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Bandwidth check of my oscilloscope with simple means ?
Fungus:
--- Quote from: BravoV on December 11, 2020, 08:39:07 am ---
--- Quote from: tggzzz on December 10, 2020, 08:18:33 pm ---The best way to verify that is to examine the rise time of a step function (not an impulse). A clean transition and flat top is important ...
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Re-emphasizing with red highlight, as these easily will be ignored or missed by beginners.
Guess where did I knew/learned that. :palm:
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I wonder if there's a reason why oscilloscopes measure "rise time" by discarding the top/bottom 10% of the signal?
Berni:
No the 10% 90% is because you can't possibly accurately tell where exactly that edge begins and ends. If you want to cheat the rise time then you define it as 20% to 80% (some do use this trick)
Getting bandwidth from rise time is also not always completely accurate because it depends on what shape the frequency response has at the cutoff point. But the shapes that optimize a faster rise time out of the same bandwidth will also generally have more overshoot and ringing. Still the resulting bandwidth number is going to be about in the ballpark of +/- 20% so its good enough.
As people have said square waves are better for testing oscilloscopes because it not only shows the approximate bandwidth but also shows if the cutoff response of the input introduces extra overshoot. A lot of scopes will have a little bit of overshoot and that is fine, but if the overshoot is something like 20% of the signal amplitude there is definitely something wrong.
As a source for generating these fast rise time square waves its easiest to use a fast digital logic chip driving the coax trough a 50 Ohm termination. One popular circuit for doing this uses parallel Hex inverters https://www.epanorama.net/circuits/tdr.html to produce a fast clean square wave (Typically used for TDR measurements of cables, but is also an excellent oscilloscope test signal)
nctnico:
In reality using risetime is a very poor method because it assumes a certain filter response for the oscilloscope. This isn't standard so using the risetime may give false results. The only way to be sure is to use a levelled sinewave generator (together with a 50 Ohm coax cable and 50 Ohm terminator on the oscilloscope input if the oscilloscope doesn't have a 50 Ohm input mode).
tggzzz:
--- Quote from: Fungus on December 11, 2020, 08:38:47 am ---
--- Quote from: tggzzz on December 11, 2020, 08:30:34 am ---
--- Quote from: Fungus on December 11, 2020, 07:58:59 am ---
--- Quote from: tggzzz on December 10, 2020, 09:10:24 pm ---Don't forget to take account of how the inductance of the lead inductance (~1nH/mm) and probe tip capacitance will affect the waveform!
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Not much - it has to work at over 100MHz!
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A 6"/15cm ground lead (or equivalent) and 15pF tip capacitance will visibly resonate at ~100MHz.
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a) A properly terminated BNC cable would be better, yes, but OP is asking for "simple means".
b) Resonance won't affect the "rise time" measurement shown on screen - they discard the top/bottom part of the signal when they calculate that.
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On most scopes it won't be correctly terminated, of course: it would be 50ohm//15pF. Whether that has an effect depends on the length of the cable - the standard standing wave stuff that I won't repeat here.
The resonance will affect a risetime measurement, to some extent. The sharper the resonance the greater the overshoot - and correspondingly shorter time taken to reach 90% of the final amplitude.
Another issue, especially with scopes that digitally process the samples, is the scope's internal filter response. That can be optimised (or pessimised!) in different ways for different purposes.
tggzzz:
--- Quote from: Berni on December 11, 2020, 09:18:01 am ---No the 10% 90% is because you can't possibly accurately tell where exactly that edge begins and ends. If you want to cheat the rise time then you define it as 20% to 80% (some do use this trick)
Getting bandwidth from rise time is also not always completely accurate because it depends on what shape the frequency response has at the cutoff point. But the shapes that optimize a faster rise time out of the same bandwidth will also generally have more overshoot and ringing. Still the resulting bandwidth number is going to be about in the ballpark of +/- 20% so its good enough.
As people have said square waves are better for testing oscilloscopes because it not only shows the approximate bandwidth but also shows if the cutoff response of the input introduces extra overshoot. A lot of scopes will have a little bit of overshoot and that is fine, but if the overshoot is something like 20% of the signal amplitude there is definitely something wrong.
As a source for generating these fast rise time square waves its easiest to use a fast digital logic chip driving the coax trough a 50 Ohm termination. One popular circuit for doing this uses parallel Hex inverters https://www.epanorama.net/circuits/tdr.html to produce a fast clean square wave (Typically used for TDR measurements of cables, but is also an excellent oscilloscope test signal)
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Circuit with imperfect but useful measurements (<300ps risetime): https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941
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