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Oscilloscope compensating capacitance

ianb1469

Feb 22, 2010
6
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Feb 22, 2010
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Hi,

I recently acquired a HP 54510A 250MHz scope (without probes). The scope inputs have a 7pF shunt, but most "reasonable cost" 10:1 probes can only be compensated down to at least 10pF or 15pF.

Is there anything else I can do to avoid paying more for the probes than the whole scope?!

For example, what the the practicalities of modifying the shunt in the scope, or modifying a standard probe?

Or perhaps for lower frequencies, an incorrectly compensated probe is "good enough"?

Has anyone any thoughts?
Many thanks

Ian
 

55pilot

Feb 23, 2010
434
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Feb 23, 2010
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Let me start by saying that I have no experience with that particular model scope. I am a Tektronics fan and do not think highly of HP scopes and have not used one in the last 15 years (used to work at a place that owned one before that). Based on my general experience, here's what I can offer:

If you want to use the scope for only low speed signals (60MHz or below) you can just use cheap Chinese P6100 knock off probes from eBay for about $15/pair new. They actually do work up to 60MHz. They are cheap and cheesy and will most probably fall apart even sooner than the genuine Tektronix P6060 probes do, but they are cheap and they do work.

If you really want to use the scope to its full potential, you will have to spend several hundred dollars a probe to get the right high speed probes. There is no getting around that.

You may want to have the low cost probes as well as some high performance ones and bring out the high performance ones only when you actually do need them. That is how we treat our 1GHz probes. We use the cheap 350MHz probes, unless we need the higher performance.

Now that you have made me think of how much we will have to pay if I break this 1GHz probe, my message to you is: quit whining about the price of those 250MHz ones :)

---55p
 
Last edited:

55pilot

Feb 23, 2010
434
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Messages
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Let me add another thought. There is noting worse than having iron clad proof of something screwing up, caught red handed on the scope. Taking a scope capture and confronting the culprit responsible for it. Only to be told that what you saw on the scope was not what was actually going on. What you saw was an artifact of the probe or your improper measurement.

As you get into these fast frequencies, you have to pay A LOT of attention to things like having the right probe, properly compensating it and properly grounding the tip. A lot of what this scope is going to show you can be your poor probe handling/grounding and not some crazy waveforms in the board.

The first thing you should do on your fast probes is to remove and throw away the alligator clip grounds. Grounding is done only with a grounding clip to something within a mm or two of where you pick up the signal..

---55p
 

neon

Oct 21, 2006
1,325
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Oct 21, 2006
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most good probes have endeed a capacitor trimmer to compensate for different of input.
 

ianb1469

Feb 22, 2010
6
Joined
Feb 22, 2010
Messages
6
@55pilot. Thank you. I've ordered some moderately expensive probes.

@neon. Thanks for your reply. I was really asking what if the trimmer doesn't go low enough for the scope - is there anything that can be done to (e.g. put a smaller trimmer in the probe) or will that start to mess something else up?

i.
 
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