Tried a new oscilloscope on a 2017 G650GS and got a voltage spike reading that made no sense

I was tracing a intermittent comms dropout on a customer's plane last Tuesday and decided to use my new Siglent SDS1104X-U instead of my old Fluke. The thing showed a 28V spike on a 12V line which had me chasing my tail for two hours. Turns out the probe ground clip was loose and picking up noise from the starter generator. Learned to double check my test equipment setup before blaming the airframe. Has anyone else had a new tool give you false readings like that?

2 comments

2 Comments

jaken231mo ago

A 28V spike on a 12V line from a loose ground clip on a Siglent is a rookie mistake we've all made at least once. But I'd gently push back on the part about the starter generator being the culprit for that reading. Most alternators and generators put out AC ripple at best, not a clean 28V DC spike like you saw. More likely, your loose ground clip was acting like an antenna, picking up magnetic field noise from the high current draw of the starter motor. That magnetic field can induce a voltage in the probe lead itself, not a real voltage on the wire. Next time, twist the probe ground lead tight around the tip or use a ground spring to keep it clean.

the_eva1mo ago

Wait, so are you saying the 28V spike was just a phantom reading from the magnetic field, not actual noise on the line? I'm trying to wrap my head around that because when I saw it, the trace was real clean and steady at 12V for a few seconds, then bam, a sharp 28V pulse that looked way too stable and repeatable to just be random interference. Did yours show a similar pattern or was it more jittery and all over the place?