Suodatin Pussi wrote:> Rob Hutchinson wrote: > > >>What is the preferred method for removing 60 hz hum from a signal without >>wiping out signal info around 60 hz? A 60 hz notch filter would not be >>useful because it would attenuate the signal as well. I'm interested in >>doing this for sampled data, so all filtering would be done in the digital >>domain. >>Thank you in advance, >>Rob >> >> > > You could try using some active noise reduction. Use a 60 Hz 'generator' > of which you can alter phase & amplitude and a little freq and a small > bandpass filter around 60 Hz. Now adjust the parameters so that the > output of the filter minimizes. (but I guess its earier said than done) > For me 150 Hz is always annoying me, they manage to deliver a clipped mains.And a 120 Hz generator. And 180, 240, and maybe more. An instrumentation amplifier with proper attention to where the ground returns run is the best answer for difficult cases. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
60 Hz Hum removal
Started by ●July 1, 2004
Reply by ●July 1, 20042004-07-01
Reply by ●July 2, 20042004-07-02
On Thu, 1 Jul 2004 21:50:33 -0400, "Evert Rozendaal" <evert20@adacs.com> wrote:>It depends on how the hum is picket up but I bed the amplitude is not >constant. Most likely it is not a perfect sine wave either. Depending on >your setup sometimes it helps to match the impedance and remove the signal >you are interested so you can look at the hum only.An example would be magnetic coupling from a PSU transformer to a guitar pickup. The phase of the induced signal will reverse as the guitarist spins around (doin' those rock moves). Coupled mains signals tend to have a heavy harmonic content, particularly for odd harmonics. They may sound more like 'buzz' ranther than 'hum'. Regards, Allan.
Reply by ●July 2, 20042004-07-02
"Rob Hutchinson" <rhutch7@kdsi.net> wrote in message news:10e8scqobsj7f3@corp.supernews.com...> What is the preferred method for removing 60 hz hum from a signal without > wiping out signal info around 60 hz? A 60 hz notch filter would not be > useful because it would attenuate the signal as well. I'm interested in > doing this for sampled data, so all filtering would be done in the digital > domain.Subtraction is the best bet. That is, active noise cancellation. That is, if you can establish a reference for the offending signal. So, if the source of the 60Hz signal is available to you then you could do something like this: +--------------------+ | | | | | | input------+---------------------|------------>(+)----+----> e[n] | ^ | | v | 60Hz >----------------->[LMS]------------+ Adaptive canceller LMS adaptive filter adjusts to minimize e[n] which cancels signal and adds no additional noise. Now, the input to the LMS filter will be best if it's a replica of the offending signal. Then it's subtracted from the input to get e[n]. Since you know that the offending signal is at 60Hz, you might bandpass the input to the LMS filter at 60Hz. Assuming the 60Hz interference is of constant amplitude then the LMS filter can change slowly or perhaps not at all. If the 60Hz interference is not of constant amplitude then you won't be able to completely separate it from the desired "signal info around 60Hz". The filter will work to minimize the output e[n]. If the input to the LMS filter is bandpassed to 60Hz, then the only work the LMS filter has to do is to adjust the amplitude and phase of the approximately 60Hz signal that is subtracted. If the filter adjusts too quickly then it will take out more of the desired signal. If the filter adjusts too slowly then it will cancel the interference less. So, the rate of change of the LMS filter may be something you can adjust to get best performance. It will have its own rate limit just to work and can be made slower... If the 60Hz information you want to keep is correlated with the interference then it won't help in keeping that "other 60Hz information". Fred
Reply by ●July 2, 20042004-07-02
Rob Hutchinson wrote:> What is the preferred method for removing 60 hz hum from a signal > without > wiping out signal info around 60 hz? A 60 hz notch filter would > not be > useful because it would attenuate the signal as well. I'm > interested in doing this for sampled data, so all filtering would > be done in the digital domain. > Thank you in advance, > RobTwo approaches, first having been mentioned already: 1) PLL Frequency and phase of the hum should be long-term stable. Even amplitude should change slowly - depending on how it couples into the signal path. You might try to generate a signal which equals the detected 60Hz and substract it - probably best before digitizing. Even subtracting it digitally after conversion might help. PLL capture and tracking must be tailored to meet your goals, and an AGC should control the amplitude. With tracking speed of both, PLL and AGC at an optimum (slow enough), even 60Hz signals would pass your filter, and signals close to 60Hz, either. 2) Reference path If possible, set up an identical path as your signal path, but without any signal applied to it. There's a good chance, that it catches an equal hum. Then build the difference between both channels. There you are with a far better signal without distorting your real 60Hz signal content. Bernhard
Reply by ●July 2, 20042004-07-02
