Oscilloscope noise? (Siglent SDS 1202X-E)

[LeoTech]

Hey,

So, here is the thing; after doing a lot of low noise measurements on some circuits recently I noticed that most of the noise still stayed, even after switching the circuit of. I therefore decided to have a look at the internal noise of the scope, by short circuiting the probes - I had no idea what to suspect.

To be honest, the result was pretty disappointing, so I am starting to suspect a potential fault in the scope. Below are some screenshots from the measurements:

(The scope in question is a Siglent SDS 1202X-E)

 


For some strange reason the noise seems to be way better today than yesterday, where it was like 1 mVRMS and 20 mVPk-Pk.

My question:
Is this normal for a scope, or is this specific unit faulty?

I am pretty inexperienced with scopes, so please be gentle.

Thanks in advance,

Leo

[Performa01]

What you see is not noise from the scope, but from your environment, caught by the nice loop antenna you've built by shorting your probe, supposedly using the unshielded ground lead.

It's all sorts of switching noise from your household appliances , but also radio stations from VLF up to VHF. That's also why the noise depends on the time of the day.

If you want to see the noise of the scope itself, just disconnect the probe. You will see that the Siglent SDS1000X-E DSOs are actually pretty low noise.

[LeoTech]

What you see is not noise from the scope, but from your environment, caught by the nice loop antenna you've built by shorting your probe, supposedly using the unshielded ground lead.

It's all sorts of switching noise from your household appliances , but also radio stations from VLF up to VHF. That's also why the noise depends on the time of the day.

If you want to see the noise of the scope itself, just disconnect the probe. You will see that the Siglent SDS1000X-E DSOs are actually pretty low noise.

Ahhhhh, thank you! This explains a lot!

I feel very relieved right now, and more confident in my previous measurement. And I did test the scope with the probes disconnect, and the noise appeared to be way, way lower - thank you for clearing this up!

Leo

[Performa01]

I feel very relieved right now, and more confident in my previous measurement.

Well, of course the unshielded probe tip (probably extended by the hook) together with the ground lead will always act as antenna for radio signals, also during real measurements. If you really have to accurately measure signal levels that low, you should try to use your probe without hook and with the ground spring instead of the lead.

A direct coax connection (BNC cable assembly) would be even better, with a 50 ohm through termination at the scope input - provided you have a proper test point in your circuit that can drive a 50 ohm load without feedback effect. It might be necessary to particularly design such a test-point connected to a BNC socket. You might need a dedicated buffer amplifier for that.

[tautech]

Ahhhhh, thank you! This explains a lot!

I feel very relieved right now, and more confident in my previous measurement. And I did test the scope with the probes disconnect, and the noise appeared to be way, way lower - thank you for clearing this up!

Leo

Now you have some basic understanding there's more to understand;
Your 200 MHz DSO will display more noise than a lower BW DSO.
500uV inputs are quite sensitive.

Watch Dave's vids for more understanding of how to work with DSO 'apparent' noise:
https://www.eevblog.com/2014/04/10/eevblog-601-why-digital-oscilloscopes-appear-noisy/
https://www.eevblog.com/2014/04/27/eevblog-610-why-digital-scopes-appear-noisy-part-2/