Electronics > Projects, Designs, and Technical Stuff
Switching Noise
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jmelson:

--- Quote from: engrguy42 on May 28, 2020, 09:50:23 pm ---Hey, wait a minute...

That same trace occurs with a normal (not differential) probe plugged into the scope, and power supply turned on, but the probe itself is connected to NOTHING !!!

--- End quote ---
Welcome to the wonderful world of working with power switching circuits.  Huge amounts or radiated magnetic and electromagnetic fields, pulses on the power mains, it gets into everything.  And, a lot of scopes don't have enough filtering on the mains input to keep it out of their sensitive internal circuits.  I bet a good old Tek scope would at least show less of this noise.

Jon
engrguy42:
Okay, well I didn't want to do it, but I guess I have to. Rolling out the big guns on this one.

Just ordered some ferrite cores to snap over the wires. BAM !! Now we'll see who's boss.  :D
engrguy42:

--- Quote from: jmelson on May 29, 2020, 02:11:34 am ---
--- Quote from: engrguy42 on May 28, 2020, 09:50:23 pm ---Hey, wait a minute...

That same trace occurs with a normal (not differential) probe plugged into the scope, and power supply turned on, but the probe itself is connected to NOTHING !!!

--- End quote ---
Welcome to the wonderful world of working with power switching circuits.  Huge amounts or radiated magnetic and electromagnetic fields, pulses on the power mains, it gets into everything.  And, a lot of scopes don't have enough filtering on the mains input to keep it out of their sensitive internal circuits.  I bet a good old Tek scope would at least show less of this noise.

Jon

--- End quote ---

With the probe lead totally disconnected from the scope (unscrewed from the BNC) the trace is flat. But with the lead connected at the BNC but not connected to any devices, the same noise appears on the trace. Which implies to me that it's common mode noise generated by the power supply and being induced on the lead (even with the ground lead disconnected, BTW). Unless the BNC connector on the scope has some internal mechanism to zero the trace when nothing connected?

So it seems the scope goodness (ie, the "old scopes are awesome" fanboyism  :D ) is kinda irrelevant isn't it, and it's more about cleaning up the emitted noise from the power supply? Which is why I'm going the ferrite core route.

EDIT: BTW, if I move the offending power supply from the bench down onto the floor (maybe 5 feet away), with the scope lead plugged into the scope but the probe connected to nothing, the noise on the trace drops down to almost nothing.   

 
engrguy42:
Another data point...

Covering the power supply in a tin foil hat (aluminum foil) cuts the peak switching noise viewed on the scope (with a lead connected to no devices) from around 1 volt peak to about 0.25 volts peak.

Yes, I actually got a roll of Reynolds Wrap and covered the power supply.  :-+

So yeah, a tin foil hat is sometimes necessary.  :D

Anyone know where I can buy a Faraday cage?  :D
TimNJ:
The loop area of your probe is acting as a nice antenna for the switching noise radiating off of the power supply and the common-mode noise currents flowing on the output cables. The common-mode noise currents cause a changing magnetic field in the vicinity of your power supply, which in turn get converted to a differential mode voltage that shows up on your oscilloscope input.

When power supply manufacturers measure ripple, usually they use a "ripple probe", that is a probe with a very small enclosed loop area loop area. The best way to do this is to use a probe tip BNC socket which allows you to plug the oscilloscope probe directly into a coaxial connector which is then wired to the power supply output. Manufacturers typically also specify a small capacitance at the end of the cable (10uF-100uF + 0.1uF ceramic). Most power supply manufacturers also use a 20MHz bandwidth limit on the oscilloscope. To further reduce EMI pickup, make sure to use a 1x probe.

With this setup, you have a more realistic idea about the power supply's actual ripple. Of course, even with the "real" ripple of the power supply, near-field radiated emissions can still cause system problems. As you saw with your accidental antenna probe, it can be quite easy to pick up noise, and that noise is real. If you have a system with lots of high sensitivity (high impedance) nodes and/or large PCB loop areas, then radiated switching noise can still be a problem for you.
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