Hi, just wondering what may be the cause of audible buzzing noise when using an oscilloscope probe?

 

Someone calling you on a cell phone that's set to stun?

Well, it "may" be the cause...

Joking aside, where is the buzzing coming from? The probe? The circuit being probed? The scope? Does it start and stop when you connect and disconnect the probe tip to the circuit? Does it only happen with a particular circuit or when probing a particular point in a circuit? What if you probe the circuit with the probe disconnected from the scope? Or with the scope turned off?

 

Yes, I only heard this when probing the circuit. The probe ground lead was already connected to the source terminal of a MOSFET, but when the test lead connected to the circuit it began buzzing. It was only one part of the circuit. I figured something was burning... Could this have been caused by not properly grounding the probe?

 

Was the source terminal of the MOSFET tied directly to the circuit common? How was the circuit grounded to the test equipment? Remember that, unless the probe has differential inputs, which are uncommon and expensive, then the scope probe grounds are almost certainly connected to the scope's earth ground prong. So keep that in mind and how it plays with your overall test setup. Otherwise, at best you risk introducing unnecessary noise into the system and at worse you risk destroying something (up to and possibly including you).

 

Was the source terminal of the MOSFET tied directly to the circuit common? How was the circuit grounded to the test equipment?

No, the source terminal for this MOSFET was not at ground potential . I think there might have been some typo in the lab instructions. For now this is a good reminder to check before making any changes to the live circuit.

 

Someone calling you on a cell phone that's set to stun?

http://www.tbotech.com/cellphonestungun.htm