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From: Fred Bartoli on 17 May 2010 16:30
John Larkin a �crit :
> On Mon, 17 May 2010 14:17:09 -0500, Vladimir Vassilevsky
> <nospam(a)nowhere.com> wrote:
>
>>
>> John Larkin wrote:
>>
>>> I wonder how one could ever verify low distortion. I guess you could
>>> trust Audio Precision or Agilent or somebody, at their guaranteed THD
>>> floor.
>> The direct measurement with AP provides for the THD floor slightly below
>> 0.001%. I don't know if it is limited by generator or analyser, but this
>> is only -100 dB.
>>
>>> But is there any fundamental way to demonstrate the THD of a
>>> source or analyzer, or to make a sinewave source that's known to have
>>> below, say, 1 PPM distortion?
>>> How do you build a lowpass or highpass or bandpass filter that's known
>>> to have < 1 PPM distortion? Vacuum caps and toroidal inductors wound
>>> on quartz forms? Would there be mechanical issues that create
>>> distortion?
>> You just have to operate at small voltages and currents. Noise can be
>> eliminated by sufficient averaging.
>>
>>> An RC highpass filter could be switched into the signal chain and its
>>> effect on distortion noted. If it were a very good filter, and the
>>> switch/connectors were to be trusted.
>> Unfortunately, there is very little data on distortion in the passive
>> components. I did some experimentation with high voltage film capacitors
>> and good quality resistors; it is feasible to make an active filter with
>> 0.0001% distortion provided the opamp is good enough.
>>
>>
>
> I was thinking of using a rigid vacuum capacitor into a TIA as a
> differentiator/high pass filter. If some sort of analyzer reports the
> amplitudes of the fundamental and the harmonics, then you boost the
> harmonic content with a known-good highpass, that sort of crosschecks
> the analyzer. Or make a really good lowpass, ditto.
>
> We've occasionally used a passive notch filter to remove the
> fundamental of a sine wave before going into a spectrum analyzer. If
> the reported harmonics change with/without the notch, one suspects the
> analyzer.
>
>


I just have to design some mid power amplifier for 50kHz with 1 ppm
distortion.

The first task was to set up the generator. Getting bellow ppm is easy.

Sort of.

Start from an already good source (about 50ppm) then set up a passive
filter which first trap the unwanted harmonics (in my case I want bellow
ppm H2). Two cells in series gives about 60dB rejection.
Follow that with a parallel LC to reject the fundamental before getting
to the SA and I get... 5-7ppm H2.
Ahem... Even with small pulley inductors I should only have odd harmonics...

Eureka! Playing with a small toy magnet to cancel the inductors residual
magnetization get me to the bellow ppm level.

Now the surprise is that that small (4mm dia x 2mm thick) magnet easily
gets to the 10s of ppm level distortion from a two feet distance.

Time to go for an active filter...


--
Thanks,
Fred.
From: George Herold on 17 May 2010 17:01
On May 17, 4:27 pm, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
> > On May 17, 11:35 am, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote:
>
> >>I need to make a test source with THD ~ 0.0001% at the frequencies below
> >>1kHz. Leaving many details aside, the biggest problem seems to be an opamp.
> George Herold wrote:
> > How about a different opamp?  The opa134 seems to have enough zero's
> > in the THD+noise spec.  (0.00008%)
>

" That's what I planned on using initially. Unfortunately, the
availability is an issue."

Geesh, you scared me there for a moment, (All my favorite TI/BB opamps
seem to have 6 month delivery times these days.) but TI reports stock
almost everywhere. Still I'd better order a couple of tubes.

George H.

>
> David Eather wrote:
> > This might be better:
> >http://www.national.com/pf/LM/LME49990.html#Overview
>
> I thought about bipolar opamps from National also. One drawback is high
> power consumption, the other problem is high noise current. I am not
> sure if it would be possible to make them work right with ~100k
> impedances in the circuit.
>
> VLV

From: miso on 17 May 2010 21:22
On May 17, 9:15 am, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 17 May 2010 10:35:12 -0500, Vladimir Vassilevsky
>
>
>
> <nos...(a)nowhere.com> wrote:
>
> >I need to make a test source with THD ~ 0.0001% at the frequencies below
> >1kHz. Leaving many details aside, the biggest problem seems to be an opamp.
>
> >The typical THD vs frequency plot for a good opamp looks like this:
>
> >ADTL082
> >http://www.analog.com/static/imported-files/data_sheets/ADTL082_084.pdf
>
> >It saturates at ~0.001% at the frequency ~2 kHz. Why is that? According
> >to the GBV = 5MHz and G = 2e5, the corner should be at 25 Hz.
>
> >Perhaps the limit is set by Audio Precision; it can't measure THD below
> >~0.008% in direct way. But, even if 0.001% is the artifact of the
> >measurement, there is still not enough of open loop gain to get to
> >0.0001% at 1kHz. Would it be possible to make a cascaded amp to boost
> >the open loop gain and improve distortion?
>
> >Vladimir Vassilevsky
> >DSP and Mixed Signal Design Consultant
> >http://www.abvolt.com
>
> Follow Jim Williams' rule: always invert. Add JL's rule: use really
> good resistors.
>
> John

I'm not sure why Jim Williams gets credit for "always invert." It is
well known that op amp have less distortion in inverting
comfiguration. For non-inverting, especially a follower, you are at
the mercy of the common mode rejection of the differential pair.
From: George Herold on 17 May 2010 21:31
On May 17, 9:22 pm, "m...(a)sushi.com" <m...(a)sushi.com> wrote:
> On May 17, 9:15 am, John Larkin
>
>
>
>
>
> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> > On Mon, 17 May 2010 10:35:12 -0500, Vladimir Vassilevsky
>
> > <nos...(a)nowhere.com> wrote:
>
> > >I need to make a test source with THD ~ 0.0001% at the frequencies below
> > >1kHz. Leaving many details aside, the biggest problem seems to be an opamp.
>
> > >The typical THD vs frequency plot for a good opamp looks like this:
>
> > >ADTL082
> > >http://www.analog.com/static/imported-files/data_sheets/ADTL082_084.pdf
>
> > >It saturates at ~0.001% at the frequency ~2 kHz. Why is that? According
> > >to the GBV = 5MHz and G = 2e5, the corner should be at 25 Hz.
>
> > >Perhaps the limit is set by Audio Precision; it can't measure THD below
> > >~0.008% in direct way. But, even if 0.001% is the artifact of the
> > >measurement, there is still not enough of open loop gain to get to
> > >0.0001% at 1kHz. Would it be possible to make a cascaded amp to boost
> > >the open loop gain and improve distortion?
>
> > >Vladimir Vassilevsky
> > >DSP and Mixed Signal Design Consultant
> > >http://www.abvolt.com
>
> > Follow Jim Williams' rule: always invert. Add JL's rule: use really
> > good resistors.
>
> > John
>
> I'm not sure why Jim Williams gets credit for "always invert." It is
> well known that op amp have less distortion in inverting
> comfiguration. For non-inverting, especially a follower, you are at
> the mercy of the common mode rejection of the differential pair.- Hide quoted text -
>
> - Show quoted text -

Well, I first read it in either one of his app notes, or his reprints
of app notes in one of his books. And doesn't the whole 'rule' go
something like, "Always invert, except when you can't." Those are
certainly words to try and design by.

George H.

From: Mike on 18 May 2010 01:29
Vladimir Vassilevsky <nospam(a)nowhere.com> wrote:

> That's what I need. Thank you, Mike.
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultant
> http://www.abvolt.com

You are welcome. Let us know when you have measured the distortion.
:)


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