• EEVblog #396 – Bode Plotting on Your Osciloscope

    Dave shows you a neat trick on how to get a real time frequency response bode plot on your oscilloscope using your function generator. Useful for filter or system response characterisation.
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      • Dorin

        You should teach electronics at a University… if you won’t directly, I’ll use you myself :)

      • f4eru

        This is not a bode plot. A bode plot also includes phase.

        Alternatively, you can use a white noise gen and FFT of your scope, pretty darn near to a scalar network analyzer

        • http://www.eevblog.com EEVblog

          It’s a Bode magnitude plot. Often that is all that is required.

        • Manu

          As we say where I’m living: “Knowlege is like jam: the less you have, the more you spread it”.

      • vincent himpe

        That’s why some advanced signal generators actually have marker outputs. My Agilent 3325 can sweep from 0hz to 22MHz and generate up tot 40vpp it has several outputs you connect to a scope.

        There is a sweep start output that you connect to the trigger input of the scope. this makes sure the scope runs in sync with the sweep. there is a marker output that you connect to channel two of the scope. this is a 5 volts pp signal and you put the scope at 0.5 volts/div vertical and set the trace in such a way you only see the vertical portion of that pulse.

        The generator is programmed to swepp and by pressing the ‘Marker’ button on the generator , whenever the internal sweep counter crosses the marker value the immediate fruquency is latched in the generator.

        in essense where that vertical line of channel 2 is , is whatever frequency is dispayed on the function generator. the generator changes its display to show you what frequency corresponds to the marker. as the sweep is a pure digital affair the frequency indication is absolute.

        this not only allows yo to bode pots but it also allows you to have a marke ron the screen an know the exact frequency the generator is at. so you could find the -3db point ( point where only 70.7% of the amplitude is left ) by hitting the marker left/right buttons an reading the amplitude on the scope. once found you look at the generator and you know the frequency.

        these older synthesizers were built especially to do this kind of stuff as spectrum analysers invariably do not have the dynamic range required and DSA’s are bloody expensive. I have another synthesized source that is very ‘pure’ as in no harmonics and has differential output capability including floating above ground.

        The 3325 does is not an ARB but a real sine oscillator steered by a DDS circuit driving a pll. So it has excellent linearity at its output.

        We frequently use it to find the resonance point of the voice coil for example. This is a tuned LC network but as the resonance point changes depending on the position of the arm you need to measure it at multiple positions to model it.

        To do the active filters in the positional servo control loop of the harddisk we have a few of those Stanford SRS756 DSA machines.

      • huh

        Dave, with your skills you could design a small moderately high speed multichannel ADC external card to do that stuff on a PC (uADC?). That would become an instant purchase for those people who need a good quality analog (AC&DC) input and don’t want to shell out the money needed for an industrial DAQ or a professional multichannel audio card.
        Should it have a good bandwidth, most HAM radio operators would also find it immensely useful for I/Q SDR decoding.

        (minor typo in the title: “osciloscope”)

      • http://www.cleverscope.com Bart Schroder

        Use a Cleverscope – 80 dB dynamic range! Not a lousy 8 bit ADC – but 14 bit! Bode plot built in. And you can copy and paste the bode plot into your word doc – and all nicely labelled. But I know Dave doesn’t like PC scopes, ah well. And you get Gain and Phase, pretty good for stability etc. Check it out!

        By the way all those exclamation marks are to be said in Dave’s unique style!

        • http://www.eevblog.com EEVblog

          I used to design PC scopes, so it’s not that I flat out don’t like them (a common misconception!), I simply don’t like them as a replacement for a bench scope for general use. For specific uses like this, they are very useful, (almost) the right tool for the job. A DSA or network analyser is best of course, but an order of magnitude more expensive than a Cleverscope! 14bit would be plenty good for most uses.

      • Nick


      • michael

        Hi Dave,

        most function gens can output the control voltage for the VFO. Mine does 😉
        You could build up a opamp bridge-rectifier plus a logarithmic output stage. That way you can have the response and the frequency axis to be logarithmic if fed into the scope running in x/y mode. Btw. this should also help to overcome the dynamic issues with the 8-Bit adc resolution.

        Nice Blog. Took me hours to watch and keep up.

      • http://agilent.com Mike H

        Very neat trick! I just recreated this here at my desk. Just wanted to point out that the 3000 X-Series you use here has a built in function generator, so this could potentially be done with just one box. Only difference is that there is no direct sweep mode, but there is FM modulation (simulated with a 50kHz sine, ramp modulation of 50kHz). Thanks as always for the entertaining videos!

