Siglent scope probe calibrator - Page 1
Forum section: electronics / general

author: atum
author_rank: Newbie
timestamp: January 04, 2021, 10:09:56 am
content: Dear all,I'm wondering one thing, i just bought siglent siglent 300 Mhz DSO provided with, of course, 300 MHz probes.When i'm using internal calibrator, i adjusted the probe screw to get good shape on square wave.I saw BW limit capability on channels parameters, and to test something, i activated channel BW limit at 20MHz and i noticed the rise time of the square wave is exactly the same !!My question is, how can we calibrate correctly 300 MHz probe if the calibrator rise time is not short enough to provide enough high frequency harmonics in the square wave to measure correctly the probe frequency banwidth.Do i missed someting ?How do you calibrate correctly high frequency probes ?Thank's for help.Atum

author: tggzzz
author_rank: Super Contributor
timestamp: January 04, 2021, 11:06:34 am
content: It is good to see somebody that thinks and questions.A scope's cal out is for low frequency calibration only, not high frequency nor amplitude. I would hope the instruction manual would explain that.I suggest you read some of the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/You will learn that "high impedance" probes are not high impedance at high frequency, and that probing technique greatly affects the waveform you see on the display. Hint: don't use a 15cm ground lead

author: David Hess
author_rank: Super Contributor
timestamp: January 04, 2021, 11:39:29 am
content: Probe compensation matches the ratios of the 9 megohm tip resistance with its parallel capacitance to the 1 megohm resistance and its parallel capacitance, about 20 picofarads, at the oscilloscope input. The frequency where this occurs is about 10 kHz so the bandwidth can be relatively low. Probe compensation is often easier with the 20 MHz bandwidth limit engaged.Some oscilloscopes like the Tektronix 485 and many Rohde & Schwarz models do have fast transition time calibrator outputs but it does not make any difference for low frequency probe compensation.

author: atum
author_rank: Newbie
timestamp: January 04, 2021, 03:18:38 pm
content: Thanks a lot for your responses guys !It is understood, calibration has no effect on the probe high frequency response.Atum

author: StillTrying
author_rank: Super Contributor
timestamp: January 04, 2021, 04:03:39 pm
content: Quote from: atum on January 04, 2021, 03:18:38 pmIt's the other way around, the probe compensation adjusts the HF response to match the fixed near DC response.

author: David Hess
author_rank: Super Contributor
timestamp: January 04, 2021, 04:53:12 pm
content: Quote from: StillTrying on January 04, 2021, 04:03:39 pmYes, but it is only high frequency response in comparison to DC. The breakpoint frequency is about 10 kHz as I outlined above.Dual path input amplifiers have a medium frequency breakpoint at about 50 kHz which is adjusted as part of the oscilloscope calibration. Higher frequency high impedance probes have transient response calibration which is high frequency compensation above 100 MHz and if exposed, will have 2 or 3 adjustments at the probe termination box in addition to the low frequency compensation. When Keysight buys 250 MHz and faster probes from Texas, they make these adjustments which are specific to the oscilloscope model and then cover them. Making these transient response adjustments yourself requires a reference level pulse generator which is several times faster than the oscilloscope bandwidth.