How Distortion Free are the Distortion Measurers? - diyAudio
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Old 5th July 2012, 05:40 AM   #1
fas42 is offline fas42  Australia
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Default How Distortion Free are the Distortion Measurers?

Meaning, how sensitive are the supposedly high resolution distortion measurement tools to their environment?

As an example, Audio Precision is the name to mention for audio measurement, and are spec'd to resolve very low levels of distortion. But if we deliberately inject some rather nasty waveform distortion into the mains supply that the analyser is running off while taking a particularly subtle reading, will the figures and graphs remain rock steady? Or, introduce some unpleasant RF signals in the vicinity, same question ...

There is no mention of the robustness of the equipment against such factors on the AP website, I wonder if anyone has tried to assess this sort of behaviour in any way, at any time?

Or does everyone just assume the test equipment is perfect ...?

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Old 5th July 2012, 05:50 AM   #2
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Test equipment does not need to be perfect (It can't be) it just has to be better than the device that is tested.
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Old 5th July 2012, 02:41 PM   #3
macboy is offline macboy  Canada
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I have an HP 8903B, not the best, but good. It's distortion floor (measuring its own oscillator output) is about 0.0015%. If I turn on a bench power supply on the same circuit, the reading goes up to about 0.0022% or more. That supply is a good Agilent one, you wouldn't think that it would create that much noise, but it does. I don't know if it is power line noise or airborne EMI/RFI, but it is there.
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Old 5th July 2012, 02:46 PM   #4
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That's why many distortion analyzers and sine wave generators can operate from internal battery power.
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Old 5th July 2012, 03:21 PM   #5
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Quote:
Originally Posted by fas42 View Post
Meaning, how sensitive are the supposedly high resolution distortion measurement tools to their environment?

As an example, Audio Precision is the name to mention for audio measurement, and are spec'd to resolve very low levels of distortion. But if we deliberately inject some rather nasty waveform distortion into the mains supply that the analyser is running off while taking a particularly subtle reading, will the figures and graphs remain rock steady? Or, introduce some unpleasant RF signals in the vicinity, same question ...

There is no mention of the robustness of the equipment against such factors on the AP website, I wonder if anyone has tried to assess this sort of behaviour in any way, at any time?

Or does everyone just assume the test equipment is perfect ...?
That's why you do a loop-back test (i.e. running the analyzers output directly into its input) to check the baseline of the equipment before doing any measurements.

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Old 5th July 2012, 03:27 PM   #6
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fair point though, a well laid out design using modern techniques and modern parts is becoming very difficult to measure with anything but the best equipment. you can do as Steve suggests so you know the floor and then measure the design under conditions that stress it in a quantifiable way to produce more distortion and noise than it would under operating conditions. After this apply math to get the predicted result. measuring such things directly is becoming near impossible for even those that make the parts.
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Old 5th July 2012, 05:21 PM   #7
macboy is offline macboy  Canada
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Originally Posted by Steve Eddy View Post
That's why you do a loop-back test (i.e. running the analyzers output directly into its input) to check the baseline of the equipment before doing any measurements.

se
You can and should do a baseline measurement, but it only tells you the lower limit of your measurements. For something as complex as a distortion measurement, you can't do somthing like subtract the baseline from the measurement to try to get a "better" result.

For example, if the THD reading is 0.0035% and the baseline is 0.0015%, you must not conclude that the THD of the DUT is only 0.0020%. It would be more accurate to say that the THD of DUT is 0.0035% +/-0.015%. This is an important distinction. Realize that certain harmonics of the DUT may be out of phase with the residual harmonics present in the instrument, and may partially cancel each other (they will add as vectors, not arithmetically).

Baseline measurements with other equipment like an AC voltmeter can be equally deceiving. I have one TRMS AC meter that will display about 0.00150 V (on the 3 volt scale) with the input leads shorted. This is due to noise in the RMS converter. It is tempting to zero/null this out, or to manually subtract it from the reading. That would be a big mistake. The input voltage and the offset voltage add like this:
Vreading = sqrt( Vinput2 + Vnoise2 )
It is tempting to think that when measuring say 0.10000 V, that the reading would be 0.10150 V due to the offset noise observed when taking a null/baseline measurement. But using the above you can see that the reading will be 0.10001 V. So the input noise of 1.50 mV contributes only 0.01 mV of error to that measurement.

So while you can and should do baseline measurements, you need to be careful about what you do with that information.
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Old 6th July 2012, 06:55 AM   #8
richiem is online now richiem  United States
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In my experience, most commercial equipment is pretty well-shielded electrostatically, but is not well-shielded electromagnetically -- that is a *lot* harder to do. So EMI is always a possible issue.

Fortunately for us, most of the magnetic noise is power-line related, and the simple expedient of high-pass filtering can alleviate a lot of junk, as long as you're not trying to see any stuff that's filtered out. That's why most distortion analyzers have a 400Hz HP filter.

High-res spectrum analysis really helps with this sorting out process, since it lets you see the signal components of interest mixed in among the offending noise.

The ongoing larger issue for me is the ability to see the self-distortion of the analyzer -- evaluating this requires a source which is much lower in distortion than the analyzer itself, and this is fairly hard to do. A good case in point is the HP 8903B mentioned by macboy. Its analyzer section can auto null to better than -110dB relative to full scale inputs, but the notch filter's self-distortion (mostly 2nd H.) limits its resolution to -100dB, or 0.001% or so, if carefully adjusted. It has a very good oscillator, but you don't know how good because of the analyzer's self-distortion.
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Old 6th July 2012, 08:06 AM   #9
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Originally Posted by richiem View Post
In my experience, most commercial equipment is pretty well-shielded electrostatically, but is not well-shielded electromagnetically -- that is a *lot* harder to do. So EMI is always a possible issue.
That is not my experience at all.
Consumer equipment isn't shielded at all due to the use of unbalanced connections.
Pro equipment is usually shielded magnetically (twisting of the positive and negative cables) but not electronically because the shield is directly connected to the signal earth.
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Old 6th July 2012, 08:14 AM   #10
richiem is online now richiem  United States
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Sorry Dirk -- my bad -- I should have said "commercial test equipment," like that made by AP, HP, Tektronix, etc. I wasn't referring to commercial or consumer grade audio gear, nor even to pro grade audio recording equipment, although i would expect that stuff to be better. Sorry for the lack of clarity.
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