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From: Vladimir Vassilevsky on 2 Mar 2010 09:45


Jerry Avins wrote:

> Vladimir Vassilevsky wrote:

>> BTW, just recently I had to measure ~0.0001% of THD. That's seismic
>> data acqusition specmanship. HiFi nuts, you said ? :-)
>
> Can you discuss some of the subtleties involved? What kind of ADC is
> linear enough to do that?

Interestingly, ADC is not the biggest part of a problem. There are ADCs
with that kind of linearity. Also, filtering the fundamental out is the
classic way for doing such measurements. The difficult part is
generation of super clean sine. I had to use good DAC with analog post
filtering.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: evilzucchini on 2 Mar 2010 10:19
>On 1 Mar, 23:17, "evilzucchini" <cbar...(a)digitalgridinc.com> wrote:
>> Thanks for the responses.
>>
>> I don't think aliasing is a problem because our anti-aliasing filter's
>> cutoff is less than 2 times our sampling rate.
>
>Eh... less than *two* times the sampling rate?
>Not less than *half* the sampling rate...?
>
>Rune

D'oh!

Sorry, I meant half.
From: steveu on 2 Mar 2010 19:58
>
>
>steveu wrote:
>
>> Why take a roundabout approach to finding the THD (finding all the
>> harmonics with an FFT and summing them), rather than go straight for
your
>> goal? What about:
>>
>> - Phase lock a synthesised pure sine wave to your signal.
>> - Tune the amplitude of your sine wave, so that
>> original.pure == pure.pure
>
>I wouldn't recommend this for beginners.

Because Matlab doesn't pre-package this strategy? :-)

>> Now, sqrt(original.original) is your harmonic polluted RMS value, and
>> sqrt(pure.pure) is the fundamental RMS value. The difference is the
>> harmonic content.
>
>Not quite. The difference is THD + N + numeric artifacts.

The numeric artefacts are fairly easy to keep under control. Noise can be
an issue if you want this to work over a wide dynamic range. Picking out
the exact harmonic frequencies, and ignoring all the noise in between,
could be cleaner with small signals.

>> Calculate those dot products in a leaky manner, and you can perform a
>> continuous measurement, tuning the phase lock and amplitude as you go.
>
>I've heard that some digital scopes adjust their sample rate so the
>period of fundamental falls exactly in the FFT bin.

I don't know what scopes do, but if you want to do accurate mains power
harmonic measurements by FFT you'd better be prepared to tune to the
precise fundamental, and sit all the bins bang on the harmonics.

I've done power analysis by having a table of Goertzel coeffs for the exact
harmonic frequencies, for reasonably fine increments of fundamental between
47.5Hz and 52.5Hz. After doing a fundamental estimate I used the
appropriate table entries to estimate the harmonics as individual elements,
rather than an amorphous FFT bank. Its capable of very accurate and stable
results, without too much computation. Something like the 20th to 40th
harmonic is usually as far as people want to go, so you don't need to run
more than 40 Goertzels. That suits an application looking for an accurate
breakdown of the individual harmonics.

Steve

From: Vladimir Vassilevsky on 3 Mar 2010 13:38


steveu wrote:

>>
>>steveu wrote:
>>
>>

>>> What about:
>>>- Phase lock a synthesised pure sine wave to your signal.
>>>- Tune the amplitude of your sine wave, so that
>>> original.pure == pure.pure
>>
>>I wouldn't recommend this for beginners.
>
> Because Matlab doesn't pre-package this strategy? :-)

MatLab: dumb methods for dumb people (TM)

>>>Now, sqrt(original.original) is your harmonic polluted RMS value, and
>>>sqrt(pure.pure) is the fundamental RMS value. The difference is the
>>>harmonic content.
>>
>>Not quite. The difference is THD + N + numeric artifacts.
>
>
> The numeric artefacts are fairly easy to keep under control.

When subtracting two square roots? You are looking for trouble.

> Noise can be
> an issue if you want this to work over a wide dynamic range. Picking out
> the exact harmonic frequencies, and ignoring all the noise in between,
> could be cleaner with small signals.

