Products > Test Equipment
Measuring Distortions with the Scope:What you see is not what you really have..
mawyatt:
To evaluate the DSOs ability to properly represent the THD of a waveform, rather than use a waveform of unknown low THD (sine-wave for example), use a waveform with known THD such as a square-wave or triangle-wave. Altho these waveforms may have an uncertainty in creation from the generator like the sine-wave they have a THD that can be computed theoretically and not ideally zero, so the results from a DSO FFT evaluation can be compared to the theoretical THD of the applied waveform.
Because these waveforms naturally possess higher levels of controlled THD compared to a uncontrolled abet low THD sine-wave, they can be measured by the DSO with the lower resolution ADCs & performance Input amplifiers/attenuators within their span of limited performance.
Here's a few THD for common waveforms derived from this IEEE paper:
https://www.researchgate.net/publication/260672713_Analytic_Method_for_the_Computation_of_the_Total_Harmonic_Distortion_by_the_Cauchy_Method_of_Residues
Square-wave THD = Sqrt[(pi^2)/8 -1] ~ 48.3%
Triangle-wave THD = Sqrt[(pi^4)/96 -1] ~ 12.1%
Sawtooth-wave THD = Sqrt[((pi^2)/6 -1} ~ 80.3%
Pulse-wave THD = Sqrt[((D(1-D)*(pi^2))/2(sin^2(pi*D))) -1] which varies from 48.3% to 191% as D (Duty Cycle) varies from 0.5 to 0.9, or 0.1 as THD function is symmetrical about D = 0.5 (Square-wave).
We are unable to verify our DSO FFT against these theoretical waveform THD readings at this time, so others may want to utilize this information and report results. Hopefully we'll be able to perform these measurements ourselves and report the results soon.
Of course this introduces another "uncertainty" with the waveform "quality", some of the modern AWGs likely are good sources at lower frequencies (as shown with sine-waves), and we do have an accurate by design custom low frequency squarewave source with precise duty cycle and amplitude available.
Anyway, hopes this helps with the FFT evaluations regarding THD.
As always, YMMV.
Best,
_Wim_:
--- Quote from: JeremyC on January 07, 2023, 01:48:39 am ---- Don’t use any kind of averaging, ERES, highres, etc.
....
As good start I suggest to check THD + Scope published by W2AEW about 3 years ago:
--- End quote ---
I think you need to re-watch the referenced video by W2AEW (#65: Basics of using FFT on an oscilloscope), because it perfectly makes sense to use HI-RES or ERES to increase the effective number of bits. This way an 8-bit scope can have a larger effective number of bits at a much lower bandwidth.
A good comparison between ERES and HI-RES can be found here: https://teledynelecroy.com/doc/differences-between-eres-and-hires
markone:
--- Quote from: _Wim_ on January 07, 2023, 06:36:23 am ---
--- Quote from: JeremyC on January 07, 2023, 01:48:39 am ---- Don’t use any kind of averaging, ERES, highres, etc.
....
As good start I suggest to check THD + Scope published by W2AEW about 3 years ago:
--- End quote ---
I think you need to re-watch the referenced video by W2AEW (#65: Basics of using FFT on an oscilloscope), because it perfectly makes sense to use HI-RES or ERES to increase the effective number of bits. This way an 8-bit scope can have a larger effective number of bits at a much lower bandwidth.
A good comparison between ERES and HI-RES can be found here: https://teledynelecroy.com/doc/differences-between-eres-and-hires
--- End quote ---
The point is that any math process that extend dynamic range with ADC sampling add artifacts and alter BW, so what JeremyC is saying makes perfect sense if you want to be sure to no catch phantom FFT peaks or miss real ones with BW cut.
I personally agree with that, while I often use ERES equivalent functions in YT mode I have no interest to see FFT fake deep dynamic ranges in 8-12 bits scopes, I find all those small false frequency components quite distracting and when I see 100-120dB FFT screenshot taken from ordinary scopes I make a smile.
elecdonia:
--- Quote from: gf on January 06, 2023, 12:14:57 pm ---
--- Quote from: nctnico on January 06, 2023, 11:32:44 am ---After doing some testing (with a Tektronix AFG31000 ) the question is more like: how useful is long FFT where it comes to showing valid results or not. I did some testing with the RTM3004 which is limited to 128kpoints. As a result the FFT noise floor is about -80dB and shows no distortion products. On my GW Instek with 1Mpts FFT (resulting in a lower noise floor) I can see the front-end distortion products just fine.
--- End quote ---
Even if you consider anything below (say) -50dBc as potentially invalid, a long FFT is still useful if you need to separate closely spaced frequencies.
--- End quote ---
This earlier post describes FFT measurement of the AWG signal from a Siglent which worked out very well, clearly displaying harmonic amplitudes -100dB below the fundamental amplitude. OP lists most of the measurement parameters.
https://www.eevblog.com/forum/testgear/comparison-between-siglent-sdg1000x-and-2000x/msg4615288/#msg4615288
In my opinion the use of 24-bit A-D to analyze the signal from the AWG is significant. Until recently 24-bit A-D converters were nearly unobtainable at any price. But today they are mass-produced and affordable. My thinking is that having 24-bit resolution for the analysis system is valuable.
_Wim_:
--- Quote from: markone on January 07, 2023, 02:03:31 pm ---The point is that any math process that extend dynamic range with ADC sampling add artifacts and alter BW, so what JeremyC is saying makes perfect sense if you want to be sure to no catch phantom FFT peaks or miss real ones with BW cut.
I personally agree with that, while I often use ERES equivalent functions in YT mode I have no interest to see FFT fake deep dynamic ranges in 8-12 bits scopes, I find all those small false frequency components quite distracting and when I see 100-120dB FFT screenshot taken from ordinary scopes I make a smile.
--- End quote ---
I agree that the very low noise floor shown is artifact, but if you take into account the effective number of bits (including the bits created by enhancement from HIRES or ERES), than the actual dynamic range can be calculated (20*log(2^enob)) and is in my opinion "real".
Many high resolutions ADCs use oversampling to achieve their high resolution, and for me oversampling and HIRES/ERES are essentially the same. Bandwidth reduction and bit-enhancement can be seen from the attached tables (copied from the Lecroy explanation)
So for a 10 bit scope a 3 bit enhancement is certainly possible for these low frequency ranges in the posts above, giving around 78db of dynamic range. Anything below that must indeed be taking with a grain of salt, but I just wanted to point out that an 8-bit scope has more that 48db of dynamic range...
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