Talk:Intermodulation

From Wikipedia, the free encyclopedia

This article contains material from the Federal Standard 1037C (in support of MIL-STD-188), which, as a work of the United States Government, is in the public domain.

	Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
Start	This article has been rated as Start-Class on the assessment scale.
???	This article has not yet received an importance rating within physics.
Help with this template Comments: edit – history – watch – refresh As for the audio aspect: While some engineers may introduce more intermodulation distortion into a recording, I doubt this is intentional. Additional harmonic distortion is what will add to the "pleasantness" of a sound. Usually, IM distortion is regarded as a loss of clarity and depth, something that isn't desirable in a recording.

	This article is within the scope of the Professional sound production WikiProject, a collaborative effort to improve Wikipedia's coverage of the technology, equipment, companies and professions related to professional sound production. If you would like to participate, you can visit the project page, where you can join the project and see a list of open tasks.
Start	This article has been rated as Start-Class on the assessment scale.

Contents 1 Not just radio 2 Agreed 3 IMD in music? Really? 4 Detech Intermodulation system 5 Changes 5.1 Zone number 6 Linear systems 7 "Problems in live audio" section

[edit] Not just radio

Can't "intermodulation" apply to other things such as audio? Also it can be a signal in a piece of electronics, such as intermodulation distortion in an amplifier (or electromechanical IMD in a loudspeaker), not just broadcasted radio. I think it needs some work to broaden the article. --Howdybob 13:18, 1 July 2006 (UTC)

[edit] Agreed

It does apply and I've added a bit of what I know on the subject under the "Intermodulation Distortion" header.

--- jason | sterly 20:44, 8 July 2006 (UTC)

Thanks. I wrote that comment in part because this article mentioned "radio signals" which I later changed to just "signals." I'm wondering about the new material though, which I'll discuss at Talk:Intermodulation distortion. --Howdybob 21:53, 8 July 2006 (UTC)

[edit] IMD in music? Really?

Is it really IMD per se that is used in music and so forth, rather than just harmonic distortion in general? --Howdybob 02:30, 28 November 2006 (UTC)

[edit] Detech Intermodulation system

developed in Invercargill, New Zealand by ASI limeted this system finds the cause of intermodulation in comercial ventures such as broadcast towers where rusty joints are the cause. This system can be carried on site and then a small handheld antena run over the tower until the cause/s has been found this saves time and effort only correcting the problem

[edit] Changes

I've attempted a first sweep at tidying up this article; here are a list of things that I've altered:

• Removed claim that the largest and second-largest IM products are second and third-order. This depends entirely on the coefficients of the Taylor series.

• Removed dubious distinction between "active" and "passive" intermodulation

• Removed claim that non-linearity will cause an infinite number of harmonics.

• Removed claim that higher-order harmonics will have less energy; again this is entirely dependent on the non-linear function in use.

• Changed subscripts from 1, 2, 3 to a, b, c, so as to reduce confusion with coefficients taking on integer values.

• Removed unnecessary introduction of "centre frequency".

• Removed distinction between "dominant" and "specific"; I've never heard of these distinctions, and can find no reference on the web to these terms being used in this sense.

• Removed discussion of "zone number"; again, I can find no reference to such a concept.

• Removed discussion of "conservation of power"; this is only true for passive devices; it is not so for general non-linear functions (e.g. amplifiers).

• Removed "Intermodulation distortion" section, as this is completely redundant.

I have not yet edited the "Intermodulation noise", "Use in music production", "Use in music production" and "Passive intermodulation" sections, but they also definitely need work.

If I have gone too far with something, or removed terminology for which sources can be found, please feel free to re-edit. Any comments would be appreciated also.

Oli Filth 00:15, 7 September 2007 (UTC)

Please allow me to respond to your comments:

1. Perhaps you will be OK with a phrasing that second and 3rd order intermods are usually the largest ones, if that's the case, just add that word.
2. The article's intro should allude to the existence of passive intermods, as intermodulation is not expected in passive structures
3. Since the Taylor Series is infinite, so is the number of intermodulation terms
4. Since the power is finite, but the number of intermod terms is infinite, obviously higher order terms must die out
5. Center frequency is a very useful concept to explain the higher order intermods
6. If you would like to challenge use of the terms "dominant" and "specific", please use the {{fact}} template, but do not delete it.
   • Same for the Zone Number
7. Active devices must also obey the principle of conservation of power
8. The "Intermodulation distortion" section was merged from its own article, and should be preserved, because it helps to give more practical applications to the intermodulations. Feel free to better incorporate it, but please do not delete it.

