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| DIY Differential probe again again - please help :-) |
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| FriedMule:
You are all a great bunch of guys!! :-) Okay I did not know that my project was so outrageous, if it is a futile goal for me right now, then let be refrace my question then. What would be a doable probe attachment that can handle up to 65V with isolated ground and ???MHz? Functioning but not necessary looking like this: Could I use either: http://blog.weinigel.se/2016/02/26/ghz-differential-probe.html Or: https://xellers.wordpress.com/electronics/1ghz-active-differential-probe/ As template, they both have circuit design and component list. |
| MagicSmoker:
--- Quote from: FriedMule on October 06, 2019, 03:29:50 pm ---... Could I use either... --- End quote --- You want us to spend considerable time assessing projects for you but still haven't answered the most basic question which has been asked several times already: What Are You Trying To Measure? |
| FriedMule:
--- Quote from: MagicSmoker on October 06, 2019, 03:41:39 pm --- --- Quote from: FriedMule on October 06, 2019, 03:29:50 pm ---... Could I use either... --- End quote --- You want us to spend considerable time assessing projects for you but still haven't answered the most basic question which has been asked several times already: What Are You Trying To Measure? --- End quote --- Sorry if I have not explained that in a good enough manner, I did try at my comment number 6: https://www.eevblog.com/forum/beginners/diy-differential-probe-again-again-please-help-)/msg2725412/#msg2725412 But please ask any question I may not have answered. |
| Marco:
Coax is inherently asymmetric, that makes a differential measurement pretty much impossible at high frequencies with a single oscilloscope probe. A 500 MHz amplifier which can directly take +/- 65 V on the inputs without dividers is implausible too, so you need dividers. So you need either a small probe with 2 pins directly going to dividers and then a differential amplifier. Or two divider probes going to a differential amplifier. For the divider probes you can have oscilloscope probes, or coax divider probes. The only existing design which somewhat meets your requirements is Smith's balanced coax probe using a power combiner. But it can't work down to DC, in fact you probably need to add blocking capacitors so the common mode doesn't blow the combiner. You could hybridize this, by adding two extra wires and using a normal low frequency differential probe (ie. high impedance dividers with an instrumentation amplifier) for the low frequencies and Smith's probe for the high frequencies. |
| FriedMule:
--- Quote from: Marco on October 06, 2019, 04:57:34 pm ---Coax is inherently asymmetric, that makes a differential measurement pretty much impossible at high frequencies with a single oscilloscope probe. A 500 MHz amplifier which can directly take +/- 65 V on the inputs without dividers is implausible too, so you need dividers. So you need either a small probe with 2 pins directly going to dividers and then a differential amplifier. Or two divider probes going to a differential amplifier. For the divider probes you can have oscilloscope probes, or coax divider probes. The only existing design which somewhat meets your requirements is Smith's balanced coax probe using a power combiner. But it can't work down to DC, in fact you probably need to add blocking capacitors so the common mode doesn't blow the combiner. You could hybridize this, by adding two extra wires and using a normal low frequency differential probe (i.e. high impedance dividers with an instrumentation amplifier) for the low frequencies and Smith's probe for the high frequencies. --- End quote --- Okay, thanks for the link, I hav not read it all yet but can see that it do not contain any diagram, so it may be great to read but maybe also hard for me to copy from, like the other say I have to, to get a little chance for something useful. :-) hmm... What if I use my standard 10:1 probe infront of the probe circuit? That will give about 6.5V and then make a divider that I have to calculate (I know that it may not be easy) and maybe end up with a 10-100MHz. I shall not pretend that I know what a coaxial divider is, but that is only fun to find out:-) |
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