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From: John Larkin on 20 Jun 2010 14:07
On Sun, 20 Jun 2010 07:18:15 GMT, glennkenroy(a)radcliff.com (Glenn
Kenroy) wrote:

>I am looking for a 60Hz active low pass filter, either discrete or IC,
>that will automatically compensate for the phase delay it imposes.
>
>IOW so that the LP filtered signal matches in phase that of the
>original input one.
>
>The type or order of the filter is not relevant at this stage.
>
>Can anyone please provide a circuit or technical reference for this
>application?
>
>Glenn Kenroy

An ideal lowpass filter passes everything, unaltered, within its
passband and nothing outside. That can be described mathematically but
is physically impossible, because it's non-causal: its impulse
response has outputs before the input, which means it predicts the
future.

You can make an approximation to an ideal lowpass, but you have to add
time delay to keep it causal. The better the approximation, the more
delay you have to add, whether the implementation is analog or
digital.

So you can make a lowpass filter whose phase changes little with
frequency *after* you allow for the time delay. Even than it's a
nuisance to do analog.

What's the application?

In addition to conservation of energy, our universe seems to have a
law that prevents predicting the future. Both laws can be handy in
short-cutting a lot of electronic analysis.

John


From: John Devereux on 20 Jun 2010 14:58
John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> writes:

> On Sun, 20 Jun 2010 07:18:15 GMT, glennkenroy(a)radcliff.com (Glenn
> Kenroy) wrote:
>
>>I am looking for a 60Hz active low pass filter, either discrete or IC,
>>that will automatically compensate for the phase delay it imposes.
>>
>>IOW so that the LP filtered signal matches in phase that of the
>>original input one.
>>
>>The type or order of the filter is not relevant at this stage.
>>
>>Can anyone please provide a circuit or technical reference for this
>>application?
>>
>>Glenn Kenroy
>
> An ideal lowpass filter passes everything, unaltered, within its
> passband and nothing outside. That can be described mathematically but
> is physically impossible, because it's non-causal: its impulse
> response has outputs before the input, which means it predicts the
> future.
>
> You can make an approximation to an ideal lowpass, but you have to add
> time delay to keep it causal. The better the approximation, the more
> delay you have to add, whether the implementation is analog or
> digital.
>
> So you can make a lowpass filter whose phase changes little with
> frequency *after* you allow for the time delay. Even than it's a
> nuisance to do analog.
>
> What's the application?
>
> In addition to conservation of energy, our universe seems to have a
> law that prevents predicting the future. Both laws can be handy in
> short-cutting a lot of electronic analysis.


I thought this was interesting:

http://cds.linear.com/docs/Application%20Note/an56.pdf



--

John Devereux
From: Tim Wescott on 20 Jun 2010 14:58
On 06/20/2010 12:18 AM, Glenn Kenroy wrote:
> I am looking for a 60Hz active low pass filter, either discrete or IC,
> that will automatically compensate for the phase delay it imposes.
>
> IOW so that the LP filtered signal matches in phase that of the
> original input one.
>
> The type or order of the filter is not relevant at this stage.
>
> Can anyone please provide a circuit or technical reference for this
> application?

You've gotten the responses that explain why you can't have a zero phase
delay filter in this causal world of ours.

Look for "Bessel" filter, which gives a pretty good approximation to a
constant group delay. It won't be _no_ delay, which you can't get, but
it'll be as close to _constant_ as you can get with a mininum-phase filter.

If you really build one be careful with component tolerances -- IIRC
from the last time I considered using one (a long long time ago) the
group delay vs. frequency relationship is fairly sensitive to component
values. So you can't just take the nominal filter and run with it --
you have to be good and do your Monte Carlo analysis of the filter.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: John Larkin on 20 Jun 2010 16:11
On Sun, 20 Jun 2010 19:58:58 +0100, John Devereux
<john(a)devereux.me.uk> wrote:

>John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> writes:
>
>> On Sun, 20 Jun 2010 07:18:15 GMT, glennkenroy(a)radcliff.com (Glenn
>> Kenroy) wrote:
>>
>>>I am looking for a 60Hz active low pass filter, either discrete or IC,
>>>that will automatically compensate for the phase delay it imposes.
>>>
>>>IOW so that the LP filtered signal matches in phase that of the
>>>original input one.
>>>
>>>The type or order of the filter is not relevant at this stage.
>>>
>>>Can anyone please provide a circuit or technical reference for this
>>>application?
>>>
>>>Glenn Kenroy
>>
>> An ideal lowpass filter passes everything, unaltered, within its
>> passband and nothing outside. That can be described mathematically but
>> is physically impossible, because it's non-causal: its impulse
>> response has outputs before the input, which means it predicts the
>> future.
>>
>> You can make an approximation to an ideal lowpass, but you have to add
>> time delay to keep it causal. The better the approximation, the more
>> delay you have to add, whether the implementation is analog or
>> digital.
>>
>> So you can make a lowpass filter whose phase changes little with
>> frequency *after* you allow for the time delay. Even than it's a
>> nuisance to do analog.
>>
>> What's the application?
>>
>> In addition to conservation of energy, our universe seems to have a
>> law that prevents predicting the future. Both laws can be handy in
>> short-cutting a lot of electronic analysis.
>
>
>I thought this was interesting:
>
>http://cds.linear.com/docs/Application%20Note/an56.pdf

Williams' filter book has some interesting designs. Some are
"transitional", Gaussian in the passband but rolling off faster than a
Gaussian after 6 or 12 dB down, similar in concept to the LTC things.
We use things like this in our waveform generators, where we want to
lowpass a DAC output without making it ring a lot. He also has filters
that have a Chenbychev-like equiripple pattern, but in phase, not
amplitude.

John

From: krw on 20 Jun 2010 17:43
On Sun, 20 Jun 2010 10:06:34 -0700, VWWall <vwall(a)large.invalid> wrote:

>MooseFET wrote:
>> On Jun 20, 11:44 pm, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
>>> MooseFET wrote:
>>>> On Jun 20, 3:18 pm, glennken...(a)radcliff.com (Glenn Kenroy) wrote:
>>>>> I am looking for a 60Hz active low pass filter, either discrete or IC,
>>>>> that will automatically compensate for the phase delay it imposes.
>>>> If you want no phase shift in a linear filter, you need a band pass
>>>> filter.
>>> So, if I take one bandpass filter at 60 Hz, then connect another
>>> bandpass filter at, say, 55 Hz, in parallel, then another at 50, 45,40
>>> and so on, so forth to zero Hz... this will make a lowpass filter with
>>> no phase shift :)))))
>>
>> Actually, no it won't because you left out the bandpass filter at
>> 57.923145067Hz. You need to add that one to the list and then
>> check it again. :)
>>
>The components get pretty big for the one at zero Hertz!

Depends on the Q.
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