[time-nuts] Mains frequency filter

[Didier Juges]

For a non Time-Nuts application, I needed a narrow bandpass filter that would provide essentially zero phase shift (no more than 10 or 20uS was desired) over a frequency range of 55 to 65 Hz while providing useful reduction of the harmonics, particularly in the range of 400 to 1kHz. This was to filter the line voltage. I needed an output that was  a clean sine wave but with essentially no phase or amplitude error compared to the input signal, even when the input signal changed in frequency.
Of course, there is no "conventional" filter topology that will provide zero phase shift over a range of frequency, you will be lucky to find one that provides zero phase shift at ONE frequency. I knew the switched capacitor filter could be set to provide zero phase shift near the center frequency, but the phase shift would change rapidly when the input frequency changed, like any other filter.
I came across the LTC1060 and I found out that when driving its clock from a PLL that multiplies the line frequency by the proper factor (50 or 100 for the LTC1060), a single potentiometer can then be used to adjust the phase shift to zero nominally, and the phase shift remains below a few uS over the range I was interested in.

Being a switched capacitor filter, it can be made as narrow as desired without affecting the other characteristics too much.

Reading this thread, I just realized that this filter would be very useful to filter the 60Hz before feeding a counter for the purpose of measuring the line frequency and phase without being too much affected by noise and other disturbances.

Didier KO4BB


Charles Steinmetz <csteinmetz at yandex.com> wrote:
>Chuck wrote:
>
>>In the case of a 60Hz mains derived signal, most of the noise is
>>going to be riding on the signal, and will be amplified with your
>>gain stage.
>
>The potential evils of bandpass filters in a timing chain are well 
>known, but as long as you can accept the delay of a filter (or 
>correct for it, which should be trivial with a PIC or other uC), you 
>may be much further ahead with a noisy signal like the AC mains if 
>you use a sharp bandpass filter on the incoming 60 Hz then amplify & 
>clip the signal to increase the slew rate.  Active filters with fast, 
>quiet op-amps should do the job well.  For the lowest jitter, a 
>Collins-style multi-stage zero cross detector may be helpful.
>
>Best regards,
>
>Charles
>
>
>
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