      • David

        Dave, please pronounce the surname “Bode” properly:

        Scraping from Wikipedia…

        Hendrik Wade Bode (pronounced Boh-dee in English, Boh-dah in Dutch).

        Born 24 December 1905, Madison, Wisconsin, Died 21 June 1982 (aged 76) Cambridge, Massachusetts

        “Something should be said about his name. To his colleagues at Bell Laboratories and the generations of engineers that have followed, the pronunciation is boh-dee. The Bode family preferred that the original Dutch be used as boh-dah.”

        • http://www.eevblog.com EEVblog

          I am pronouncing it “properly”. That’s how I was taught, and how it’s been commonly spoken in Australia for several generations.
          The English language is a living creature, it changes and morphs in it’s own way in different regions and cultures. So my pronunciation is not wrong, nor uncommon, just how it’s spoken here.

          • David

            Hi Dave,

            My post on the Bode pronounciation is just friendly constructive critisism. Perhaps I should have formatted the post im a more diplomatic way. But I’ll further my case a bit – if allowed…

            Keep in-mind, you are broadcasting to an International Audience. You will gain better acceptance by adhering to standards where possible.

            So why did “Bode”, “Boh-dee”, and “Boh-dah” peak my interest and my response?

            I live in Indonesia (U.S. Expat). Here, I use the pronounciation “Boh-dee” as learned when taking my EE in the U.S.; and most accepted Internationally today IMO.

            But I am sometimes surprised when I’m corrected by listeners here in Indonesia which use the Dutch-proper pronounciation “Boh-dah”. After-all, Indonesia was a Dutch Colony.

            There are Dutch language roots all through Bahasa Indonesia that even carry into the dialect of bi-lingual Standard-English/Bahasa-Indonesia speakers.

            Again, it is not surprising, the Bode Family in Wisonsin, an area with strong Dutch immigrant roots, prefers the pronounciation “Boh-dah” when referencing Hendrik Wade Bode in-legacy. But I defer to “Boh-dee” and always make reference to “Boh-dah” here in Indonesia; for clarity.

            As an aside: The Japanese language also colors Bahasa Indonesia quite a bit due to the forced use of Japanese during the Japanese occupation of Indonesia (then still a Dutch Colony) during World War-II.

            I only include these Indonesian tid-bits as Australia and Indonesia are closely tied in both culture and commerce these days, but the Indonesian language nuances are often over-looked by Australians. You are after-all in Australia.

            Best Regards, David

            • http://www.eevblog.com EEVblog

              Sorry, no sale. I’m going to pronounce it the way I was taught, and the way it’s been said here in Oz for generations.
              If I changed my ways just to be politically correct for a majority of the worldwide audience, then people will never know that people in other countries pronounce it other ways.
              So there is value in not following the “standard” IMO.
              Everyone knows exactly what I mean when I pronounce it the way I do, so why does anyone care?
              Should I change my “annoying” voice too?, for “better acceptance”, like suggested by the dozen comments I get a week about it?
              Answers on the back of a postcard please 😛

      • Robert D

        Dave, why didn’t you use your scope in X/Y mode and use the sweep output of your generator for the Y axis?

        • Pete Higgins

          Perhaps, because he would not then have been able to have a logarithmic frequency scale. The sweep output is proportional to frequency and there’s probably no way to make it proportional to the log of frequency.

          Incidentally, in my 42 years working as an analogue electronics engineer in the UK I don’t ever remember hearing Bode pronounced Boh-dee or Boh-dah, it’s always Bode as in Spode.

          I’m steadily working my way through these videos, perhaps in another six months I’ll be commenting on current ones.

      • Diego

        Was anyone able to reproduce this with one of the low end scopes or even a PC scope ?

        Can that even been done ?
        I am not sure if you cac zoom out enought (not enought memory I guess)

      • euzer

        As someone commented on youtube for this video, be aware of overdrive recovery when adjusting the scope’s vertical scaling such that some of the waveform is off screen. I think you could see the resultant distortion when first changing to a log frequency sweep. Tektronix used to have an online comparison video showing how there scopes didn’t suffer from it to the same extent. I’ve seen the problem on a 5000 & 6000 and wouldn’t be surprised if it’s still present on the DSOX range.

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