Author Topic: Help with oscilloscope probes please  (Read 2846 times)

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Offline orbiter

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Help with oscilloscope probes please
« on: June 30, 2010, 07:26:49 pm »
Hi all,

I've just been a plonker and managed to stand on one of my oscilloscope probes :(

Problem is I've never had to buy these before so i don't know what the score is here.. The probes that came with my scope are rated for 60MHz and are x10/x1, 600V. Now do I have to stick with 60MHz probes or can I go higher on that number? The reason I'm asking is that I was considering these 150MHz probes and wondered if they would be ok or are there other things I have to look out for?

http://uk.farnell.com/_/probe150/probe-oscilloscope-150mhz/dp/4284057

Thanks for your advice guys


alm

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Re: Help with oscilloscope probes please
« Reply #1 on: June 30, 2010, 07:41:18 pm »
Nothing wrong with buying probes with more bandwidth than your scope, the only issue is that it's often desired for less input capacitance and higher price. You don't need a switchable 1x/10x probe either, the 1x mode is rarely used in my experience. You can also buy probes with less bandwidth than your scope, but you will obviously be limited to their bandwidth, and some aberrations beyond their rated bandwidth might show up and be confusing. Some useful info: Choosing a probe.
 

Offline orbiter

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Re: Help with oscilloscope probes please
« Reply #2 on: June 30, 2010, 07:51:43 pm »
Hi alm,

Thanks for the assistance. One more question though please.. At present the scope I have is only a 25MHz model which came with 60MHz probes. Am I still ok to go with the 150MHz ones I linked to even though I can't use that bandwidth. I mean using the 150MHz ones won't knock out the lower end frequency detection or anything will it? The reason I ask is that in the page you kindly linked to It mentions only using probes which are 'a little faster'.

Quote
Bandwidth
Scope probes have a maximum bandwidth, just like scope input circuits. Use a slow probe on a fast circuit and you won't see the high frequency portions of the signals. To avoid this problem, you should choose a probe that is about the same bandwidth (or perhaps a little faster) than your scope. If you are measuring slow signals, you can use a low bandwidth probe on a fast scope without introducing problems.

alm

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Re: Help with oscilloscope probes please
« Reply #3 on: June 30, 2010, 08:15:19 pm »
I don't see why not, the scope will basically ignore (severely attenuate) anything beyond 25MHz. The only issue is that faster probes often have a lower input capacitance, which limits their compensation range. As long as the compensation range includes the input capacitance of your scope (as described in that article), you should be fine.
 

Offline orbiter

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Re: Help with oscilloscope probes please
« Reply #4 on: June 30, 2010, 08:37:40 pm »
OK thanks again alm,

I think I'm ok here? Especially on the x10 range.

I can only find the 'Input impedance' (NOT capacitance) of my scope which shows.. 1M?±2%, in parallel with 20pF ±3pF

The new probes shows a compensation range on the x10 scale.. 10 - 35pF

However on on the x1 scale the probes show 46pF (plus scope capacitance)

orbiter

alm

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Re: Help with oscilloscope probes please
« Reply #5 on: June 30, 2010, 09:13:26 pm »
20pF ±3pF is the capacitance, the probe has a compensation range of 10-35pF, so that should be fine. There is no divider in 1x mode, so no need to compensate anything. The probe will work with almost any scope in 1x mode (assuming the input is something like 1Mohm).
 

Offline orbiter

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Re: Help with oscilloscope probes please
« Reply #6 on: June 30, 2010, 09:16:05 pm »
Ok brilliant, thanks again for your help alm.

Much appreciated

orb


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