The SDS2000X Plus series has 1MΩ/50Ω selectable inputs and FFT functionality. I have an D-130 antenna connected to the 50Ω input of my old HP 8591E Spectrum Analyzer. Can I use an SDS2000X Plus scope in similar manner to a dedicated spectrum analyzer by switching the input to 50Ω and then connecting my antenna and running the FFT to show me an on-screen display similar to my HP8591E? (For example, on my HP, I can press the button of a say a 303.875MHz RF transmitter and see that on the scope, and I can then zoom in on that frequency and limit the horizontal display range to 10MHz, 1MHz, 100kHz, etc. Wonderfing what's possible on the Siglent when using FFT an a 50Ω antenna.)
Short answer: yes, you can do it within the bandwidth of the DSO (max. 500 MHz).
Slightly longer answer: for achieving the best results, the setup of the DSO is a little more involved as that of an SA. To learn about the basics of FFT in a modern Siglent DSO, you can study the corresponding chapters of my review of the SDS1104X-E. This scope doesn't have internal 50 ohms termination, but an external through terminator does the job for lower bandwidths like this. The article also contains some examples, including a broadcast overview of the FM radio band:
https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/msg1371782/#msg1371782Of course, this is a bit outdated by now and additional tools and features have been added even to the low end SDS1004X-E series since then. For example, the setup is a bit easier now, because we can choose any FFT length independently, as long as it is not longer than the record length. Consequently, all the extensive tables in the article have become meaningless by now.
Because of this, the basic setup for a contemporary Siglent DSO is better described in the following post:
https://www.eevblog.com/forum/testgear/rohde-schwarz-rtb2002-vs-siglent-sds2104x-plus/msg3239832/#msg3239832Compared to the HP8591 the bandwidth is lower of course, but the potential RBW can be narrower. For analyzing the modulation spectra at higher frequencies, the max. FFT length of 2 Mpts might be insufficient, but for an exclusive signal, as from a nearby transmitter, we can always resort to undersampling as demonstrated in the following post:
https://www.eevblog.com/forum/testgear/siglent-sds2000x-hd-12bit-(published-for-chinese-domestic-market-only)/msg4320658/#msg4320658