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Power line hum question

Started by DJT November 10, 2008
On Mon, 10 Nov 2008 19:58:47 -0600, Vladimir Vassilevsky wrote:

> DJT wrote: > >> Hello everybody! >> >> I'm trying to remove power line hum from a signal and have two >> questions concerning this. >> >> 1. How high do the harmonics go? (In general, round numbers) > > Infinitely high. The rolloff factor ballpark is ~10dB per octave.
And you can't get more general (or round!) than infinity.
> >> 2. How much frequency variations (+/-) should I expect in the >> fundamental 50 Hz component? > > Very accurate in the long term; short term accuracy ~ 1e-4. > >> The reason for asking these questions is that I am having a hard time >> choosing between a fixed comb-notch filter and something that tracks >> the frequency variations by adapting... > > No point to do an adaptive filter. The random fluctuations of the > waveform smear the spectrum. >
I would disagree on the last two points. If you're running off of a continent-wide grid then the frequency variations are very small indeed. But if you're making a product that may ever have to run off of a generator, or on a small grid (and there are some, even in out-of-the-way places in developed countries) then this isn't going to hold true. And if the frequency varies, then a fixed-frequency comb filter isn't going to help much. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
Robert Adams wrote:
(snip)

> On a slightly related note; did you know that if you are analyzing a > recording and wish to know when and where it was made, you can extract > the hum on the recording, track the tiny fluctuations in frequency > over the duration of the recording, and correlate these with the > frequency variations that occur in the power grids of various > countries? These fluctuations are recorded (for just this purpose), so > you can tell both the time it was recorded as well as the country > (just search the database for a correlation peak). Since all > generators are syncronously linked, the frequency variations are valid > for the entire country (or continent, even).
> This doesn't work if you are running off a battery, and are living in > a cave .....
Or if your recorder has a synchronous motor run off the power line. Well, if you had a constant frequency signal in the recording other than the power line (assumed not constant) then you could do it directly from a synchronous motor powered recording even without hum. -- glen
>On Mon, 10 Nov 2008 19:58:47 -0600, Vladimir Vassilevsky wrote: > >> DJT wrote: >> >>> Hello everybody! >>> >>> I'm trying to remove power line hum from a signal and have two >>> questions concerning this. >>> >>> 1. How high do the harmonics go? (In general, round numbers) >> >> Infinitely high. The rolloff factor ballpark is ~10dB per octave. > >And you can't get more general (or round!) than infinity. >> >>> 2. How much frequency variations (+/-) should I expect in the >>> fundamental 50 Hz component? >> >> Very accurate in the long term; short term accuracy ~ 1e-4. >> >>> The reason for asking these questions is that I am having a hard time >>> choosing between a fixed comb-notch filter and something that tracks >>> the frequency variations by adapting... >> >> No point to do an adaptive filter. The random fluctuations of the >> waveform smear the spectrum. >> >I would disagree on the last two points. If you're running off of a >continent-wide grid then the frequency variations are very small indeed.
>But if you're making a product that may ever have to run off of a >generator, or on a small grid (and there are some, even in out-of-the-way
>places in developed countries) then this isn't going to hold true. > >And if the frequency varies, then a fixed-frequency comb filter isn't >going to help much. > >-- > >Tim Wescott >Wescott Design Services >http://www.wescottdesign.com > >Do you need to implement control loops in software? >"Applied Control Theory for Embedded Systems" gives you just what it
says.
>See details at http://www.wescottdesign.com/actfes/actfes.html >
Found the thread and I think this idea looks interesting assuming the frequency varies significantly... "If there's a need to preserve the signal much above the line rate and if you have a sample of the line frequency available, an cheap adaptive filter that acquires the line frequency and uses that to set the spacing on a comb filter would work well." However I don't really understand one of the answers I got concerning my idea to use an adaptive filter (I was thinking NLMS)... "No point to do an adaptive filter. The random fluctuations of the waveform smear the spectrum." I can buy this, but still is it not a little bit harsh to say that there is no point to use an adaptive filter... I mean this should be higly dependent on the application, the characteristics of the signals the amount of "smearing" that can be tolerable and so on. Am I completely lost here or? JT

Tim Wescott wrote:
> On Mon, 10 Nov 2008 19:58:47 -0600, Vladimir Vassilevsky wrote: > >
>>No point to do an adaptive filter. The random fluctuations of the >>waveform smear the spectrum. >> > > I would disagree on the last two points. If you're running off of a > continent-wide grid then the frequency variations are very small indeed. > But if you're making a product that may ever have to run off of a > generator, or on a small grid (and there are some, even in out-of-the-way > places in developed countries) then this isn't going to hold true. > > And if the frequency varies, then a fixed-frequency comb filter isn't > going to help much.
Some time ago I developed VLF instruments. The harmonics of the power frequency are the major source of interference in VLF; typically they are dominating over the ambience up to the 1000th harmonic orders. Of course, I took advantage of the comb filtering, and got a substantial improvement. However the random variations of the power waveform due to the loads switched on/off, etc. are the natural limitation to that improvement, and this limitation happens much earlier then the frequency instability comes into play. So, I don't see much benefit in making the filter frequency adaptive, unless someone is running from the autonomous alternator. BTW, those cheap alternators are terrible: the frequency can be off by as much as 10 Hz or so; I had issues with that on the different project :) Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
On Mon, 10 Nov 2008 18:26:44 -0800 (PST), Robert Adams
<robert.adams@analog.com> wrote:

