Phase Self-Correcting LP Filter

[Glenn Kenroy]

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

 

[John Devereux]

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 Note/an56.pdf

 

[[email protected]]

The components get pretty big for the one at zero Hertz!

Depends on the Q.

 

[Glenn Kenroy]

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

 

[Glenn Kenroy]

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

 

[Paul Keinanen]

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

Or even a comb filter at 10 Hz, that will take out both 50 and 60 Hz
(depending on location) and their harmonics.

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.

The PLL would solve the network frequency drift during the day, which
can vary more than 1000 ppm, depending on the load.

 

[Glenn Kenroy]

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.

This appears to be the most promising response so far to the elusive
zero-delay issue.

What might it take for you to draw this up as a conceptual circuit
(untested OK) to get me started on the right track?

Glenn Kenroy