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From: MooseFET on 14 Dec 2009 10:00
On Dec 13, 10:04 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?

If you connect a 3 phase bridge to a constant load, the power drawn
goes through 6 complete cycles in a cycle of the mains. This makes
the 3rd harmonic in the mains zero. It doesn't do the same to higher
harmonics.

>
> Regards,
> Martin Brown

From: miso on 14 Dec 2009 13:47
On Dec 14, 3:51 am, legg <l...(a)nospam.magma.ca> wrote:
> On Sun, 13 Dec 2009 07:07:19 GMT, 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.
>
> >Harold Keller
>
> Combining the outputs of high pass and low pass filters would leave
> 'passed' components pretty phase coherent to the input.
>
> As the 50Hz notch is usually addressing hum, avoiding it in the first
> place is a better idea, where practical.
>
> RL

A high pass has no delay at high frequencies A low pass as delay near
DC. Seriously, this idea won't work, and it's not really a notch
unless you spread the HP and LP corners a bit.

There is no free lunch.



From: miso on 14 Dec 2009 13:49
On Dec 14, 6:50 am, MooseFET <kensm...(a)rahul.net> wrote:
> On Dec 13, 10:02 am, Leon <leon...(a)btinternet.com> wrote:
>
>
>
> > 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.
>
> You need quite a bit of memory and a lot of math to do it.  Each RC
> looks like:
>
>    V[n] += (V[m] - V[n]) * A
>
> Where A is less than one.
>
> You also need to gather all the data in a computer with significant
> storage.  It make more sense to do the correction after the fact than
> on the fly because you have the entire record to work with.

Often people pre-filter for dynamic range issues. Since the original
poster has left the scene, we don't know his objective.
From: George Herold on 14 Dec 2009 14:47
On Dec 13, 11:11 pm, "m...(a)sushi.com" <m...(a)sushi.com> wrote:
> On Dec 13, 6:32 pm, "john jardine" <zen177...(a)zen.co.uk> wrote:
>
>
>
>
>
> > "Harold keller" <haroldkel...(a)quantum.com> wrote in message
>
> >news:4b2491c4.5812453(a)news.tpg.com.au...
>
> > > 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
>
> > This any help? ...http://i48.tinypic.com/zsv4nc.jpg
>
> > (From Ebay, I'd bought a defunct research institute's complete collection of
> > that EE magazine. The above circuit was one of about 100 items that were
> > begging to  be scanned to a printable PDF, for light bedtime reading :)
>
> That is just a notch filter implemented in a different manner. You get
> no phase shift at the center frequency, so the subtractor makes a
> notch. Really is it no better than building a notch filter in the
> first place.
>
> What the original poster failed to mention is over what bandwidth does
> he need linear phase. Far away from the notch, the phase error is
> small.- Hide quoted text -
>
> - Show quoted text -

Thanks Miso, I was confused by the bandpass then subtract circuit.
As you say it's no better than a notch... and may be worse since now
you have to 'balance' the signal going to the subtraction part. (oh
the non-inverting input to the second opamp should be tied to ground
and not left floating.)

George H.
From: Nemo on 14 Dec 2009 17:57
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
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