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From: Fred Bartoli on 29 Jul 2010 16:59
Vladimir Vassilevsky a �crit :
>
>
> Fred Bartoli wrote:
>
>> Vladimir Vassilevsky a �crit :
>>
>>>
>>> I just tested an active filter with THD ~ 0.0001%. Wow. This filter
>>> is intended for distortion measurements.
>>>
>>> There are two key things about it:
>>>
>>> 1. Using good quality film capacitors at the swings of less then 1%
>>> of the rated voltage.
>>>
>>> 2. All opamps should be in the inverting mode.
>>>
>
>> Well, a few weeks ago I just measured levels just 20 times below that
>> at 50kHz. OK, I cheated a bit since it was only H2 that I needed to
>> measure :-)
>
> At 0.00001% levels, SNR and signal coherence could represent a problem :-)

Lock-ins easily deal with that.

>
>> Seems impossible, but doing it the right way it's not that difficult...
>> It's even deceptively simple once you've figured how to do it.
>
> :-)
>
>> The real fun is to generate the signal and make sure that your
>> measurement is not pure illusion :-)
>>
>> Again, H2 only made that easier (a ladder of notch filter does the job
>> pretty well). The funny thing was, for the first cut experiment, to
>> tweak the RLC filter distortion with magnets to set the inductor (last
>> stage) cycle mid point to a pure odd function this cancels any even
>> component.
>
> I can easily bring the distortion down to the numeric accuracy floor by
> digital correction, however I am suspicious about repeatability of this
> kind of cheating.
>

You're right to be cautious. There's no reason at all for your DAC
errors to be repeatable well below the LSB level.


>> A small NeB magnet at half a meter distance did the job but you had to
>> be really careful to not blow the distortion up to the ppm levels :-)
>
> That looks very professional and reliable. I wonder how would you make
> it in production :-)

Did I mention prod somewhere ?-)

It was a quick and dirty lash-up to do some tests down to the 10s of ppb
level.

>
>> (RM8 cores)
>
> Why not an air core?
>
>

'coz I had the RM8 cores already wound and trimmed to the percent level
and I had to find a way of doing the test as soon as I had the idea for
the low distortion notch filter...


--
Thanks,
Fred.
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