Rigol DS1000 triggering problem

[alexthe5th]

I've run into an issue with my fairly new Rigol DS1000 scope. If I connect it up to my function generator outputting a 1kHz sine wave @ 500mV amplitude, and triggering set to 0V rising edge - I get two waves superimposed on one another (one 180 degrees phase shifted from the other) whenever the vertical scale is set to 200mV/div or less.

Here's a video of the problem:

Anyone encountered anything like this before? Just wondering if it's a dumb error on my end or if this is a problem with the scope.

[TurboTom]

There may be noise / RF superimposed to the signal. Try enabling the bandwidth limiting function for the vertical channel you used.

[alexthe5th]

There may be noise / RF superimposed to the signal. Try enabling the bandwidth limiting function for the vertical channel you used.

It's a little bit improved with the bandwidth limiting function on (the out-of-phase wave is now flickering as opposed to being almost continuously on), but the problem's still there.

[alexthe5th]

Aha! I found a noise rejection setting buried one level below the trigger menu - that solved the problem. Thanks for your help TurboTom - that led me in the right direction. 

[TurboTom]

Great that you found a solution!    In case of these or similar "trigger quirks", it's also always a good idea to crank up the time base until you actually see on what the scope triggers. Especially if you are connecting the probe to another, "far away" grounded source, you may end up with all kinds of noise picked up via the connection and the ground loop.

For comparison, you may try to sync your scope on the internal generator outputting the same sine wave but linked back-to-front just with a BNC cable. This should work without any difficulty and display a nice and stable sine wave, even without any bandwith limitation or trigger noise reject function.

In general, especially if you're new to fairly modern digital scopes, it's very useful to spend some time experimenting with the vast amount of trigger functions. There's virtually always something available that allows you to get a stable display of even the weirdest waveforms  .

[alexthe5th]

Especially if you are connecting the probe to another, "far away" grounded source, you may end up with all kinds of noise picked up via the connection and the ground loop.

I think you're right - that's almost certainly the problem. The function generator source I've been using is a USB-powered Analog Discovery 2, which has a long ground loop through the PC, around the wall, and back to the scope. There must be a ton of noise picked up along the way. I tested it with the internal generator as you mentioned, and it seems perfectly fine with a much cleaner sine wave. None of the noise rejection / bandwidth filtering options are needed for it to trigger properly.

Also, same goes for using the AD2's internal scope + the AD2's internal generator - very clean signal.

I'm not sure what exactly can be done to reduce the ground noise. It's unfortunate since it'd be nice to be able to use the two devices with each other.

[Fungus]

I'm not sure what exactly can be done to reduce the ground noise. It's unfortunate since it'd be nice to be able to use the two devices with each other.

You're using the ground clip on the probe, right?

[alexthe5th]

Yup.

[Fungus]

Yup.

So ... how is the ground path " through the PC, around the wall, and back to the scope"?