Products > Test Equipment
Probing at high speeds. Will the 10x probe overload the circuit?
tggzzz:
--- Quote from: joeqsmith on May 09, 2024, 07:11:51 pm ---When ballsystemlord titled Probing at high speeds, they never defined what they were attempting to probe. I provided what I considered the topic to be about based on their mention of resistive probes, thinking the OP would provide more details. I really have no idea what that were actually after and assumed after reading some of the posts, lost interest. A shame really.
--- End quote ---
Agreed.
Quite a few people tried to extract information from him, to no effect.
Doesn't look good, but perhaps he has been "unavoidably detained in a real life situation".
G0HZU:
Note that up at RF frequencies, the loading effect of a typical 10Meg x10 scope probe isn't as simple as 10Meg in parallel with about 12-15pF.
The 10Meg Rp will fall quite steadily with increasing frequency. By 10MHz it could fall to about 20k ohm and by several hundred MHz it can fall to maybe 100 ohms Rp or less.
By contrast, the parallel capacitance Cp seen at the tip will start to fall through VHF and into UHF. It might be 12pF at 1MHz but this could easily fall to 8pF or even less up at 200MHz.
So once you get well into the VHF region, the Rp loading effect can also become significant. The quality of the compensation also matters as this can affect how a fast pulse appears on the scope display.
Generally speaking, x10 scope probes don't perform very well for critical stuff like this. It is possible to buy x10 scope probes that use 2Meg resistive dividers rather than 10Meg and this can help improve the high frequency response as flatter compensation should be easier to achieve.
joeqsmith:
--- Quote from: G0HZU on May 10, 2024, 12:41:09 pm ---Note that up at RF frequencies, the loading effect of a typical 10Meg x10 scope probe isn't as simple as 10Meg in parallel with about 12-15pF.
...
--- End quote ---
It's not just the probes performance, the user also plays a part. I couldn't tell you how many times I have seen someone using that long ground clip lead and hooked tip.
tggzzz:
There was an interesting and long thread on https://groups.io/g/HP-Agilent-Keysight-equipment
This message shows the measured tip R and C for an HP10073 probe - a quite decent *10 "high" impedance probe.
https://groups.io/g/HP-Agilent-Keysight-equipment/message/143362
And this shows the same for an old Tek probe https://groups.io/g/HP-Agilent-Keysight-equipment/message/143360
There are more measurements later in that thread, too.
tggzzz:
--- Quote from: joeqsmith on May 10, 2024, 01:08:27 pm ---I couldn't tell you how many times I have seen someone using that long ground clip lead and hooked tip.
--- End quote ---
Oh yes :(
It is always worth asking for a photo of the experimental setup, to rule out a likely cause being long leads and solderless breadboards.
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