Electronics > Beginners

1X probe acts weird (needs compensation?)

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beowulfenator:
Hi!

I got my hands on a 1X probe (the manual says "Elditest GE1502") and hooked it up to Tektronix TAS 485. This is what I get when I probe the "probe comp" input.

My 10X probes (HP 10071A) work fine, and I can adjust them to make the waveform square. But this probe (and a piece of any wire for that matter) produces this skewed waveform. Am I doing something wrong here?

ArthurDent:
Check what the input specs are on the two scopes. I noticed that different scopes have different capacitance loading on the probes that would cause the problem. You probably can't adjust a 1X probe to work without changing the coax length or perhaps a low value resistor that may be in series with different 1X probes.

The Tek may be 1Mohm/20Pf

beowulfenator:
The scope is indeed 1MOhm/20pF. According to the datasheet, the probe's input impedance is 65 pF. But then there are two issues: first, a short piece of wire gives the same picture and second, the P6101B probe that is mentioned in the scope's manual as compatible has input impedance of 1MOhm/100pF.

ArthurDent:
Dave has a good explanation of what your 1X scope probe problem is. Watch especially from minute 18 onward which will describe the difference you are seeing between the 10X probe designed to work with your scope and some other 1X probe not designed to match your scope. 

beowulfenator:
Oh, I've seen that video before. I've just watched it again, and he describes a switchable 1X-10X probe that's optimized for 10X and works poorly at 1X. I've measured DC resistance of my probe, it's roughly 180 Ohm. There should be no attenuation at 1 MHz.

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