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Does turning a volume knob create sidebands?

>D> >>John Devereux wrote: >> >>>D>>> >>> Amplitude modulation creates sidebands. >> >> Yes it does, so what is the non-linear device in the amp >> that will do the multiplication required of AM? > >Non-linear with respect to what? You can configure an analog >multiplier as a frequency multiplier, by connecting both >inputs to the signal. If the signal is x then the output is >x^2, there is your non-linearity. The same thing would happen, >conceptually, if one input was an input to an amplifier, and >the other one a servoed volume pot. In both cases the output >is one input, multiplied by the other. > >>> This remains true if the modulation is done in an analog >>> multiplier, or by varying the power supply, or by turning a >>> volume control knob. Although obviously the frequency offset >>> will be tiny with a manually operated control, they are still >>> there. >> >> How do you know this to be true? > >Because the volume control is a perfect analogy, conceptually, >to the second input of an analog multiplier. Analog multipliers >can be used as mixers. > >>> AIUI even Eeyore doesn't think the effect is non-existent, >>> just that it is so tiny it would not be heard (which is >>> true no doubt). >> >> Why "no doubt?" > >Because it's only a ~1 Hz away from the fundamental and the >volume is going up and down at the same time. OK, maybe it >*could* be heard, with the right test signal. I guess I >don't really know the limits of audio perception.

Don, assume for the sake of argument that there is a 1000 Hz pure sine wave added to a identical-amplitude 1001 Hz pure sine wave in the normal way that sine waves combine to make more complex waveforms. These can be voltages, radio waves, or sound waves -- the principle[1] remains the same.

As the two sine waves cancel and reinforce each other, the combined output will vary from zero amplitude to full amplitude at a 1 Hz rate. You will, no doubt, recognize what I just described as a SSB (single sideband) AM signal. Multiply the frequency of both signals by a factor of 1000 and you will have a nice ~1KHz SSB modulation of a ~1MHz carrier[2] that any radio engineer would be proud to see from his transmitter.

Now assume that the *exact same signal* was created by running a ~1 KHz signal through a standard audio amplifier and having a servo move a fader up and down in a sinusoidal fashion at a ~1 Hz. rate. Exact same signal = exact same harmonic content.[3] Therefore we can conclude that moving the fader creates sidebands.

Footnotes:

[1] no fair using multiple sources that are 1/4 wave apart, 1/2 wave apart, etc. They have to be at the same point otherwise we will get bogged down thinking about interference patterns, lobes, etc. [2] You may have noticed that I failed to specify the exact frequency. You tell me: is it 1000 Hz? 1001 Hz? 1000.5 Hz? Other? To keep the carrier at exactly 1000 Hz despite changes in the modulation signal, you really need a *double* sideband AM signal, which would have complicated my description and made the basic principle less clear. I also cheated a bit by assuming a pure sine wave for the modulation... A question for you to ponder: assuming that "~" means "approximately", would our radio engineer be able to look at a signal with a ~1KHz amplitude modulation of a ~1MHz carrier and tell you whether is was SSB AM or DSB AM? How about the case where the modulation was a human voice? [3] I still recall being amazed a a small child when my dad showed me using all-tube test equipment and a blackboard that any periodic waveform can be created out of pure sine waves. As a teenager, when I learned about the Fourier series and the various Fourier-related transforms, it was the first time it really hit me that math wasn't just something teachers use to torture students but was instead a way to gain a deeper understanding of exactly how the world really works. 
--
Guy Macon  Guy Macon 
Guy Macon  Guy Macon 
Guy Macon  Guy Macon 
Guy Macon  Guy Macon
Reply to
Guy Macon
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"Guy Macon"

** But does NOT have the same envelope shape as a ( 100%) amplitude modulated wave.
** Hold on just a mo, you are being very loose with terminology.

A radio signal is *either* SSB or it is AM - it cannot be both.

** What are you on about ??

SSB radio ( ie ham ) transmitters output no carrier at all - the popular term " SSB" is shorthand for SSBSC = Single Side Band Suppressed Carrier.

So, when the transmitter's input signal is a simple 1000Hz sine wave, the RF output will also be a simple sine wave at an RF frequency equal to the suppressed ( ie non existent) carrier plus 1000Hz - assuming upper sideband mode.

BTW

You are way off track and merely obfuscating the bleeding OBVIOUS - well obvious to everyone except that congenital and incorrigible nit-wit Bowey.

...... Phil

Reply to
Phil Allison

snip

I got a slightly diffement theery

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martin

Reply to
Martin Griffith

On a sunny day (Tue, 28 Oct 2008 14:44:35 +0100) it happened Martin Griffith wrote in : .

