While reading Tektronix's "ABCs of Probes" manual, I was told that "there's an inductance associated with the ground lead [of an oscilloscope probe]. This ground-lead inductance increases with increasing lead length."

However, according to the inductor sizing equation, length is inversely proportional to inductance, so longer ground leads should DECREASE inductance, right?

These two statements contradict so I must be missing something. I appreciate any help.

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ABCs of Probes Primer by Tektronix (I'm referring to page 27, right column)
http://www.tek.com/Measurement/App_Notes/A.../60W_6053_8.pdf

Inductor Sizing Equation on AllAboutCircuits.com
http://www.allaboutcircuits.com/vol_5/chpt_1/6.html

 

Hi

It may appear that the formula is telling you that, but the lead inductance is caused by the magnetic field that surrounds it. It will increase with length, and the effect may easily be demonstrated by using two different-length ground clips (at some suitably high frequency). We used a single loop in the ground lead to pick up RF. The free end of the lead was electrically clipped to the probe tip. Got good results above 150 MHz.

Most 10X scope probes have compensating caps built in. A very careful adjustment will aid in presenting high frequency waveforms as well.

 

Attached is a URL that includes inductance formulas for various physical components, straight wires, coax, etc. I think you will see that inductance increases with lead length...it has to, that's the physics, as pointed out in the first append. Length only enters the denominator when it affects flux density i,e.(spacing of the turns, such as an "air core coil"). In this case lenght is also implicit in the numerator (# of turns + turn diameter)....

 

Originally posted by curious@Aug 8 2004, 05:23 PM
Attached is a URL that includes inductance formulas for various physical components, straight wires, coax, etc.

I can't find the link. I appreciate your help.

 

http://www.rfcafe.com/references/electrica.../inductance.htm


Oops, missed the URL... sorry