What are those spikes after transients?

[uliano]

Thanks everybody, lots of stimulating info here!

I'm waiting for external 50 Ohm (chinese) terminators as cheap scopes are 1M only. Then I would very much try the "alternative probe"

[David Hess]

Your blocking capacitors are foil - not suitable for decoupling in your case - you have to use ceramics instead.

....

PS: foil capacitors are good for audio or "low" frequency apps.
For decoupling of fast transients in digital circuits you want capacitors with low parasitic inductance - the best are smd ceramic capacitors (like 10nF, 100nF), or through-hole one..

If those are SMF (stacked metal film) capacitors, then they are fine for decoupling.  The construction and layout on a breadboard dominates anyway.

[tggzzz]

Ah, tggzzz has alternative facts…

So what then is the characteristic impedance of your ‘very lossy transmission line’ in your probe cables? 1 Mohm??

The central conductor's resistance is around 300ohms/m.
Don't believe me, cut open a probe and measure it with a multimeter.

Basically your understanding is based on, to repeat your phrase, alternative facts.
Don't believe me, but do believe Tek, p14 of http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf

This a good reference for other classes of probes, too.

[uliano]

Maybe it was not probing, after all! 

Same probing. (also,  no caps at all!  )

Different outcome. (with a 100 Mhz scope and a 150 Mhz probe I guess I can't see anything steeper than this)

[sicco]

Ah, tggzzz has alternative facts…

So what then is the characteristic impedance of your ‘very lossy transmission line’ in your probe cables? 1 Mohm??

The central conductor's resistance is around 300ohms/m.
Don't believe me, cut open a probe and measure it with a multimeter.

Basically your understanding is based on, to repeat your phrase, alternative facts.
Don't believe me, but do believe Tek, p14 of http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf

This a good reference for other classes of probes, too.

Yes, great Tek manual from the old days. The resistance of the coax center wire is there in decent probes and helps in certain scenarios. But it cannot prevent the reflections at the two impedance jumps (the two ends) in the transmission line. Assuming the coax is still 50 ohms characteristic impedance (or maybe it's 20 or maybe 200 ohms or whatever - but it will not be a Z of kohms or Mohms, and the resistance of the center wire is not to be confused with characteristic impedance!), then at both ends, seen from a wave travelling within the cable, a wave that's about to hit the impedance jump, the refection back is still huge. Reflection coefficient is (Zload-Zcable)/(Zload+Zcable). Only way to get that to zero is when Zcable = Zload. There are no cables with a Z of 1 Mohm...
Assuming A is the probe and B is the scope's BNC, 1 Mohm input, at say one or two meters displaced from A, one cannot claim that the signal goes beautifully, unchanged, from A to B, with nothing bouncing back into the same cable at B. What bounced at B will propagate back to A (if the wire resistance didn't kill it on the way out, then why would it kill it on the way back?), and there again an impedance jump, so again a reflection back into the cable. Plain physics.
Only matched impedance at B would do the trick. The Finnish DIY probe does approach that. The Tek Z0 probes with 50 ohms cables into a 50 ohms scope input do that. But no probe into a 1 Mohm input can do that, no matter how thin you make the center wire in the coax cable.       

[sicco]

Maybe it was not probing, after all! 

Same probing. (also,  no caps at all!  )

Different outcome. (with a 100 Mhz scope and a 150 Mhz probe I guess I can't see anything steeper than this)

Is that the same ic as signal source as before? It doesn’t look like it is. Different ic = different output drive characteristic = different rise and fall times in the transients.

[uliano]

No, not the same, but I didnt'expect so much difference!

they're both recent 8 bit avr MCU

[tggzzz]

Ah, tggzzz has alternative facts…

So what then is the characteristic impedance of your ‘very lossy transmission line’ in your probe cables? 1 Mohm??

The central conductor's resistance is around 300ohms/m.
Don't believe me, cut open a probe and measure it with a multimeter.

Basically your understanding is based on, to repeat your phrase, alternative facts.
Don't believe me, but do believe Tek, p14 of http://www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf

This a good reference for other classes of probes, too.

Yes, great Tek manual from the old days. The resistance of the coax center wire is there in decent probes and helps in certain scenarios. But it cannot prevent the reflections at the two impedance jumps (the two ends) in the transmission line. Assuming the coax is still 50 ohms characteristic impedance (or maybe it's 20 or maybe 200 ohms or whatever - but it will not be a Z of kohms or Mohms, and the resistance of the center wire is not to be confused with characteristic impedance!), then at both ends, seen from a wave travelling within the cable, a wave that's about to hit the impedance jump, the refection back is still huge. Reflection coefficient is (Zload-Zcable)/(Zload+Zcable). Only way to get that to zero is when Zcable = Zload. There are no cables with a Z of 1 Mohm...
Assuming A is the probe and B is the scope's BNC, 1 Mohm input, at say one or two meters displaced from A, one cannot claim that the signal goes beautifully, unchanged, from A to B, with nothing bouncing back into the same cable at B. What bounced at B will propagate back to A (if the wire resistance didn't kill it on the way out, then why would it kill it on the way back?), and there again an impedance jump, so again a reflection back into the cable. Plain physics.
Only matched impedance at B would do the trick. The Finnish DIY probe does approach that. The Tek Z0 probes with 50 ohms cables into a 50 ohms scope input do that. But no probe into a 1 Mohm input can do that, no matter how thin you make the center wire in the coax cable.       

I'm impressed that you know more than Tek engineers, and look forward to seeing your explanation for how any standard *10 "high" impedance probes can ever work at up to 500MHz.

Start by explaining these measured results (400MHz scope, ~300ps source risetime, 250MHz probe)...



FFI: https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/