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From: John Fields on 20 Jun 2010 19:50
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?

---
What does your input signal look like?

From: Phil Hobbs on 20 Jun 2010 21:04
On 6/20/2010 2:07 PM, John Larkin wrote:
> 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 inadvertently tried that once--in my first engineering job, when I was
21, and had a newly-minted physics and astronomy bachelor's degree. I
was building a high performance PLL for satcom--this was the first PLL I
had ever seen, let alone designed--and badly needed some more loop
bandwidth.

I took the classical all-pass phase shift filter trick,

*--RRRR---*
| |
| |
| |\ |
0-*-RRRR-*-|+\ |
| | \ |
| | >---*---0
| | /
*-RRRR-*-|-/
| |/
|
---
---
|
GGG

and convinced myself by doing some algebra that changing the capacitor
for an inductor would change the sign of the phase shift, which would
give me a bunch more loop bandwidth.

It failed to work, which I didn't understand until my boss, a very smart
guy called Joe Fikart, pointed out to me that I'd made a math
mistake--the phase shift has the opposite sign, but also goes the
opposite way with frequency, so that the group delay d(phi)/d(omega) is
always positive.

You can't go back in time, even if you are a young enthusiast. ;)

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Glenn Kenroy on 21 Jun 2010 03:36
On Sun, 20 Jun 2010 18:50:42 -0500, John Fields
<jfields(a)austininstruments.com> wrote:

>What does your input signal look like?
>

It is analog output direct from an Earth micropulsation sensor coil,
all below 50Hz.

What is the best I can do to preserve the real time signal
characterisitics with minimum phase delay and distortion?

Glenn Kenroy

From: Glenn Kenroy on 21 Jun 2010 03:48
On Sun, 20 Jun 2010 07:44:51 -0700 (PDT), MooseFET
<kensmith(a)rahul.net> wrote:

>This sounds like a job for the PLL. If you have a signal you can lock
>it onto, they do this sort of thing easily.
>

Could you please provide a brief description of the design approach
you would take using this technique (PLL)?

The object is minimum delay and distortion in 50/60Hz LP for real time
analog data from an Earth micropulsation sensing coil.

Glenn Kenroy
From: MooseFET on 21 Jun 2010 07:20
On Jun 21, 3:48 pm, glennken...(a)protech.com (Glenn Kenroy) wrote:
> On Sun, 20 Jun 2010 07:44:51 -0700 (PDT), MooseFET
>
> <kensm...(a)rahul.net> wrote:
> >This sounds like a job for the PLL. If you have a signal you can lock
> >it onto, they do this sort of thing easily.
>
> Could you please provide a brief description of the design approach
> you would take using this technique (PLL)?
>
> The object is minimum delay and distortion in 50/60Hz LP for real time
> analog data from an Earth micropulsation sensing coil.

You may want to consider a different sort of magnetometer than a
coil.

Your biggest noise sources are the mains and the 3rd harmonic.
I will assume that the mains are 60Hz. If you are in a place with
50Hz, you will need to change things.

The 60Hz and 180Hz are fairly constant in most locations. They
will be on the order of 100nT in amplitude. Your micropulsations
will be on the order of 1nT. You need a filter that takes the 60Hz
down by about 60dB and passes perhaps 10Hz with little
distortion.

The trick is to make a very tight notch at the mains frequencies
and use a normal low pass for the general junk.

The first step is to make a PLL that locks onto the 60Hz. You want
the VCO in the PLL to be running at many times the 60Hz frequency.
I am going to suggest 7200 times, but faster is likely better. 7200
times just makes the explanation easier.

Important frequencies:

60*8*3*5 = 7200

7200 / 15 = 60*8
7200 / 3 = 5*60*8
7200 / 5 = 3*60*8

I will assume that you have the PLL locked to the 60Hz.


You will be making the same circuit 3 times. It uses the CD4051
The 8 times the frequency goes to a counter that makes the
CD4051 scan through a group of capacitors.

Each of the 8 outputs of the CD4051 connects to one end of
a capacitor. The other end of the capacitor is grounded.

If the common point is fed with a resistor. This makes a circuit
that will charge up the capacitors until they match the 60Hz
input waveform.
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