Linear phase 50Hz notch filter
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From: Harold keller on 13 Dec 2009 02:07 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. Harold Keller
From: Jim Thompson on 13 Dec 2009 09:58 On Sun, 13 Dec 2009 07:07:19 GMT, haroldkeller(a)quantum.com (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. > >Harold Keller > What do you mean, "...without phase distortion"? You will certainly have phase shift going thru the notch. Go to the SED/Schematics page of my website and look at GyratorFilterNotch.pdf. ...Jim Thompson -- | James E.Thompson, CTO | 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 | Help save the environment! Please dispose of socialism properly!
From: MooseFET on 13 Dec 2009 12:26 On Dec 12, 11:07 pm, haroldkel...(a)quantum.com (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. How high is the highest frequency you need to have linear phase for? "Linear phase" also means "constant time delay". As you go through the notch, the phase shift later very quickly. Above the notch, the phase must continue to go later. To do this you need to add "all pass" filter stages that shift the phase without changing the gain. If you want to go much above 100Hz, you are going to need a great many such stages. Since this is for data acquisition, you are almost certainly better off taking the phase shift out of the data after the fact. You need to take a lot of data from both before and after the time you care about for this to work. The simplest way to think about the software method is to imagine that you have taken the data and flipped it end of end. This inverts the sign of the phase shift in the data. If you make a software filter just like your hardware notch and feed the data through it, the total phase shift will now come out to zero. In real life, you don't go to the work of flipping the data. You just code a filter with the needed phase shift and apply it to the data. One really nice side effect of doing this way is that you can end up with better dynamic range for burst like inputs. Spreading the pulse out in time as it is going through the ADC can hide some of the artifacts of the ADC.
From: Leon on 13 Dec 2009 13:02 On 13 Dec, 17:26, MooseFET <kensm...(a)rahul.net> wrote: > On Dec 12, 11:07 pm, haroldkel...(a)quantum.com (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. > > How high is the highest frequency you need to have linear phase for? > > "Linear phase" also means "constant time delay". As you go through > the notch, the phase shift later very quickly. Above the notch, the > phase must continue to go later. To do this you need to add "all > pass" filter stages that shift the phase without changing the gain. > If you want to go much above 100Hz, you are going to need a great many > such stages. > > Since this is for data acquisition, you are almost certainly better > off taking the phase shift out of the data after the fact. You need > to take a lot of data from both before and after the time you care > about for this to work. > > The simplest way to think about the software method is to imagine that > you have taken the data and flipped it end of end. This inverts the > sign of the phase shift in the data. If you make a software filter > just like your hardware notch and feed the data through it, the total > phase shift will now come out to zero. > > In real life, you don't go to the work of flipping the data. You just > code a filter with the needed phase shift and apply it to the data. > One really nice side effect of doing this way is that you can end up > with better dynamic range for burst like inputs. Spreading the pulse > out in time as it is going through the ADC can hide some of the > artifacts of the ADC. I'd use a cheap DSP, like a dsPIC. Leon
From: Martin Brown on 13 Dec 2009 13:04
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 |