal ......

..... Phil

Thanks all for the nice discussion, I know very little about side bands and I tend to get lost in the mathematical analysis, I really like a picture of something. My understanding of side bands and the phase relationships was much improved when I saw phasor diagrams of the addition of sine waves. I saw this in the "Physics of Vibration, Vol. 1" by A.B. Pippard, though I assume it's been done else where. Being able to see the vectors added graphically made everything much clearer. I'm sorry I don't have a link to post.

George Herold

You should check with someone who has a spectrum analyzer, and watch the display as you try various forms of modulation and stuff. If you're only turning a volume control, you'd probably have to crank the resolution way up (i.e., look at the narrowest band your SA can look at), because if all you're doing is turning a knob, the sidebands won't be very far from the carrier.

Cheers! Rich

On a sunny day (Mon, 03 Nov 2008 19:12:38 GMT) it happened Rich Grise wrote in :

You are totally wrong. It depends on *how* you move the slider or knob.

If you move it reasonably fast in a linear way, like a ramp, then: Maybe this will HELP:

The ... ramp waveform (or n-ramp) is made up of a fundamental component and all of its harmonics, each in phase with the fundamental.

So, generated is (fc is carrier, fm is modulating frequency): fc - fm, fc - 2fm, fc - 3fm, fc - 4fm, fc - 5fm, fc - 6fm ....... fc + fm, fc + 2fm, fc + 3fm, fc + 4fm, fc + 5fm, fc + 6fm .......

The spectrum should be full, and much wider than 2 x fm. Of course the further away from Fc, the weaker the sideband.

So that was a linear fade out.

It certainly does. The smooth S-shaped transition curve generates minumum of the artifacts. You can generate the transition curve as a lookup table, however it will take several thouthands of samples. The trick is how to generate such curve in place efficiently. Another problem is what to do if the slider is moving while the S-shaped transition is initiated already.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Interestingly, that same S shaped curve (a section of a sine wave, actually) is really effective at reducing vibration and acoustic noise in servoactuators.

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How can signals at different frequencies be 'in phase' with one another?

Chris

On a sunny day (Tue, 4 Nov 2008 17:17:18 -0000) it happened "christofire" wrote in :

Although the signals are at different frequencies, those frequencies are in this case always *harmonics*, so multiples of the base frequency. You can phase shift an harmonic, but its frequency will always be

Dark Fader

There is no "approximately" about right and wrong. And you perhaps shouldn't lecture about "basic concepts" that you don't understand... especially in a public forum.

When you're wrong, the easiest thing to say is "oops, I was wrong" and end it there.

John

So what do you get if you are 90 degrees lagging from in sync?

Nice. I even understand the unexpected envelope for the 90 degree carrier. But something else is happening also, isn't it?

In that case you get an ugly AM/PM thing. The sum of the two sidebands is always at 0 or pi phase from the carrier in normal AM, so in your situation they always have a +-pi/2 phase. The resulting waveform has some phase modulation and some amplitude modulation, both badly distorted--the envelope is sqrt(1+sin(2 pi f_m t)**2) instead of 1+sin(2 pi f_m t), and the phase is atan(sin(2 pi f_m t)cos(2 pi f_c t)/sin(2 pi f_c t)), which isn't anything pretty either.

Cheers,

Phil Hobbs