But this is a totally different story. :-)

>>>Calculate those dot products in a leaky manner, and you can perform a
>>>continuous measurement, tuning the phase lock and amplitude as you go.
>>
>>I've heard that some digital scopes adjust their sample rate so the
>>period of fundamental falls exactly in the FFT bin.
>
>
> I don't know what scopes do, but if you want to do accurate mains power
> harmonic measurements by FFT you'd better be prepared to tune to the
> precise fundamental, and sit all the bins bang on the harmonics.
>
> I've done power analysis by having a table of Goertzel coeffs for the exact
> harmonic frequencies, for reasonably fine increments of fundamental between
> 47.5Hz and 52.5Hz.

Why is that? That's very narrow for mobile generator and too wide for
power grid.

> After doing a fundamental estimate I used the
> appropriate table entries to estimate the harmonics as individual elements,
> rather than an amorphous FFT bank. Its capable of very accurate and stable
> results, without too much computation. Something like the 20th to 40th
> harmonic is usually as far as people want to go, so you don't need to run
> more than 40 Goertzels. That suits an application looking for an accurate
> breakdown of the individual harmonics.

The 8-bit thinking is very touching; however it seems to be getting less
and less popular with all that internets and matlabi :-)



Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: steveu on 4 Mar 2010 05:59
>
>
>steveu wrote:
>
>>>
>>>steveu wrote:
>>>
>>>
>
>>>> What about:
>>>>- Phase lock a synthesised pure sine wave to your signal.
>>>>- Tune the amplitude of your sine wave, so that
>>>> original.pure == pure.pure
>>>
>>>I wouldn't recommend this for beginners.
>>
>> Because Matlab doesn't pre-package this strategy? :-)
>
>MatLab: dumb methods for dumb people (TM)
>
>>>>Now, sqrt(original.original) is your harmonic polluted RMS value, and
>>>>sqrt(pure.pure) is the fundamental RMS value. The difference is the
>>>>harmonic content.
>>>
>>>Not quite. The difference is THD + N + numeric artifacts.
>>
>>
>> The numeric artefacts are fairly easy to keep under control.
>
>When subtracting two square roots? You are looking for trouble.
>
>> Noise can be
>> an issue if you want this to work over a wide dynamic range. Picking
out
>> the exact harmonic frequencies, and ignoring all the noise in between,
>> could be cleaner with small signals.
>
>But this is a totally different story. :-)
>
>>>>Calculate those dot products in a leaky manner, and you can perform a
>>>>continuous measurement, tuning the phase lock and amplitude as you go.
>>>
>>>I've heard that some digital scopes adjust their sample rate so the
>>>period of fundamental falls exactly in the FFT bin.
>>
>>
>> I don't know what scopes do, but if you want to do accurate mains power
>> harmonic measurements by FFT you'd better be prepared to tune to the
>> precise fundamental, and sit all the bins bang on the harmonics.
>>
>> I've done power analysis by having a table of Goertzel coeffs for the
exact
>> harmonic frequencies, for reasonably fine increments of fundamental
between
>> 47.5Hz and 52.5Hz.
>
>Why is that? That's very narrow for mobile generator and too wide for
>power grid.

Its wider than the power grid gets in even the worse run grids, but its the
usual spec you are required to work to.

>> After doing a fundamental estimate I used the
>> appropriate table entries to estimate the harmonics as individual
elements,
>> rather than an amorphous FFT bank. Its capable of very accurate and
stable
>> results, without too much computation. Something like the 20th to 40th
>> harmonic is usually as far as people want to go, so you don't need to
run
>> more than 40 Goertzels. That suits an application looking for an
accurate
>> breakdown of the individual harmonics.
>
>The 8-bit thinking is very touching; however it seems to be getting less
>and less popular with all that internets and matlabi :-)

It still tends to be efficient solutions that get the volume sales. Many
utilities are interested in charging based on fundamental power energy
consumption, rather than the active energy consumption they use today.
They'll put really detailed harmonic analysis into every consumer
electricity meter if its cheap enough. You'd better make the cost delta
over a simply electricity meter close to zero, though, if you don't want to
be confined to a niche.

Steve

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