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  • EEVblog #601 – Why Digital Oscilloscopes Appear Noisy

    Posted on April 10th, 2014 EEVblog 12 comments


    Why do digital oscilloscopes appear noisier than traditional analog oscilloscopes?
    Dave busts the myth that digital scopes are noiser than analog scopes, and demonstrates what inherent advantages digital scopes can have over analog scopes in terms of true waveform capture. And also why your analog scope may be hiding important signal detail from you.
    Demonstrations of how memory depth, analog bandwidth, averaging, and intensity graded displays can all effect the signal detail you see on your digital oscilloscope.
    And how long exposure camera shots on analog oscilloscopes can reveal detail you can’t see with your eyes.

    Demonstrations are done on the new Tektronix MDO3000, the Rigol DS1052E, the Tektronix TDS220, and Tektronix 2225 analog oscilloscope.
    Previous video on common mode noise measurement
    Forum HERE

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    • Phil Tirino

      This is the kind of EEVBlog video I love to see. Stuff you wouldn’t pick up from a textbook and refines your expertise in using equiptment and tools of the trade. I LOVE IT! Very interesting. I think I saw some waveform/function generators in the background there. That’s going to be my next big purchase. I have even thought about building a microcontroller based waveform gen, but it’d still be nice to have a professionally made one. That could be an idea for a new video. It’s something a little different from the typical Dave Jones multimeter/oscilloscope/power supply video. Tell us what makes the Rigol DG1000′s different from the DG4000′s that we wouldn’t find by reading the marketing materials on the products.

      Anyway, thanks for #601. I enjoyed it. I’m still looking forward to some follow-ups on #600 too. Cheers!

      • http://www.eevblog.com EEVblog

        I’ve got the Rigol and Siglent sig gens, so a video comparing those and other has been on the cards for a while.

    • Pierre lavoie

      So does analog are still useful today, is there a situation where my digital scope wońt do the job ?

      Thank´s

      Pierre lavoie

      • tchicago

        I’d say the digital scope will do the job of the analog one just fine. It is only the cheaper or older digital scope will be a little harder to use in a some rather small number of situations.

        Example of a situation: you are just made a DAC, say using the R-2R resistors. Then you are feeding it a digital sine wave and expecting to use the scope to see how smooth the sine wave on the output is, like seeing any bumps and nonlinearities. If the digital scope does not have an “analog-like” averaging brightness mode, the random noise may prevent you from seeing some of the DAC-induced noise. So you’ll need to go to the scope settings and enable the averaging. The non-repeatable random noise will cancel itself out, whereas the repeatable DAC noise will be visible on the averaged signal.

        To enable averaging in Rigol DS1102E or DS1052E: press Acquire button, then the top button next to the screen – it will let you switch from the Normal acquisition mode to Average.

      • http://www.indigresso.com JP Norair

        A digital scope with 2x higher bandwidth of an analog scope can theoretically do the same job. This is the Nyquist rate. In reality, there is no ideal low-pass filter, but today there are really great digital FIR filters, so if your bandwidth is maybe 4x, then, theoretically you can do everything just the same.

    • Circlotron

      So… upgrading my DS2072 from 70MHZ to 200MHz has actually made it “noisier”. Hmmm. I just love the boxcar averaging though.

    • http://www.indigresso.com JP Norair

      I love how your old Tek digital scope says “Australian Defence Force” on it. Love it!

    • http://www.indigresso.com JP Norair

      I love how your old Tek digital scope says “Australian Defence Force” on it. Love it!

    • Circlotron

      I wonder if there is some correlation between this scope situation and sound recording where you trace an analogue wiggle on a vinyl disc and the disc cutter cannot respond to extremely sudden noise blips in kind of the same way the analogue scope hides them. A CD recording might have some of this single sample noise and maybe it might sound different to some ears.

    • http://none Michael

      Dave, take the analog scope and trigger externally in normal mode to try to captute only ONE trace a time to make a photo (like a DSO does it). Possibly we can see the same result!
      Best regards from Germany
      Michael

      • Wim Libaers

        That can be done. At slow sweep speeds, you can even see it with your eyes.

        At high sweep speeds, that’s no longer possible except when you have an amplified screen. I’ve used a Tektronix 2467B (microchannel plate CRT) to observe single pulses at 2ns/div. Single shot mode. Easily visible, of course only as a brief flicker, it’s not a storage scope, but good enough to see the shape of the signal. Even visible at faster speeds than that.

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