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Oscilloscope bandwidth simulated with Octave/Matlab

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ik1zyw:
I, the OP, spent the last day reading and watching the referenced material, then trying to improve the overall response of my scope+probe.

It turns out that my fastest high-impedance probe came with a BNC adapter so I could get rid of the 15 cm ground lead and repeat the rise-time experiment (still to be calculated with the freshly learned formula). The attached picture shows the result and can be compared to the one in reply #7. The edge shape doesn't change with the generator frequency (10 kHz vs 200 kHz).

According to gf's reply while typing this text, I need 1) to repeat the experiment with proper sampling and 2) get a generator with a known rise time (suggestions for a DIY solution?). The impulse is generated with this circuit https://ik1zyw.blogspot.com/2011/05/comb-generator-full-diagram.html and the duty cycle is calculated considering average values in data sheet (rise/fall time, propagation delay). At that time I needed a wide comb and now I understand it is not the right circuit to check the performance of an oscilloscope (still I'd like to see that output!).

I am glad I am not the only one learning from my own initial question, and I thank everyone who has spent precious time writing the software. I will play with it to visualise answers to other questions I have in mind, while it is clear that the simulation I was after would not match the real world system. All this while learning the important details behind a DSO, as I come from analog scopes.

I won't mind if this thread continues regardless it gets away from my question. This is the Programming forum after all!

Paolo

tggzzz:

--- Quote from: ik1zyw on September 02, 2021, 09:45:22 am ---I, the OP, spent the last day reading and watching the referenced material, then trying to improve the overall response of my scope+probe.

--- End quote ---

Good to see someone that listens and thinks. That's not always guaranteed!


--- Quote ---It turns out that my fastest high-impedance probe came with a BNC adapter so I could get rid of the 15 cm ground lead and repeat the rise-time experiment (still to be calculated with the freshly learned formula). The attached picture shows the result and can be compared to the one in reply #7. The edge shape doesn't change with the generator frequency (10 kHz vs 200 kHz).

--- End quote ---

It wouldn't. Anthropomorhic explanation: the scope and signal don't "know" when the next transition will occur. Mathematical explanation and experiment, see https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

Nonetheless, and importantly, the short ground lead has significantly reduced the ringing.

You might also care to understand the theory and practice of "low" impedance resistive divider Z0 probes. You can make them at home, unlike conventional probes, and at 100MHz their resistance is ~7 times higher than so-called high impedance *10 probes.

RoGeorge:
Well done, the latest waveform already looks better.   :-+

For a DIY pulse generator, the one shown by w2aew is good and easy to make #88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter, then you'll remain with a nice tool to measure cables by Time Domain Reflectometry, too.

Since the generator fits on a small PCB, you can put a BNC on the same board and plug it straight into the oscilloscope's BNC connector, without any wires or probes in between.  Do not use a breadboard, use a PCB clad and solder all the parts in a compact way, Manhattan/dead bug stile.  Having a ground plane on the PCB is very important, too.  Do not skip the Vcc decoupling capacitors either.  You must have both uF range and nF range capacitors in parallel.  Usually there is a 10nF, 100nF, 10uF or similar, all in parallel.  Keep any connection as short as possible, and the signal edges will look even better.

The frequency is not important, can be 10 kHz or 10 MHz, it doesn't matter much.  What matters is how fast the edges are.  The sharper the edge, the widest the generated frequency comb, though a very wide spectrum (caused when the generator gives a train of very narrow Von spikes) might confuse a digital oscilloscope if it's DSP part is not well crafted.

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