--Mr. PIM 20:23, 12 September 2007 (UTC)

Thanks for your comments. Here, in turn, are my responses (I've numbered your points for ease of reference):

1. We'll need to think of an appropriate wording if we want to discuss 2nd and 3rd usually being the largest. Specifically, in many radio applications, even-order products are often insignificant. Perhaps something along the lines of "In many practical electronics applications, low-order IM products contain the most energy."
2. Who says it's not expected? It's a critical design point, in say, audio transformers. At any rate, there is still a section devoted to this topic.
3. This article is about intermodulation in general, not practical examples specifically. Whilst in most real-world applications, the transfer function will best be expressed by an infinite series; this is not a requirement for a non-linear system. In the general case, the power series expansion is not necessarily infinite. The simplest case is self-modulation, i.e. .
4. Same as above.
5. I don't see how talking about centre frequency helps. On the contrary, it just introduces another variable that the reader will have to keep track of.
6. There is no requirement to leave in material that one thinks dubious. I have never seen these terms used, and can find no reference to them used in this context on the web. If you can show otherwise, then by all means provide a ref, and I'll be happy to re-add the discussion of them to the article.
7. For active devices, it is simply not true (almost by definition) that "Conservation of power requires that the total power of the output signal must be less than or equal to the total power of the input signal".
8. I've realised that I screwed up the section hierarchy in my series of edits. The majority of the "Intermodulation distortion" is still there; all I removed was the intro to that section, which was completely redundant. I'm in the process of sorting that out.

Oli Filth 21:07, 12 September 2007 (UTC)

[edit] Zone number

On another look, it seems there are a few relevant references to e.g. "first zone" + intermodulation. However, I'm not sure that in this context, the "zone number" is defined by the sum of the intermod coefficients; instead, the "m^th zone" would seem to be defined (I'm guessing here) as "the region of spectrum surrounding the m'-th harmonic of the carrier frequency".

Now, in the case where all the original frequency components are closely bunched around the carrier (e.g. in a radio context), then I can see that the formula for would more or less tally with this definition I just made up. However, in the more general case, where there is no bunching around a carrier (e.g. an audio signal, or a wideband transmission), such a relationship isn't meaningful.

Therefore, I'm still sceptical of the definition of zone number being "the sum of the intermod coefficients". Oli Filth 07:01, 13 September 2007 (UTC)

[edit] Linear systems

I made two changes to the section "causes of intermodulation".

• First, I deleted the leading words "By definition" because the definition of a linear system is that if f(x)=a and f(y)=b then f(x+y)=a+b and that f(kx)=kf(x). The definition doesn't mention the presence or absence of intermodulation.
• I also added the qualifier "time invariant" because this is a necessary condition for the statement to be true. A counter-example is a mixer, which multiplies its input by sin(wt). This is a linear, time varying system, with frequencies not present in the input.

Serrano24 15:03, 12 September 2007 (UTC)

You are missing the point of the definition of a linear system. If a linear system has a sinusoidal input, the output by definition, is a sinusoid of the same frequency as the input sinusoid, but possibly a different amplitude or phase. Therefore, a linear system cannot have intermodulations, since the output will be the exact same frequencies of the input signals. --Mr. PIM 20:09, 12 September 2007 (UTC)

I'm assuming that we're talking about LTI systems here, not just linear. I agree with Serrano24; the definition of an LTI system is, of course, that it obeys the properties of linearity and time-invariance. One resultant property of such systems is that complex exponentials (and therefore sinusoids) are eigenfunctions; but this is not in any way the definition. Oli Filth 20:18, 12 September 2007 (UTC)

OK, changed from "by definition" to "a consequence". --Mr. PIM 20:29, 12 September 2007 (UTC)

"The consequence of a linear system is that it cannot produce intermodulations" doesn't really read properly. This could be re-worded as "One consequence of the property of linearity is that such a system cannot produce intermodulation". But what's wrong with just leaving it as it was: "A linear system cannot produce intermodulation"? Oli Filth 20:33, 12 September 2007 (UTC)

[edit] "Problems in live audio" section

I'm not convinced of the factual accuracy or relevance of this section. It seems to be more interference in general, rather than intermodulation. Can someone please tidy this up, and/or add an appropriate reference.

Regards, Oli Filth 21:16, 12 September 2007 (UTC)

I've now removed the section. It may be retrieved, if necessary, from this edit. Oli Filth 18:34, 14 September 2007 (UTC)

This section, as of May 2008, is completely incorrect. It is clearly describing gain compression and harmonic distortion, which should not be confused with intermodulation distortion. Nimur (talk) 16:11, 22 May 2008 (UTC)

I have rewritten this section, I hope it clears up the important distinction between harmonic distortion in general, and intermodulation in particular. Nimur (talk) 16:15, 22 May 2008 (UTC)