Oscilloscope probe voltage derating @ high frequency - Page 1
Forum section: electronics / testgear

author: G-son
author_rank: Regular Contributor
timestamp: May 25, 2024, 09:23:29 am
content: In the documentation with oscilloscope probes we get a derating curve, but when do we really have to care about it? Does it apply only to a constant voltage & frequency sine wave, or do we have to handleoccasional pulses the same way? In other words, do we have to follow the same voltage limit for one single 1µs sine wave per second with zero volts the rest of the time as we have to with a constant 1MHz sine wave signal? (Obviously assuming 1MHz is well above the frequency where that hypothetical probes derating curve starts going down.)

author: mag_therm
author_rank: Frequent Contributor
timestamp: May 25, 2024, 01:12:13 pm
content: It was my fault when I destroyed a pair of the generic P6100 200MHz probes.I did not check to see the P6100 derating curve is about 4 times lowerat 14MHz, than the (standard for scope) Instek GTP-200B-4 200 MHz which are rated from the curve: 100 [Vdc+Vpk] @ 14MHzAs there are no thermal time curves, I would answer you question in the conservative, meaning to stay below the curve even for intermittent pulse.I since spent a bit more on a pair of Keysight N2140A 200 MHz.The quick start guide does not have a curve, just "10:1 - 300V rms CAT II"

author: G-son
author_rank: Regular Contributor
timestamp: May 26, 2024, 05:01:24 pm
content: That is no doubt the safest option. We'll see how safe I can stand being...

author: mag_therm
author_rank: Frequent Contributor
timestamp: May 26, 2024, 05:36:27 pm
content: Better option: 30dB long tail Tee, homebrew needing lathe/milldrill and a fair amount of spare time.https://app.box.com/s/gnnkla2v5gktpgkzgul0dwypl0cfzh8h

author: 2N3055
author_rank: Super Contributor
timestamp: May 26, 2024, 07:32:57 pm
content: Quote from: mag_therm on May 25, 2024, 01:12:13 pmAre you saying that you believe there is no derating needed because there is no curve?Do you realize Keysight considers those "low cost" probes, and those are delivered with 1000X series?It would not be surprising if they had derating curve similar to cheap Hanteks..Or do you have some official specifications to base your opinion on?

author: mag_therm
author_rank: Frequent Contributor
timestamp: May 26, 2024, 08:19:18 pm
content: Hi 3055,I hope you can re-read or re-translate my posts #1 & #3Theonly "opinion" I stated was in line 4 "..in the conservative..".If you need rating info on the Tee I made, I can give details.

author: 2N3055
author_rank: Super Contributor
timestamp: May 26, 2024, 08:31:32 pm
content: Quote from: mag_therm on May 26, 2024, 08:19:18 pmI'm sorry if I did not understand...Just want you to be on the safe side.

author: G-son
author_rank: Regular Contributor
timestamp: May 26, 2024, 11:25:43 pm
content: Quote from: mag_therm on May 26, 2024, 05:36:27 pmI currently don't have those machines, but I'm interested in a bit more info about that project anyway.

author: mag_therm
author_rank: Frequent Contributor
timestamp: May 27, 2024, 03:35:04 pm
content: Hi G-son,I re-measured the Tee today after it has been monitoring transmitting at 50W ft8 intermittently for over a year.Before this project,I read some ham reports of overheating and also compensation components needed for spectrum analyser Tees built in a single die cast box with a single dropping resistor.That lead to the present housing, with Tee of 3 series resistors in a tight bore resembling a section of very lossy coax.At 50 Watt through, (50 Volt rms) the three resistors are dissipating 3.3 Watt total ( ~1.1Watt each).The resistors have a nominal rating of 2 Watt each.I have no way of measuring the resistor temp rise in the bore at the end of each 15 second ft8 transmission. Max ft8 duty cycle is 50% of 30 sec cycles but in operation, that duty cycle would rarely be approached.NanoVNA results ( I use it to monitor the harmonics of 14MHz from the homebrew Linear Amp so the vna scan today was 1 MHz to 100 MHz:dB of S21https://app.box.com/s/gxff3w1ks6t74n7m52681563o47wufjjphi of S21https://app.box.com/s/78wocx5dbecpjho6vkprp6l38qyow2ltZ of S11https://app.box.com/s/ma249p1ydpw7h10bo54wce5d91xiwk0fSketch of buildhttps://app.box.com/s/pe17juibjai989w7wmmktrndefsat9k6