Linear phase 50Hz notch filter

[Harold keller]

Can anyone provide information on implementing a 50Hz notch filter for
data acquisition that operates without phase distortion?

This could be a circuit or dedicated linear phase filter IC.

Harold Keller

 

[Martin Brown]

Can anyone provide information on implementing a 50Hz notch filter for
data acquisition that operates without phase distortion?

This could be a circuit or dedicated linear phase filter IC.

The only way you could do this would be by mixing in the right amount of
phase locked 50Hz signal to null out the unwanted hum component. A
sharp notch filter necessarily has to change phase across the null point.

You might be able to trade the depth of the notch for a smaller phase
error if you don't need too much attenuation. Does it have to track
mains frequency in real time? Or will it be wide enough to cover
expected variations? What sort of high frequency has to be passed?

High precision DVMs use mains synchronous ADC conversion to eliminate
hum components from the measured data. 100mS being the first common
multiple that works for both UK & US mains frequencies.

Reminds me. Anyone have any thoughts on why the fifth harmonic on UK
mains at 250Hz is so strong these days?

Regards,
Martin Brown

 

[Martin Brown]

On a sunny day (Sun, 13 Dec 2009 18:04:22 +0000) it happened Martin Brown
<[email protected]>:

Global warming?

I suspect switched mode PSU's but I can't see why the 5th harmonic is
now so strong or for that matter where the roughly 623+/-3Hz component
in my local electrical interference is coming from.

There is a heck of a lot of crossover distortion on our local mains - it
doesn't look all that much like a sine wave!

I can see other higher frequencies from CFLs but the 5th harmonic of
mains remains a bit odd. Some days it is the strongest harmonic component.

Regards,
Martin Brown

 

[john jardine]

Can anyone provide information on implementing a 50Hz notch filter for
data acquisition that operates without phase distortion?

This could be a circuit or dedicated linear phase filter IC.

Harold Keller

This any help? ...
http://i48.tinypic.com/zsv4nc.jpg

(From Ebay, I'd bought a defunct research institute's complete collection of
that EE magazine. The above circuit was one of about 100 items that were
begging to be scanned to a printable PDF, for light bedtime reading

 

[Paul Keinanen]

The only way you could do this would be by mixing in the right amount of
phase locked 50Hz signal to null out the unwanted hum component.

You have to phase lock the PLL to the 50 Hz in the input signal and
not to the power supply voltage, since the input signal hum can be
from a different mains phase than the one powering the instrument.

High precision DVMs use mains synchronous ADC conversion to eliminate
hum components from the measured data. 100mS being the first common
multiple that works for both UK & US mains frequencies.

Reminds me. Anyone have any thoughts on why the fifth harmonic on UK
mains at 250Hz is so strong these days?

How strong is the 150 Hz component compared to the 250 Hz component ?

In a three phase system with equal resistive loads, the neutral wire
current will cancel.

When single phase rectifier+capacitor input loads are operated with
conduction angles less than 60 degrees on all phases, the return
current peaks do not overlap in the neutral wire, thus no cancellation
will occur in the neutral wire and the neutral current can be as high
as the phase current. This has caused some cable failures in some
underground cables, when the cross section area of the neutral
conductor was less than the cross section of a phase conductor.

Anyway, the mainly 150 Hz current peaks in the neutral wire will also
cause voltage drop in the neutral wire. When you measure the harmonic
content between one phase and the neutral, quite strange values might
be visible. However, I would expect the 150 Hz component to be much
stronger than the 250 Hz.

 

[Martin Brown]

You have to phase lock the PLL to the 50 Hz in the input signal and
not to the power supply voltage, since the input signal hum can be
from a different mains phase than the one powering the instrument.

Yes. I should have made that clear.

How strong is the 150 Hz component compared to the 250 Hz component ?

Third harmonic is barely measurable. The fifth harmonic most times is
way stronger than the second harmonic and all other even harmonics are
absent. After that in ordering of decreasing strength is 7, 11, 9, 15
,13 sponsored by a convenience store maybe...

In a three phase system with equal resistive loads, the neutral wire
current will cancel.

When single phase rectifier+capacitor input loads are operated with
conduction angles less than 60 degrees on all phases, the return
current peaks do not overlap in the neutral wire, thus no cancellation
will occur in the neutral wire and the neutral current can be as high
as the phase current. This has caused some cable failures in some
underground cables, when the cross section area of the neutral
conductor was less than the cross section of a phase conductor.

Anyway, the mainly 150 Hz current peaks in the neutral wire will also
cause voltage drop in the neutral wire. When you measure the harmonic
content between one phase and the neutral, quite strange values might
be visible. However, I would expect the 150 Hz component to be much
stronger than the 250 Hz.

So would I - hence the comment about this anomolous 250Hz component.

Turns out it is due to a sensor in the process of going bad. I just
tried a brand new one before posting this and it gives much cleaner more
reasonable results. The harmonics now look like I would expect.

Regards,
Martin Brown

 

[legg]

Can anyone provide information on implementing a 50Hz notch filter for
data acquisition that operates without phase distortion?

This could be a circuit or dedicated linear phase filter IC.

Harold Keller

Combining the outputs of high pass and low pass filters would leave
'passed' components pretty phase coherent to the input.

As the 50Hz notch is usually addressing hum, avoiding it in the first
place is a better idea, where practical.

RL

 

[Nemo]

Harold keller writes

Can anyone provide information on implementing a 50Hz notch filter for
data acquisition that operates without phase distortion?

This could be a circuit or dedicated linear phase filter IC.

It's hard, because if you're trying to filter out 50 / 60 Hz you are
probably trying to measure down to DC. The first thought is to use a
notch filter, but as others have noted that usually gives a phase shift
across the notch. The next thought is to use a band-reject by using
parallel lowpass and highpass filters, and summing their outputs; you
set one's cutoff to 45Hz and the other to 55Hz and you'd expect these to
cut out everything from 45 to 55Hz. In practice though they have finite
sharpness (not as sharp as a notch filter), so you have to set the
cutoffs at least 20% away from the centre frequency to give decent
attenuation to the target frequency, so you're probably seriously
affecting your measurement bands.

So next you think hmm Bessel filters? And a little looking around leads
you to Maxim App note AN431, which details a low freq notch filter based
on their own 8 pin low pass clocked chip MAX7410 (which is cheap) and
specifically mentions the 180 degree phase shift as input sweeps across
its notch freq. But there are versions, the MAX7409 and MAX7413, which
are Bessel filters. I asked their tech support department about this
recently and they said the 7409 would work albeit possibly with a
slightly different ratio of clock-to-notch frequency, but I never got a
chance to experiment with this idea so please let us know if it works
8) . Its sister part the 7410 is quoted as having an 85:1 clock:notch
ratio in the app note, ie it needs a few kHz clock for a 50Hz notch.

Switching this filter into circuit will reduce circuit bandwidth to the
(15kHz?) max of this IC. It will also eliminate any offset as there are
DC blocking capacitors in this path – this may not always be desirable.

 

[JosephKK]

I suspect switched mode PSU's but I can't see why the 5th harmonic is
now so strong or for that matter where the roughly 623+/-3Hz component
in my local electrical interference is coming from.

There is a heck of a lot of crossover distortion on our local mains - it
doesn't look all that much like a sine wave!

I can see other higher frequencies from CFLs but the 5th harmonic of
mains remains a bit odd. Some days it is the strongest harmonic component.

Regards,
Martin Brown

It is driven by two things, simple rectifiers and third harmonic reduction
as a result of delta windings in three phase transformers.

It should mostly go away in "say" ten years as many things get power factor
and harmonic corrected.