LOL

Reply to
Jan Panteltje

All that is so totally whacked-out, so insanely wrong, that it should disqualify you from ever doing any non-trivial signal-processing engineering, ever.

But moving the fader does create new frequencies.

John

Reply to
John Larkin

I just want to know, when he says "we can conclude", who is "we"? There could be more than one of them out there! ;-)

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
There are only 10 kinds of people in this world,
those who understand binary and those who don\'t.
Reply to
Paul Hovnanian P.E.

As Yogi Berra said, "include me out."

John

Reply to
John Larkin

I may be incorrect in my understanding in an area (signal processing) that is not my specialty, or I may have written poorly and given a wrong impression, but mere insults without any explanation as to *why* I am wrong do not give me any opportunity to learn from my mistakes. When I disgreed with Don Bowey, I treated him with respect, told him why I think he is wrong, and responded to his arguments rather than simply engaging in personal attacks.

Although I fail to follow my own advice at times, my preference is to approach technical discussions on the basis of logic and evidence, not emotion.

Reply to
Guy Macon

On a sunny day (Tue, 28 Oct 2008 22:34:34 +0000) it happened Guy Macon wrote in :

I have to work from memory, but I think what you said was: 'Take 1000 and 1001 Hz, and *add* those.'

What is not correct, is that that is the _same_ as AM modulating 1000 Hz with 1Hz.

If you AM modulate 1000Hz(fc) with 1Hz(fm), you get fc - fm, fc, and fc + fm, so 999, 1000, and 1001 Hz. The math is described here, it is simple gonio multiplication:

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Reply to
Jan Panteltje

Sno-o-o-o-ort! Today has been so strange I was beginning to think there had been a time warp to April 1.

Maybe Guy can give Jon Slaughter a hand with fixing his schematic-less preamp ?:-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food
Reply to
Jim Thompson

Then it might be prudent to not make pedantic technical statements about things that you don't understand.

John

Reply to
John Larkin

Cheers?

John

Reply to
John Larkin

Guy,

(I'm home sick today, so I have more time to post.)

(a) There's no such thing as a 'SSB AM signal'. If you stick a sine wave into a SSB modulator, what comes out is another sine wave--the SSB modulator maps f_audio->f_c+f_audio (USB) or f_c-f_audio (LSB).

(b) What you're describing is a DSB signal, as produced by a 0.5 Hz modulation frequency on a 1 kHz carrier. You'd need to add a 1.0005 kHz signal of twice the amplitude, properly phased, to make it 100% AM.

(c) No RF engineer would be proud to see a 'SSB' signal with 0 dB unwanted sideband rejection. He'd get fired.

(d) The DSB signal you are in fact describing has an envelope that goes negative, which means that it cannot be generated by any unipolar gain change whatsoever. In particular, the servo-controlled fader couldn't generate it.

(e) The tone of assurance you adopt might lead people to believe you have some idea of RF signal processing, which (with 4 serious mistakes in a

19-line post) you apparently don't possess.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Yep. Phil is sipping his "medication" ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

"The American Republic will endure until the day Congress
discovers that it can bribe the public with the public\'s money."

                 - Alexis de Tocqueville
Reply to
Jim Thompson

"Phil Hobbs"

** This is not correct.

Guy's example involved the *sum* of two equal amplitude sine waves of 1000 Hz & 1001 Hz.

This just happens to be similar to the *multiplication* of two frequencies ( of equal amplitude again) whose sum and difference are equal to the two frequencies that were summed. In this case, that makes the two frequencies

0.5 Hz and 1000.5 Hz.

However, the upper and lower sidebands of AM modulation are phase related while two arbitrary frequencies are not.

IOW - you will NOT get an AM signal buy adding three sine waves whose frequencies are related by just A, B and (A+B)/2.

..... Phil

Reply to
Phil Allison

"...in the normal way that sine waves combine to make more complex waveforms" was my exact phrase. Not very clearly written, obviously. I apologize for that.

I apologize for again being unclear. When I wrote

"...you really need a *double* sideband AM signal, which would have complicated my description and made the basic principle less clear."

I was attempting (poorly) to convey the above concept. Given how the thread had been going, I came to the perhaps mistaken conclusion that discussing signals at three frequencies would not have gotten my point across to Dan. So I tried to get him to visualize the case with one sideband, thinking that once he saw the pattern of cancellation and reinforcement, I could move on to discussing two sidebands at once.