That's the whole point - the "hum" can have any number of harmonics, and the only way to cut them all back is attention to the analog circuitry. But if it's too late for that, something like an inverse comb filter will do it. On Thu, 01 Jul 2004 22:59:17 -0400, Jerry Avins <jya@ieee.org> wrote:>Suodatin Pussi wrote: > >> Rob Hutchinson wrote: >> >> >>>What is the preferred method for removing 60 hz hum from a signal without >>>wiping out signal info around 60 hz? A 60 hz notch filter would not be >>>useful because it would attenuate the signal as well. I'm interested in >>>doing this for sampled data, so all filtering would be done in the digital >>>domain. >>>Thank you in advance, >>>Rob >>> >>> >> >> You could try using some active noise reduction. Use a 60 Hz 'generator' >> of which you can alter phase & amplitude and a little freq and a small >> bandpass filter around 60 Hz. Now adjust the parameters so that the >> output of the filter minimizes. (but I guess its earier said than done) >> For me 150 Hz is always annoying me, they manage to deliver a clipped mains. > >And a 120 Hz generator. And 180, 240, and maybe more. An instrumentation >amplifier with proper attention to where the ground returns run is the >best answer for difficult cases. > >JerryTony (remove the "_" to reply by email)
Reply by ●July 2, 20042004-07-02
Jerry Avins wrote:> Suodatin Pussi wrote: > >> Rob Hutchinson wrote: >> >> >>>What is the preferred method for removing 60 hz hum from a signal without >>>wiping out signal info around 60 hz? A 60 hz notch filter would not be >>>useful because it would attenuate the signal as well. I'm interested in >>>doing this for sampled data, so all filtering would be done in the digital >>>domain. >>>Thank you in advance, >>>Rob >>> >>> >> >> You could try using some active noise reduction. Use a 60 Hz 'generator' >> of which you can alter phase & amplitude and a little freq and a small >> bandpass filter around 60 Hz. Now adjust the parameters so that the >> output of the filter minimizes. (but I guess its earier said than done) >> For me 150 Hz is always annoying me, they manage to deliver a clipped mains. > > And a 120 Hz generator. And 180, 240, and maybe more. An instrumentation > amplifier with proper attention to where the ground returns run is the > best answer for difficult cases. > > JerryYeah, you're right as ever, but you get the idea, find an 180 degrees shifted signal (using LMS, prediction, tapped from the source...) which matches that you need to get rid off and add that to the inputsignal. I totally agree with you that one should find the source of all troubles and solve that first. (Return the groundsignal where it originates from (I don't mean use long wires to 1 point, but keep the ground close to the 'hot' wire). Keep chassis and ground separated and soft couple them somewhere using a 10 ohms resistor. Soft couple digital and analog ground etc..
Reply by ●July 2, 20042004-07-02
Tim Wescott wrote:> Rob Hutchinson wrote: > >> What is the preferred method for removing 60 hz hum from a signal without >> wiping out signal info around 60 hz? A 60 hz notch filter would not be >> useful because it would attenuate the signal as well. I'm interested in >> doing this for sampled data, so all filtering would be done in the >> digital >> domain. >> Thank you in advance, >> Rob >> >> > > Your signal processing algorithm won't be able to tell the hum from the > real data -- you're stuck with either a really narrow 60Hz notch (which > you can do digitally if you're really careful about numeric precision) > or trying to remove the hum before you acquire. Neither of these is a > good prospect, but there you are. > > Perhaps you should move to Germany, or Brazil? Then you'll only have to > worry about 50Hz, or 25 :). >Just a small correction, in Brazil we do use 60Hz. :D
Reply by ●July 2, 20042004-07-02
Ricardo wrote: ...> Just a small correction, in Brazil we do use 60Hz. :DSlight correction? Tell that to the motor on my phonograph turntable! Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●July 2, 20042004-07-02
Ricardo wrote:> Tim Wescott wrote: > >> Rob Hutchinson wrote: >> >>> What is the preferred method for removing 60 hz hum from a signal >>> without >>> wiping out signal info around 60 hz? A 60 hz notch filter would not be >>> useful because it would attenuate the signal as well. I'm interested in >>> doing this for sampled data, so all filtering would be done in the >>> digital >>> domain. >>> Thank you in advance, >>> Rob >>> >>> >> >> Your signal processing algorithm won't be able to tell the hum from >> the real data -- you're stuck with either a really narrow 60Hz notch >> (which you can do digitally if you're really careful about numeric >> precision) or trying to remove the hum before you acquire. Neither of >> these is a good prospect, but there you are. >> >> Perhaps you should move to Germany, or Brazil? Then you'll only have >> to worry about 50Hz, or 25 :). >> > Just a small correction, in Brazil we do use 60Hz. :DI'm sorry, I didn't check my facts. When I was in school 20 years ago there were a few pockets somewhere in South America that still used 25Hz -- I'm nearly certain they were in Brazil, but I imagine the drive to modernize them was pretty strong. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●July 2, 20042004-07-02
But sometimes they do. Check out http://www.saultstar.com/webapp/sitepages/content.asp?contentID=70613&catname=Local+News A small town in Canada where everyone's clocks gained 10 minutes. Cheers, Syms. "Jerry Avins" <jya@ieee.org> wrote in message news:40e4cee3$0$23332$61fed72c@news.rcn.com...> filter may be useful if the interfering frequency drifts slowly, but > power lines rarely do. >
