Strange scope issue(I think it's the probe but not sure)

Sep 08, 2008 6 Replies

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Jon Slaughter 17 years ago

I have some digital logic going on(basically 3 ic's that convert a "truth table" to another one). It consists of an xor, and, and invertor wired up relatively simply.

In any case on the AND ic, an 74HCT08, I get what looks to be like a differentiated signal that sometimes works and sometimes doesn't(in that it gives the correct result and sometimes the differentiated result).

It almost certainly is the probe but I'm not sure why. If I move the probe slightly it sometimes works or doesn't. The other probe works fine. I haven't switched the channels to test because 99% likely the probe(if it was the scope then it shouldn't react to position).

What could be wrong with the probe to cause it to act like that?

Thanks, Jon

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PeterD 17 years ago

What type of probe are you using? What frequency is the circuit running at?

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Jon Slaughter 17 years ago

Shoudln't matter. As I said, it worked fine with the other probe which is the same make. It's your standard probe.

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John Devereux 17 years ago

Sounds like the core of the coax is broken. (I think they use very thin cores, like hair thickness, to get low capacitance).

John Devereux

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tmoranwms 17 years ago

was

Check for oxidized contacts (my original Tek probe has a threaded segment you adjust for compensation), cable looseness (sometimes, sometimes not, I get this behavior, I suspect around the BNC connector), twitchiness in the scope's attenuator (at the moment, my Tek 475 is growing tin whiskers in channel 1 -- I can't imagine why they pure-tin-plated the things, they know better) and that sort. It could also be later in the scope, vertical amps for instance, especially if the trigger is stable but the voltage waveform isn't.

Tim

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JeffM 17 years ago

You've answered your own question. When you flex the cable of the probe and the damaged conductor in the probe breaks contact, it becomes a (very low value) series capacitor.

Series capacitor; shunt resistor: the definition of a differentiator.

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PeterD 17 years ago

Since you know the answer, why did you post the question?

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