Linear phase 50Hz notch filter
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From: miso on 14 Dec 2009 20:43 On Dec 14, 2:57 pm, Nemo <z...(a)nospam.nospam.nospam.nospam.co.uk> wrote: > Harold keller writes > > >Can anyone provide information on implementing a 50Hz notch filter for > >data acquisition that operates without phase distortion? > > >This could be a circuit or dedicated linear phase filter IC. > > It's hard, because if you're trying to filter out 50 / 60 Hz you are > probably trying to measure down to DC. The first thought is to use a > notch filter, but as others have noted that usually gives a phase shift > across the notch. The next thought is to use a band-reject by using > parallel lowpass and highpass filters, and summing their outputs; you > set one's cutoff to 45Hz and the other to 55Hz and you'd expect these to > cut out everything from 45 to 55Hz. In practice though they have finite > sharpness (not as sharp as a notch filter), so you have to set the > cutoffs at least 20% away from the centre frequency to give decent > attenuation to the target frequency, so you're probably seriously > affecting your measurement bands. > > So next you think hmm Bessel filters? And a little looking around leads > you to Maxim App note AN431, which details a low freq notch filter based > on their own 8 pin low pass clocked chip MAX7410 (which is cheap) and > specifically mentions the 180 degree phase shift as input sweeps across > its notch freq. But there are versions, the MAX7409 and MAX7413, which > are Bessel filters. I asked their tech support department about this > recently and they said the 7409 would work albeit possibly with a > slightly different ratio of clock-to-notch frequency, but I never got a > chance to experiment with this idea so please let us know if it works > 8) . Its sister part the 7410 is quoted as having an 85:1 clock:notch > ratio in the app note, ie it needs a few kHz clock for a 50Hz notch. > > Switching this filter into circuit will reduce circuit bandwidth to the > (15kHz?) max of this IC. It will also eliminate any offset as there are > DC blocking capacitors in this path this may not always be desirable. > -- > Nemo Maxim and LTC make DC accurate filters. Pretty noisy for use with ADCs by modern standards. [I designed some of the Maxim DC accurate parts and the ladder filters, but not those 5th order filters.] You are trying to get a free lunch here. This parallel filter scheme isn't going to fly.
From: amark on 15 Dec 2009 01:43 On Dec 14, 5:04 am, Martin Brown <|||newspam...(a)nezumi.demon.co.uk> wrote: > Harold keller wrote: > > Can anyone provide information on implementing a 50Hz notch filter for > > data acquisition that operates without phase distortion? > > > This could be a circuit or dedicated linear phase filter IC. > > The only way you could do this would be by mixing in the right amount of > phase locked 50Hz signal to null out the unwanted hum component. A > sharp notch filter necessarily has to change phase across the null point. > > You might be able to trade the depth of the notch for a smaller phase > error if you don't need too much attenuation. Does it have to track > mains frequency in real time? Or will it be wide enough to cover > expected variations? What sort of high frequency has to be passed? > > High precision DVMs use mains synchronous ADC conversion to eliminate > hum components from the measured data. 100mS being the first common > multiple that works for both UK & US mains frequencies. > > Reminds me. Anyone have any thoughts on why the fifth harmonic on UK > mains at 250Hz is so strong these days? > > Regards, > Martin Brown Perhaps the supply transformers are nearing overload?
From: JosephKK on 20 Dec 2009 02:23 On Sun, 13 Dec 2009 21:50:07 +0000, Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote: >Jan Panteltje wrote: >> On a sunny day (Sun, 13 Dec 2009 18:04:22 +0000) it happened Martin Brown >> <|||newspam|||@nezumi.demon.co.uk> wrote in >> <B0aVm.68146$de6.1720(a)newsfe21.iad>: > >>> Reminds me. Anyone have any thoughts on why the fifth harmonic on UK >>> mains at 250Hz is so strong these days? >> >> Global warming? > >I suspect switched mode PSU's but I can't see why the 5th harmonic is >now so strong or for that matter where the roughly 623+/-3Hz component >in my local electrical interference is coming from. > >There is a heck of a lot of crossover distortion on our local mains - it >doesn't look all that much like a sine wave! > >I can see other higher frequencies from CFLs but the 5th harmonic of >mains remains a bit odd. Some days it is the strongest harmonic component. > >Regards, >Martin Brown It is driven by two things, simple rectifiers and third harmonic reduction as a result of delta windings in three phase transformers. It should mostly go away in "say" ten years as many things get power factor and harmonic corrected.
From: miso on 20 Dec 2009 13:48 On Dec 19, 11:23 pm, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > On Sun, 13 Dec 2009 21:50:07 +0000, Martin Brown <|||newspam...(a)nezumi.demon.co.uk> wrote: > >Jan Panteltje wrote: > >> On a sunny day (Sun, 13 Dec 2009 18:04:22 +0000) it happened Martin Brown > >> <|||newspam...(a)nezumi.demon.co.uk> wrote in > >> <B0aVm.68146$de6.1...(a)newsfe21.iad>: > > >>> Reminds me. Anyone have any thoughts on why the fifth harmonic on UK > >>> mains at 250Hz is so strong these days? > > >> Global warming? > > >I suspect switched mode PSU's but I can't see why the 5th harmonic is > >now so strong or for that matter where the roughly 623+/-3Hz component > >in my local electrical interference is coming from. > > >There is a heck of a lot of crossover distortion on our local mains - it > >doesn't look all that much like a sine wave! > > >I can see other higher frequencies from CFLs but the 5th harmonic of > >mains remains a bit odd. Some days it is the strongest harmonic component. > > >Regards, > >Martin Brown > > It is driven by two things, simple rectifiers and third harmonic reduction > as a result of delta windings in three phase transformers. > > It should mostly go away in "say" ten years as many things get power factor > and harmonic corrected. I think I read the cheap Chinese offline switchers are half wave rectified. If that is the case, I can see harmonic distortion on the AC as a way of life.
From: Mark on 20 Dec 2009 14:03
On Dec 20, 1:48 pm, "m...(a)sushi.com" <m...(a)sushi.com> wrote: > On Dec 19, 11:23 pm, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > > > > > On Sun, 13 Dec 2009 21:50:07 +0000, Martin Brown <|||newspam...(a)nezumi.demon.co.uk> wrote: > > >Jan Panteltje wrote: > > >> On a sunny day (Sun, 13 Dec 2009 18:04:22 +0000) it happened Martin Brown > > >> <|||newspam...(a)nezumi.demon.co.uk> wrote in > > >> <B0aVm.68146$de6.1...(a)newsfe21.iad>: > > > >>> Reminds me. Anyone have any thoughts on why the fifth harmonic on UK > > >>> mains at 250Hz is so strong these days? > > > >> Global warming? > > > >I suspect switched mode PSU's but I can't see why the 5th harmonic is > > >now so strong or for that matter where the roughly 623+/-3Hz component > > >in my local electrical interference is coming from. > > > >There is a heck of a lot of crossover distortion on our local mains - it > > >doesn't look all that much like a sine wave! > > > >I can see other higher frequencies from CFLs but the 5th harmonic of > > >mains remains a bit odd. Some days it is the strongest harmonic component. > > > >Regards, > > >Martin Brown > > > It is driven by two things, simple rectifiers and third harmonic reduction > > as a result of delta windings in three phase transformers. > > > It should mostly go away in "say" ten years as many things get power factor > > and harmonic corrected. > > I think I read the cheap Chinese offline switchers are half wave > rectified. If that is the case, I can see harmonic distortion on the > AC as a way of life. to op... if you can implement your filter in DSP, then use a FIR filter. They are linear phase by definition. Mark |