Earlier today I dug up some polaroid photos I made of what I saw on the spectrum anayser the last time I worked with RF, and two things jumped out at me:

[1] It has been a *long* time since I did anything with RF, and I never did get enough experience to be any good at it. Which is why my carreer focus is on low-cost high-volume uCs. [2] What I remembers as being a bog-standard AM signal was really double-sideband reduced carrier, so my mental image of the sidebands and carrier was way off. Again, I apologize and I will make sure I hit the books before ever describing modulation in the frequency domain again.

Thanks! The discussion of Single Side Band AM vs. Vestigial Side Band AM was especially helpful. I had forgotten that.

For some reason I find the above to ba a lot easier to follow that the one I found using a Google search: [

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]. Thanks for bringing it to my attention.

--
Guy Macon
Reply to
Guy Macon

Sure, cheers.

I've made worse mistakes than those myself, just not in quite such a concise fashion. (Like Zaphod Beeblebrox, I made the mistake of looking into my soul, and discovered that it wasn't the totally wonderful object that a man in my position would expect.)

Cheers again,

Phil Hobbs

Reply to
Phil Hobbs

In my defense, there are a great many educated technical people who use the following phrases, as can be verified with a Google search:

" Single Sideband AM " " Upper Sideband AM " " Lower Sideband AM "

I agree that it is not technically correct usage.

Which, of course, I do not. What I was *trying* to convey was that I have a great deal of assurance that Dan's theory about volume knobs and modulation is wrong. I was trying to get him to see why using a simplified thought experiment using two signals, not addressing those who not only know that already, but are experts on the subject. I apologize for any impression of assurance about any expertise in RF. For the record, my expertise is in other areas entirely. If I need a RF design, I hire a RF designer.

The next time I edit my resume I will add a "I suck at RF" disclaimer to go with the existing "I suck at C/C++" notice. In fact, I will make a long list of things that I suck at. We all have our limitations...

--
Guy Macon
Reply to
Guy Macon

Hey, I'm a recovering Episcopalian, cut me some slack. ;)

Cheers,

Phil Hobbs

(now in the Anglican Church of Uganda, believe it or not)

Reply to
Phil Hobbs

On Tue, 28 Oct 2008 23:13:52 GMT, Jan Panteltje wrote:

--- That's not correct either.

If you _add_ 1000 Hz. and 1001 Hz., what you get is an algebraic addition, not a multiplication. On a scope the amplitude variations _look_ like AM as the signals slide through each other, but they're not, they're just two AC signals adding in time.

Here:

Version 4 SHEET 1 880 680 WIRE 208 -16 -272 -16 WIRE 16 80 -160 80 WIRE 128 80 96 80 WIRE 256 80 128 80 WIRE 368 80 336 80 WIRE 208 144 208 -16 WIRE 16 160 -48 160 WIRE 128 160 128 80 WIRE 128 160 96 160 WIRE 176 160 128 160 WIRE 368 176 368 80 WIRE 368 176 240 176 WIRE 176 192 128 192 WIRE -272 272 -272 -16 WIRE -160 272 -160 80 WIRE -48 272 -48 160 WIRE 208 272 208 208 WIRE 368 272 368 176 WIRE -272 416 -272 352 WIRE -160 416 -160 352 WIRE -160 416 -272 416 WIRE -48 416 -48 352 WIRE -48 416 -160 416 WIRE 128 416 128 192 WIRE 128 416 -48 416 WIRE 208 416 208 352 WIRE 208 416 128 416 WIRE 368 416 368 352 WIRE 368 416 208 416 WIRE -272 480 -272 416 FLAG -272 480 0 SYMBOL Opamps\\\\LT1001 208 112 R0 SYMATTR InstName U1 SYMBOL voltage -48 256 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(0 1 1000) SYMBOL voltage -160 256 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 1 1001) SYMBOL voltage -272 256 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value 10 SYMBOL voltage 208 368 R180 WINDOW 0 24 104 Left 0 WINDOW 3 24 16 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V4 SYMATTR Value 10 SYMBOL res 112 144 R90 WINDOW 0 63 62 VBottom 0 WINDOW 3 64 64 VTop 0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res 112 64 R90 WINDOW 0 -38 56 VBottom 0 WINDOW 3 -35 58 VTop 0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res 352 64 R90 WINDOW 0 -38 56 VBottom 0 WINDOW 3 -35 58 VTop 0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL res 352 256 R0 SYMATTR InstName R4 SYMATTR Value 10k TEXT -240 448 Left 0 !.tran 2

Run it and do an FFT on the plot and, interestingly, there _do_ seem to be some sidebands about f1 and f2, but they're about 45 dB down so there must be some nonlinearities in the opamps causing them.

Even more interesting, there are two pairs of them.

---

JF

Reply to
John Fields

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