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I have prototyped the circuit in the attached photo. It is a high voltage with aiming to achieve a rise time <= 100 ns across the load R1.

In reality, the waveform I probe on R12 using a 1 Meg probe on my oscilloscope exhibits a fast rise time (< 100 ns) up to about half its amplitude, then an abrupt discontinuity with a very round and slow rise which lasts about 20 us.

I am unsure about what I am missing. The prototype is built on a classic proto-board using THT components.

Any ideas/suggestion are welcome.

simulation in ltspice

UPDATE: Thank you.

I have added an (exaggerated) 100 pF from the test-point to gnd. As expected, edges get rounded. But the actual scope trace has a totally different shape, with a first, very low rise time part, and a discontinuos slow corner.

yellow=trigger, violet=signal yellow=trigger, violet=signal added probe capacitance

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  • \$\begingroup\$ Your scope probe will have significant input capacitance. On the 10x setting it probably has about 10pF in parallel with 10 Megohm. Put that into the simulator and see how the results compare. \$\endgroup\$ Commented Apr 1 at 2:00
  • \$\begingroup\$ @KevinWhite This comment answers the question; consider copying it into an answer, perhaps adding a diagram to show the equivalent circuit and reproduce the indicated waveform. \$\endgroup\$ Commented Apr 1 at 5:27
  • \$\begingroup\$ I have added the capacitance in the sim, along with a picture of the oscilloscope traces. \$\endgroup\$ Commented Apr 1 at 8:59
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    \$\begingroup\$ @KevinWhite OP is probing R12 which has 200kOhms impedance. OP, can you check probe compensation? Are you using 1x or 10x probe? \$\endgroup\$ Commented Apr 1 at 9:30
  • \$\begingroup\$ the probe is on 1x. I struggle to understand the sharp transition between the first fast edge and the slow corner. \$\endgroup\$ Commented Apr 1 at 13:11

2 Answers 2

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Your high impedance scope probe has a non-zero capacitance (probably 10pF with a 1M probe) which will lead to the rounding off of high frequency response.

The probe effect is often modelled as a simple RC element, but in reality is considerably more complex, and this tends to be more visible when the source circuit impedance is high.

The discontinuity you are seeing looks to me like an RCRC effect from the combination of probe and scope architecture, exposed by the very high source impedance.

If you need better results, I'd suggest you look at some of the custom probing options in Howard Johnson and Martin Graham's excellent "High Speed Digital Design: A Handbook of Black Magic.".

These show how you can tailor probe characteristics for particular applications, often trading signal amplitude for veracity.

If it works in you application, the best result might come from connecting your scope in 50R mode between the bottom of R12 and Gnd. You signal will be down to 1mV, but I suspect the waveform shape will have way fewer artefacts.

(You should also check the trimmer cap adjustment of your probe, as that not being right would definitely make the system RCRC.)

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It is probably not so bad in reality, the scope just doesn’t like to measure high impedance source, 500/25M is 20uA only.

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