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From: Vladimir Vassilevsky on 13 Dec 2009 23:47


Harold keller wrote:

> Can anyone provide information on implementing a 50Hz notch filter for
> data acquisition that operates without phase distortion?

Sure. Integrate the signal per 20ms, and 50Hz components will be
cancelled out with all harmonics. This is perfecty linear phase process.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: Paul Keinanen on 14 Dec 2009 01:09
On Sun, 13 Dec 2009 18:04:22 +0000, Martin Brown
<|||newspam|||@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.

You have to phase lock the PLL to the 50 Hz in the input signal and
not to the power supply voltage, since the input signal hum can be
from a different mains phase than the one powering the instrument.


>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?

How strong is the 150 Hz component compared to the 250 Hz component ?

In a three phase system with equal resistive loads, the neutral wire
current will cancel.

When single phase rectifier+capacitor input loads are operated with
conduction angles less than 60 degrees on all phases, the return
current peaks do not overlap in the neutral wire, thus no cancellation
will occur in the neutral wire and the neutral current can be as high
as the phase current. This has caused some cable failures in some
underground cables, when the cross section area of the neutral
conductor was less than the cross section of a phase conductor.

Anyway, the mainly 150 Hz current peaks in the neutral wire will also
cause voltage drop in the neutral wire. When you measure the harmonic
content between one phase and the neutral, quite strange values might
be visible. However, I would expect the 150 Hz component to be much
stronger than the 250 Hz.

From: Martin Brown on 14 Dec 2009 03:40
Paul Keinanen wrote:
> On Sun, 13 Dec 2009 18:04:22 +0000, Martin Brown
> <|||newspam|||@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.
>
> You have to phase lock the PLL to the 50 Hz in the input signal and
> not to the power supply voltage, since the input signal hum can be
> from a different mains phase than the one powering the instrument.

Yes. I should have made that clear.
>
>
>> 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?
>
> How strong is the 150 Hz component compared to the 250 Hz component ?

Third harmonic is barely measurable. The fifth harmonic most times is
way stronger than the second harmonic and all other even harmonics are
absent. After that in ordering of decreasing strength is 7, 11, 9, 15
,13 sponsored by a convenience store maybe...
>
> In a three phase system with equal resistive loads, the neutral wire
> current will cancel.
>
> When single phase rectifier+capacitor input loads are operated with
> conduction angles less than 60 degrees on all phases, the return
> current peaks do not overlap in the neutral wire, thus no cancellation
> will occur in the neutral wire and the neutral current can be as high
> as the phase current. This has caused some cable failures in some
> underground cables, when the cross section area of the neutral
> conductor was less than the cross section of a phase conductor.
>
> Anyway, the mainly 150 Hz current peaks in the neutral wire will also
> cause voltage drop in the neutral wire. When you measure the harmonic
> content between one phase and the neutral, quite strange values might
> be visible. However, I would expect the 150 Hz component to be much
> stronger than the 250 Hz.

So would I - hence the comment about this anomolous 250Hz component.

Turns out it is due to a sensor in the process of going bad. I just
tried a brand new one before posting this and it gives much cleaner more
reasonable results. The harmonics now look like I would expect.

Regards,
Martin Brown
From: legg on 14 Dec 2009 06:51
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
>
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
From: MooseFET on 14 Dec 2009 09:50
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.

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