>On Nov 10, 8:58&#2013266080;pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com> >wrote: >> DJT wrote: >> > Hello everybody! >> >> > I'm trying to remove power line hum from a signal and have two questions >> > concerning this. >> >> > 1. How high do the harmonics go? (In general, round numbers) >> >> Infinitely high. &#2013266080;The rolloff factor ballpark is ~10dB per octave. >> >> > 2. How much frequency variations (+/-) should I expect in the fundamental >> > 50 Hz component? >> >> Very accurate in the long term; short term accuracy ~ 1e-4. >> >> > The reason for asking these questions is that I am having a hard time >> > choosing between a fixed comb-notch filter and something that tracks the >> > frequency variations by adapting... >> >> No point to do an adaptive filter. The random fluctuations of the >> waveform smear the spectrum. >> >> Vladimir Vassilevsky >> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com > >On a slightly related note; did you know that if you are analyzing a >recording and wish to know when and where it was made, you can extract >the hum on the recording, track the tiny fluctuations in frequency >over the duration of the recording, and correlate these with the >frequency variations that occur in the power grids of various >countries? These fluctuations are recorded (for just this purpose), so >you can tell both the time it was recorded as well as the country >(just search the database for a correlation peak). Since all >generators are syncronously linked, the frequency variations are valid >for the entire country (or continent, even). > >This doesn't work if you are running off a battery, and are living in >a cave ..... > >Bob Adams
Interesting. So the next obvious step would be to have each power generating station modulate their output very slightly with stuff like data and time, spread with a unique spreading code that would identify the station. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
On Nov 11, 2:26&#2013266080;pm, "DJT" <JanTorgrims...@hotmail.com> wrote:
> Hello everybody! > > I'm trying to remove power line hum from a signal and have two questions > concerning this. > > 1. How high do the harmonics go? (In general, round numbers) > > 2. How much frequency variations (+/-) should I expect in the fundamental > > &#2013266080; &#2013266080;50 Hz component? > > The reason for asking these questions is that I am having a hard time > choosing between a fixed comb-notch filter and something that tracks the > frequency variations by adapting... > > Thanks in advance > > JT
I am always surprised that people want to filter out hum when the best solution is to avoid it in the first place. Often a bad layout in a board,earthing etc has a lot to do with it. Mains hum is everywhere, but if you are careful with layout,shielding,grounding you can minimise it. Hardy
HardySpicer wrote:
> On Nov 11, 2:26 pm, "DJT" <JanTorgrims...@hotmail.com> wrote: >> Hello everybody! >> >> I'm trying to remove power line hum from a signal and have two questions >> concerning this. >> >> 1. How high do the harmonics go? (In general, round numbers) >> >> 2. How much frequency variations (+/-) should I expect in the fundamental >> >> 50 Hz component? >> >> The reason for asking these questions is that I am having a hard time >> choosing between a fixed comb-notch filter and something that tracks the >> frequency variations by adapting... >> >> Thanks in advance >> >> JT > > I am always surprised that people want to filter out hum when the best > solution is to avoid it in the first place. > Often a bad layout in a board,earthing etc has a lot to do with it. > Mains hum is everywhere, but if you are careful with > layout,shielding,grounding you can minimise it.
In humid weather, my favorite AM radio station is almost completely jammed by power-line harmonics when I am near certain poles. further away, the interference is merely annoying. (When it's very bad, I note the pole numbers and alert the power company. Spraying the insulator with WD-40 helps considerably.) Can you suggest how I can avoid this? Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
On Tue, 11 Nov 2008 09:02:21 -0600, Vladimir Vassilevsky wrote:

> Tim Wescott wrote: >> On Mon, 10 Nov 2008 19:58:47 -0600, Vladimir Vassilevsky wrote: >> >> >> >>>No point to do an adaptive filter. The random fluctuations of the >>>waveform smear the spectrum. >>> >>> >> I would disagree on the last two points. If you're running off of a >> continent-wide grid then the frequency variations are very small >> indeed. But if you're making a product that may ever have to run off of >> a generator, or on a small grid (and there are some, even in >> out-of-the-way places in developed countries) then this isn't going to >> hold true. >> >> And if the frequency varies, then a fixed-frequency comb filter isn't >> going to help much. > > Some time ago I developed VLF instruments. The harmonics of the power > frequency are the major source of interference in VLF; typically they > are dominating over the ambience up to the 1000th harmonic orders. Of > course, I took advantage of the comb filtering, and got a substantial > improvement. However the random variations of the power waveform due to > the loads switched on/off, etc. are the natural limitation to that > improvement, and this limitation happens much earlier then the frequency > instability comes into play. So, I don't see much benefit in making the > filter frequency adaptive, unless someone is running from the autonomous > alternator. BTW, those cheap alternators are terrible: the frequency can > be off by as much as 10 Hz or so; I had issues with that on the > different project :)
I wouldn't expect any modern power grid that's bigger than a small city to have enough variation to cause a hint of a problem. But after 20 years of saying "the customer did _what_?!?", sighing, and starting in on a design modification, I've gotten more willing to be defensive in my engineering. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html