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| Bandwidth check of my oscilloscope with simple means ? |
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| Fungus:
FWIW I just did another test using the little probe spring instead of the clip-on lead. As expected, the ringing was much reduced. (Note for newbies: The "ringing" on the display probably isn't in the signal, it's coming from the probe clip. Use the spring whenever possible! BNC is even better!) The measured rise times also seemed a teeny bit smaller, but there's not much in it. PS: I just noticed that the crocodile clips on these Micsig probes aren't detachable. That's annoying. >:( Edit: Re-did the screen captures after I remembered the 'scope can measure overshoot. :palm: nb. For those people just joining the thread, the capacitor is just shoved into the GND and +5V on the Arduino. I'm just using it as a 5V voltage source. |
| tggzzz:
--- Quote from: 2N3055 on December 11, 2020, 03:11:14 pm --- --- Quote from: tggzzz on December 11, 2020, 02:49:16 pm --- --- Quote from: 2N3055 on December 11, 2020, 02:01:35 pm ---Sometimes, some people don't really need to know bandwidth, they only need to check how fast is their scope in showing fast digital edges. That is what pulser is born for. --- End quote --- Usually if you are interested in bandwidth/frequency, then it is better to use a frequency domain tool such as a spectrum analyser. (Or, say it quietly, a scope with an FFT) --- End quote --- Sometimes amplitude is important... But for strict frequency domain measurements SA is primary instrument... That is not questionable. Although, i managed to some nice quick checks with a scope on occasion, at 432 MHz and 868 MHz telemetry transceivers. ^-^ --- End quote --- My 350MHz Tek 485 triggers nicely on a 1GHz signal. The beam deflection leaves something to be desired, though :) You can also estimate a signal generator's risetime using a spectrum analyser: https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/ |
| tggzzz:
--- Quote from: Fungus on December 11, 2020, 05:04:27 pm ---(Note for newbies: The "ringing" on the display probably isn't in the signal, it's coming from the probe clip. Use the spring whenever possible! BNC is even better!) --- End quote --- The ringing is the consequence of the interaction between the scope probe's tip capacitance and a ground lead's inductance. You can shift the frequency higher by reducing either the capacitance or the inductance. https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/ https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/ BNC leads have a mis-match between the 50ohm cable impedance and the 50ohm//15pF scope input. That will form a comb filter at frequencies determined by the cable's length. |
| Berni:
--- Quote from: Bud on December 11, 2020, 03:41:31 pm ---There is a neat method how to measure your oscilloscope's bandwidth that Keysight published in a couple of their Blogs. What you do is connect a fast pulser to input, apply Math "differentiate" function, then apply FFT to the result. This will produce a plot in frequency domain. Now use cursors to read the frequency point where level on the FFT plot drops to -3dB from the top. https://community.keysight.com/community/keysight-blogs/oscilloscopes/blog/2016/09/01/how-to-measure-your-oscilloscope-and-probe-s-bandwidth-yourself --- End quote --- Yep that method works too. Here it is done using Leos fast pulser on a MSO9204H First trace is the Ch 1 input, next trace is the differential of that signal (Showing a peak slewrate of about 4 GV/s) and then the FFT of that showing the bandwith. And yes i know the cutoff does not look all that nice, but it is only a 2GHz scope so getting about 3.2GHz out of it at -3dB is not bad. |
| tggzzz:
--- Quote from: Berni on December 11, 2020, 05:19:40 pm ---And yes i know the cutoff does not look all that nice, but it is only a 2GHz scope so getting about 3.2GHz out of it at -3dB is not bad. --- End quote --- It would look even better without that ringing/overshoot :) But yes, it is fun to be able to visibly apply the theory of time-frequency duality, to practically measure things indirectly. How about not differentiating, and comparing the FFT to that shown here: https://entertaininghacks.wordpress.com/2015/08/11/measuring-digital-signal-edge-rates-without-an-oscilloscope/ especially: |
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