[Solved] Newbie Q: What does the derating curve mean exactly for oscill probes? - Page 1
Forum section: electronics / testgear

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: August 28, 2022, 12:00:53 am
content: So I finally got an oscilloscope. Like a good student, which I strive to be, I read through the provided documentation. I noticed that my probes have a derating curve. Does this mean:A: The probes do not read/pass as large a magnitude of voltage when the frequency of the large magnitude voltage is higher (like 1Khz vs. 1Mhz).B: The probes pass large magnitude voltages through to the oscilloscope when the frequency of the large magnitude voltage (like 1Khz vs. 1Mhz) is high, possibly destroying the probes and the scope in the process (and hurting me!)C: Both of the above.D: Some answer I'm not aware of...Thanks!

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 28, 2022, 12:24:03 am
content: The answer is A. The probes usually fail to pass the correct magnitude of the signal past their high freq rating.It is important to match the probes to the scope, if you want to have a reasonable amplitude reading on your scope.The capacitance of the input of the scope is usually on the face of the scope near the BNC input.Make sure you have or get probes that are rated for the freq of the scope and for the capacitance of the input.Higher freq probes have lower capacitances usually.

author: Someone
author_rank: Super Contributor
timestamp: August 28, 2022, 01:12:08 am
content: Quote from: Wallace Gasiewicz on August 28, 2022, 12:24:03 amNo, that is their bandwidth rating, which is separate to thederating(mentioned in the thread title and again in the post).Quote from: ballsystemlord on August 28, 2022, 12:00:53 amThe probes have a specific rating, beyond which the performance and/or safety is not guaranteed. Rarely do data sheets or manuals separate out functional vs safety ratings just picking the most conservative of the two. Since many users do not go beyond the default probes provided with a scope, they are often specified together as a system.But, a probe might pass a voltage too high for the scopes inputs, but that is dependent on the scope and probes. You'd need to go back to the scopes specifications (few detail their input derating or make it obvious).The derating of probes with attenuators (most probes) are often constrained by the thermal limits of the attenuator components, so for short term pulse/burst measurements or in less than maximum temperature environments its possible to exceed the derating without failure or loss of accuracy.

author: S57UUU
author_rank: Regular Contributor
timestamp: August 28, 2022, 05:46:45 am
content: It is easy to fry a probe with anything more than a few watts at RF.Did it with <20W at 145 MHz. Nothing happened to the scope, but I wouldn't bet on that every time.

author: tggzzz
author_rank: Super Contributor
timestamp: August 28, 2022, 08:01:00 am
content: This is a slightly simplistic handwaving explanation, but it is sufficient for the purpose.The big clue is the derating curve looks like the frequency response of a low pass filter.The so-called high impedance *10 scope probes are typically 10Mohm//15pF at the tip. The AC current flowing through the capacitor becomes more significant at higher frequencies, and is the AC voltage is high enough that current will heat the plastic causing it to melt.The derating curve is intended to "prevents" such overheating.Note that at 100MHz the input impedance of such a "high impedance" probe will be <100ohms, whereas a passive "low" impedance Z0 resistive divider *10 probe will be 500ohms.

author: capt bullshot
author_rank: Super Contributor
timestamp: August 28, 2022, 08:22:13 am
content: "Derating" usually refers to reduction of some parameter in dependency of other parameter(s).So in this case it's the maximum voltage dependent of frequency (or the other way round). As it says "derating" it means you shall not exceed these parameters, otherwise correct function will not be given. Depending on how far you exceed the parameters, damage to the object or harm to the user can happen.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: August 28, 2022, 08:40:39 pm
content: Quote from: capt bullshot on August 28, 2022, 08:22:13 amI understood that much.What I wanted clarification for is that the probes are rated 300V Rms CAT-II (150V Rms CAT-II at 1x). Now everyone will probably understand that as meaning that frequency does not matter so long as you do not exceed the rated voltage.BUTI've actually read one or more papers about how the breakdown voltage of air and enameled wire decreases as frequency increases. So, it probably follows that the same would happen to oscilloscope probes and their leads unless there was some special shielding that prevented such breakdown from occurring.I decided to choose the wisest course of action and ask those who probably know more about this than myself.

author: tautech
author_rank: Super Contributor
timestamp: August 28, 2022, 08:59:25 pm
content: It's very simple really it is.CAT ratings ONLY apply to mains frequencies, 50 or 60Hz.For absolute Max probe ratings apply DC thinking.Everything else derate voltages applied to the probe in relation to the signal frequency as expressly outlined in the derating graph.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: August 29, 2022, 02:42:06 am
content: I think the others made it clear. I was just replying to the gentlemen above because I thought he misunderstood what I was asking and/or thinking.

author: Fungus
author_rank: Super Contributor
timestamp: August 29, 2022, 09:19:15 am
content: Quote from: ballsystemlord on August 28, 2022, 08:40:39 pmIt's nothing to do with breakdown.Anything with capacitance will heat up if you charge/discharge it. The more times per second you do it, the hotter it will get.Result: Your probes will melt......hopefully before the oscilloscope melts. Oscilloscopes have a derating curve, too.https://www.eevblog.com/forum/testgear/rigol-ds1202z-e-input-voltage-vs-frequency-derating/