Siglent SDS2104X Plus - Scope-internal frequency visible in FFT?

[RandomUser]

Hello,

I want to do some pre-compliance EMC measurements with my SDS2104X Plus. Unfortunately, it shows 2 peaks when directly terminated into a RF-50R SMA.

I'm using channel 3 as input for the FFT.
Is this my scope?
Does anyone else has such a problem with his siglent?
Do I see an internal SMPS regulator showing in my spectrum?

See attached picture.

Any feedback is appreciated.
Thanks!

[RandomUser]

Okay, did some research...

A big thanks to Dave for the HiRes photos.
I post a snippet of that below.



The power supply IC is a TI TPS65400 4ch buck.
The datasheet specifies a minimum SW-Freq. of 275kHz set by an external resistor.
When doing the FFT on ch. 1-4 I get to see 281kHz with RBW~1kHz. So basically min. frequency. Might be slightly off e.g. due to resistor tolerances.

Due to the distance of the supply to the analog input / ADC circuitry I suspect the ripple on the supply voltage to be the verdict of my FFT result.
On the other hand I get the best result on Ch3 which has the same supply as Ch4, I guess? Therefore, I would expect to see same results on those channels but when doing the same settings, this is what I get.


Any ideas?

[temperance]

Volts = 10(dBV/20

-110 dBV= 3.16 µV

Good luck.

[2N3055]

That noise floor is actually crazy good....

[lucy.sierra]

Ken Wyatt could help you out.