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Re: Keithley 2015 THD - question

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mzacharias:
I have a quick question regarding a Keithley 2015 THD multimeter. (Yes I know there's much better analyzers around, but I have neither the money nor bench space for those.)

As I recall, the loop-back distortion runs about .05%, (haven't tested lately) yet units under test routinely measure .005%, often even less, still using the internal generator.

Just wondering why this would be. BTW the 2015 owners manual specs .005.

switchabl:
1. If the DUT acts as a low-pass filter, it may suppress higher harmonics present in the test signal. E.g. if you are testing an audio amplifier with a bandwidth of 20 kHz at 10 kHz, anything above the 2nd harmonic will essentially be cut off, while in loop-back configuration the full 50 kHz bandwidth of the analyzer is available. Look at the individual harmonics instead of THD to see which ones are affected.

2. Unlike noise, distortion components add coherently. If they are out of phase, the DUT distortion can cancel out some of the distortion of the generator (or analyzer) instead of adding to it. This introduces serious measurement uncertainty as you get close to residual distortion level of your instrument. You want your generator and analyzer to have significantly lower distortion than your DUT to avoid this.

(Also, the distortion level of the generator will be somewhat dependent on the load. However, I doubt it will become significantly better with a low input impedance DUT connected compared to loop-back.)

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