Recent content by pelesl

Looks like a good reference. Thanks. I think the next thing for me to try is to carefully solder a twisted pair shielded cable to a BNC, short the two wires when it's connected to the scope, and see what I get. I should probably look for another scope just to be safe...

Figure 6.5 shows a simple ground loop: device under test plugged into a different outlet than the oscilloscope. This is definitely the case when I'm powering the test resistor from either power supply. However, it's obviously not the case when I'm just shorting the probe or powering the resistor...

Then I put a diode in front of the resistor, so current could only flow into it, but not "back into the supply". I would have expected to see an asymmetrically clipped ringing, but I don't: So I think everything points to this problem being transmitted by air and not by the supply: 1...

Here are the traces with the probe and cable connected across a resistor (both channels on the same resistor at the same time), the resistor unpowered. Here it is with the resistor powered by the linear supply.

Here are some more tests that I did last week. First, I compared an oscilloscope probe to the BNC cable I had. The probe certainly sees a very different thing---primarily, much lower voltage spikes. Here are the traces with the probes shorted. The TEK probe is on channel 2.

I think I can try this on the fluorescent light I was testing with. I'll let you know what I see. I always go to http://www.abebooks.com/ for books. If you don't care about the edition you can get it for as low as $2. I bought one for $17 (shipping included) from London (usually arrives...

If by that you mean that it has an isolation transformer between its AC input and the line, the answer is no. I tried plugging it into an isolation transformer and the problem persists. By secondary do you mean that the DC output uses Earth ground as the negative, or do you mean secondary...

An update: I've tried with an actual oscilloscope probe and the waveforms are a bit different but the problem is still there (not to mention I can see the effect in an encoder's output whether there are oscilloscopes involved or not). I've also tried using zeners and diodes in my resistor...

Test 8: ran the Oscilloscope from a UPS batter (so there was no physical connection to room wiring). Without the BNC cable attached I got nothing. With the cable attached and shorted at the clip leads I got this:

So from my point of view, the tests show the following: 1. The DC supply is affected by the phenomenon. There is a consistent difference between measurements across the resistor when powered by the DC supply and everything else. 2. The high frequency ringing seems to be traveling through the...

Then just to be sure the solderless breadboard wasn't the issue, I pulled the resistor off the breadboard. Still battery powered. Test 7: Battery powered resistor circuit, scope ungrounded, across the resistor, disconnected from the breadboard.

Then I disconnected the resistor from the DC supply and connected it to a single D-cell battery. Test 5/6: Battery powered resistor circuit, scope ungrounded, first across resistor then probe shorted.

Then I plugged the oscilloscope into the outlet through an adapter from which I cut the ground leg off. So the oscilloscope is no longer connected to Earth ground. Test 3/4: Scope ungrounded, first across the resistor and then probe shorted.

Thanks for the warm welcome. And thanks to smartphones... I was able to perform the tests you suggested and capture waveforms. Some background: this is in a 700 horsepower wind tunnel at a university. This giant 3-phase motor (run off a giant VFD) was running at the time I did the tests...

I have a situation where I see spikes (with ringing) on the output of DC power supplies whenever something AC powered is switched in the room. The spikes change in magnitude; they can be as bad as 3 volts peak-to-peak; I suspect they change "randomly" because they are related to when the device...