Two oscilloscope proves, same location different readouts…. Why???

[Rene]

Hello,

Please take a second to check out attached picture 1. You can see from the picture that I have an extraordinarily simple circuit. It is nothing more than a single resistor with two oscilloscope proves connected across the resistor in the exact same locations.

So what would you expect the oscilloscope readout to be if I were to change the signal from 0 to 5 volts on the circuit? Well, I was expecting the two wave form to be identical since the two proves are connected in identical places but to my surprise this was not the case. Checkout picture 2 to see what I am talking about.

My question is why are the two wave forms not identical? Why is there disparity between the two waves? Is there something wrong with my scope?

Thanks.

[Richard Head]

When measuring such steep risetimes you shouldn't use the ground lead. Rather use the shortwire spring contacts that are supplied with the probes. This ensures that there is no appreciable voltage drop across the series inductance of the scope ground lead.
You should also check your probe compensation for each probe. That is probably your problem in this instance.

[PA4TIM]

And do not use a breadbord.

[AndyC_772]

Frequency is too high for this to be a probe compensation problem. Probes are compensated using a 1kHz square wave, and the effect is visible on a time scale of tens of microseconds. This trace is captured over nanoseconds, so a compensation error will just appear as a difference in gain, not wave shape.

I'd be willing to bet that simply moving the probes or putting a hand somewhere near the board will make more of a difference than we're seeing in that single scope trace.

The scope, and probes, are fine.

Have a read - lots of good stuff here!
http://www.mikrocontroller.net/wikifiles/6/63/Probe_Fundamentals-_Tektronix.pdf

[David Hess]

There may be subtle differences between the probes or vertical input channels but the differences in inductance and positioning between the two ground leads is enough to explain this.

Connecting the probes coaxially to the signal source would be a better test and they make probe tip adapters for just this purpose.

[T3sl4co1l]

I dare say you're observing a see-saw oscillation between the two probes' capacitances and ground clip / tip inductances.

This is an important lesson actually, as some components can do this.  Parallel diodes, for instance: if the trace lengths are unequal, and the recovery times are unequal (even if schottky, the strongly nonlinear capacitance can produce the parametric equivalent), the junction capacitance - stray inductance loop becomes excited.  Which, if the mutual connection of the two diodes is balanced, the ringing doesn't leave the circuit -- like the handle of a tuning fork, it's neutral.  But the resonance is nonetheless present, and prone to radiate energy!

Damping the common point bridging the two capacitances can help.  In this case, you might try 100 ohms from source to each probe, rather than tying them in common over 220.

Tim