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From: Vladimir Vassilevsky on 27 Jul 2010 18:49 Jan Panteltje wrote: > On a sunny day (Tue, 27 Jul 2010 09:48:37 -0500) it happened Vladimir > Vassilevsky <nospam(a)nowhere.com> wrote in > >>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. > > > How do you measure the distortion? In my case, that was really simple: there is an ADC with less then 1 ppm of distortion. The measurement is straightforward. The purpose of the filter is generate clean sine wave to verify the performance of ADC. Actually, measuring low distortion levels is not too difficult if you have a good bandpass and same as good notch filter at the fundamental frequency. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Vladimir Vassilevsky on 27 Jul 2010 19:06 Fred Bartoli wrote: > Vladimir Vassilevsky a écrit : > >> >> 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 :-) > 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. > 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 :-) > (RM8 cores) Why not an air core? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Bill Sloman on 27 Jul 2010 22:25 On Jul 28, 9:06 am, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote: > Fred Bartoli wrote: > > Vladimir Vassilevsky a écrit : > > >> 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 :-) > > > 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. > > > 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 :-) > > > (RM8 cores) > > Why not an air core? You can't tweek the permeability of air with a NeB magnet. -- Bill Sloman, Nijmegen
From: Jan Panteltje on 28 Jul 2010 06:52 On a sunny day (Tue, 27 Jul 2010 17:49:52 -0500) it happened Vladimir Vassilevsky <nospam(a)nowhere.com> wrote in <jNudnYxOkJgN_9LRnZ2dnUVZ_jWdnZ2d(a)giganews.com>: > > >Jan Panteltje wrote: > >> On a sunny day (Tue, 27 Jul 2010 09:48:37 -0500) it happened Vladimir >> Vassilevsky <nospam(a)nowhere.com> wrote in >> >>>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. >> >> >> How do you measure the distortion? > >In my case, that was really simple: there is an ADC with less then 1 ppm >of distortion. The measurement is straightforward. The purpose of the >filter is generate clean sine wave to verify the performance of ADC. I am not sure I complete grasp that sentence. Do you digitise the filter output, then do a FFT? >Actually, measuring low distortion levels is not too difficult if you >have a good bandpass and same as good notch filter at the fundamental >frequency. I know that, in the long ago past I did measurements on Eurovision sound lines using just that procedure, cannot remember the name of the instrument, but it was sort of a weekly ritual :-) Simple bit of math gave the exact distortion. Always curious how somebody else does it. You also needed a good signal [generator].
From: Dennis on 28 Jul 2010 07:34 "Jan Panteltje" <pNaonStpealmtje(a)yahoo.com> wrote in message news:i2p24l$j54$1(a)news.albasani.net... > On a sunny day (Tue, 27 Jul 2010 17:49:52 -0500) it happened Vladimir > Vassilevsky <nospam(a)nowhere.com> wrote in > <jNudnYxOkJgN_9LRnZ2dnUVZ_jWdnZ2d(a)giganews.com>: > >> >> >>Jan Panteltje wrote: >> >>> On a sunny day (Tue, 27 Jul 2010 09:48:37 -0500) it happened Vladimir >>> Vassilevsky <nospam(a)nowhere.com> wrote in >>> >>>>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. >>> >>> >>> How do you measure the distortion? >> >>In my case, that was really simple: there is an ADC with less then 1 ppm >>of distortion. The measurement is straightforward. The purpose of the >>filter is generate clean sine wave to verify the performance of ADC. > > I am not sure I complete grasp that sentence. > Do you digitise the filter output, then do a FFT? > >>Actually, measuring low distortion levels is not too difficult if you >>have a good bandpass and same as good notch filter at the fundamental >>frequency. > > I know that, in the long ago past I did measurements on Eurovision sound > lines > using just that procedure, cannot remember the name of the instrument, but > it was > sort of a weekly ritual :-) > Simple bit of math gave the exact distortion. > Always curious how somebody else does it. > You also needed a good signal [generator]. You just made me remember doing something similar using one of these: http://www.smoke.com.au/~ic/audio/AWA-F240.html
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