PLL for Zero Delay LPF

The same problem exists. Fundamentally (>>grin

I can build you negative group delay filters. Would that be close enough?

:-)

Whats wrong with sampling the data using a pll to clean up and sync to the 60 hz ?? That or something like that has been done for ages.

You mean perhaps to use two identical sensing coils; one tuned to ELF and the second to 60Hz? The output from the second could then be adjusted in amplitude and inverted to null the 60Hz component in the output of the second.

Yes, but I am still uncertain how the phase equalization would be implemented in terms of actual circuitry.

Glenn Kenroy

Yes please, for this application.

Or, as I replied to John's post, how about deriving this signal instead from a second sensing coil tuned to 60Hz? Then adjust amplitude, invert and mix with to the output of sensor #1 to null the

Glenn Kenroy

Actually, I was thinking of using the local power line. But a divided-down crystal oscillator would work as well. It only needs to be pretty close to 60 Hz.

Ask google!

John

A zero _phase shift_ notch is easy, but a notch filter is hardly low delay, and the narrower it is the more delay it must have -- if you can't figure out why, ask.

If the device is mains powered, why not use it directly as a reference for cancellation.

You may need the ability to shift this reference phase, since the pick up coil may also pick up magnetic fields from other phases, creating a resultant, which is not in phase with your equipment power.

A second sensing coil doesn't necessarily pick up exactly the same interference as the first, so you're probably better off notch-filtering the signal from a single sensor.

Apart from that, *carefully* *read* Dave Platt's explanation. This really is fundamental. Using a 2nd sensor tuned to 60Hz does not change that.

Jeroen Belleman

Yes.

The i/q cancellation loop automatically corrects for phase and frequency variations.

John

It took a long time for this to percolate back up from old memory. About 30 or so years ago Popular Electronics did an article on a old school analog distortion analyzer. It used a tunable twin T active filter to remove the fundamental, and since it was servo'd in both frequency and phase you could get up toward 100 dB rejection. All this with little delay and well understood and simple phase response. I suppose this technique could be used here, perhaps with a phase canceling pair of servo'd twin T active filters.

I just bought an HP 339A Distortion Meter that does this. It's a great tool.

Cheers

Phil Hobbs

Unless they're using their own generator, the 60 Hz line is incredibly stable, so a fixed-frequency high-Q notch will work fine. Its width needs to be compatible with sidebands caused by any amplitude variation in the amount of 60 Hz pickup.

John