EEVblog #1213 - The Oscilloscope Interpolation Trap!

Oscilloscope Sin X/x Interpolation can be a trap for young players, find out why.

#Oscilloscope #Trap #Interpolation

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[AstralSorm]

Gotta know your sinc. Your scope at least has zero order hold ("disabled") I hope too, in addition to linear? (Bars instead of tiny dots. Or fat dots.)

One thing I'd love to see is ZOH with error bars from sinc ringing as estimate, like a blurry overlay. It's a rare case that your clock is perfectly synced with signal. Old analog scopes naturally did that with faded display due to capacitance in memory and display... Though that can lie too, it's exponential interpolation - Laplacian it's called I think?

Using a scope as a frequency counter over sample rate only works if you average enough samples. (Or take a long Fourier spectrum, similarly.) Not quite what you showed.

[pamersiel]

The ringing BEFORE the signal is a screaming sign, that we see artefacts of digital signal processing.

[Pops]

Keep these scope vids going. I teach an electronics class at work to green engineers and you seem to explain things a lot better than I can.

[simondalzell]

It's the reverse of the problem audiophiles have! Some look at linearly-interpolared digital audio samples and assume they're getting a "blocky" audio output. Then they look at the sine interpolation and complain about those wiggles around a theoretical square wave and think that's an artifact of converting analog to digital. Great demonstration in an obvious place that I'd never considered.

[PeregrineBF]

I wouldn't call sinc "smoke and mirrors". The Nyquist-Shannon sampling theorem derivation requires convolution of the sampled values with an ideal sinc function to recover the original signal. The reason for the 4x instead of the 2x limit is because of the finite number of samples and the finite (non-ideal) sinc function used. Just as the Fourier transform is only valid for infinitely repeating signals so real systems use windowing functions which add some distortion. It's important to know what the differences from the ideal case are to interpret any measurement.

[friedmule]

10:00 Genius illustration in upper right corner! Now I think that I really understand what FFT is. Thanks Dave! :-)

[RedwoodRhiadra]

The Oscilloscope Interpolation Trap - best Robert Ludlum novel ever.

[radoslawbiernacki]

Great video. This is the proper way of explaining what is the bandwidth and sample rate of oscilloscope.

[greenlight]

Basically this is just sampling theorem. Multiplying with delta pulses in the time domain (equals the sampling process) means folding in the frequency domain. If you want the analog signal back you filter in the frequency domain with an ideal rectangel to get only the low frequencies up to the maximum of 200 MHz limited by the bandwidth of the analog frontend (and not the ones that occured as copies of the real signal at higher frequencies due to folding as an effect of sampling in the time domain). This rectangel in the frequency domain corresponds to folding in time domain with sinc aka. Use a sinc-wave for every point in the time domain. If you have too low sampling the frequency copies due to the sampling interfere with the original low signal-frequencies. Trying to filter out the original signal with the ideal frequency rectangle filter aka. Sinc-interpolation in time-domain is not possible anymore and you get nonsense. The time domain sinc is therefore just coming from the ideal rectangle frequency filter. Flies away.

[Der_Arathok]

Watching this literally 2 days after checking a signal and taking data from the oscilloscope that "didn't make sense"... I always thought, the oscilloscope is always right! Greatly appreciate this Video!
A must watch for people having to use a oscilloscope in a lab maybe as scientific helper and just started out. I wrote the link down for my fellow students!

[wpherigo]

Which is why it’s always a great idea to have 2 ways to measure something, with one of them being analog. You can best understand these issues if you’ve done time with analog scopes before, and understand how digital scopes work.

[rabbibacongrease]

This is probably the most informative video on this subject that's ever been made. Great job and keep up the good work.

[Chris_Grossman]

I always maximize the memory and check the sample rate before I zoom in. On my SDS1102X the slowest sweep that will capture at 1 Gs/s is 1ms/div with the full 14M memory depth turned on. It is great for zooming in on the scope, but gives you a vary large file if you want to save the data.

[PlasmaHH]

I don't know anymore which scope it was, but I once had my hands on a scope where when you zoomed in enough, it would display little dots in addition to the line. I really loved that and thought that this should be a default on all scopes as it immediately makes you aware of such situations.

[henke]

There is an easy solution that avoids this issue while also letting the marketing people invent higher numbers. Just mark the actual samples on the plot!

[HamiltonMechanical]

I just bought my first oscilloscope! I'm so excited! I bought an auction lot with 7 oscilloscopes, a couple function generators, and a few digital power supplies. Can't wait to get ahold of them and see how many of them still work!

[jtsotherone]

I think AvE and Dave are slowly converging into one entity, they are sharing the same vocabulary these days. No bad thing!

[kalhana_photography]

real trap for young players
newfangled
come-a-gutsa
chase a red herring down a rabbit hole

[udhayakumara]

Dave, you got a new oscilloscope SDS5104X. Can you please do a full review video and if possible compare with keysight and Tektronix.
I had a plan to purchase this earlier because it's a decent spec 1GHz oscilloscope in the market with very lower cost. I don't believe in the marketing videos and I looking for your opinion on this.

[ICMpolytechLille]

Good video about an overlooked aspect. The 500GS/s advertised in the data sheet is acquisition interpolated sample rate. These samples can be saved, while display interpolated samples cannot. They are meant to fill up the screen pixels to get a continuous signal.