A question for owners of a Rohde & Schwarz RTB2000 with 70MHz bandwidth and a Siglent SDS2000x Plus with 100MHz bandwidth.
Have you ever measured the cut-off frequency at -3db ?
Is this artificial limit exactly at 70MHz/100MHz ?
Regards,
Frank
SDS2104X Plus -3dB is ~185 MHz. Comes with 200 MHz probes.
Triple checked with 3 signal sources.
Edit
Actual screenshot:
https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg2958288/#msg2958288
This is bit bad situation for start this from me but still I want rise one common question about how we measure oscilloscope frequency response and specially -3dB point what is normal.
This is also NOT any kind of complaint about your measurement, AFAIK we do it usually just using same principle what perhaps almost all do.
Usually we measure it using RF signal generator what have enough good flatness in its output connector. Then cable to scope and there scope internal 50ohm or external feed thru terminator what both are bit bad. It is simple method and if done carefully and well it may give somehow reliable result. About how to do it there can write whole book about things around this. It is not accurate method due to generator flatness errors and signal pathway impedance matching errors. But widely used and ok for normal purposes. But also this is not question.
How ever we do this test setup and so on, we use same principle. We go more and more high freq and look oscilloscope response and where level drop 3dB related to our some reference point. If we continue rising freq we are sooner or later -6dB and -10dB and so on... but principle is we are looking attenuated signal and watch how much it is attenuated until attenuation is value what we are interested, example -3dB or -6dB
Select some zero dB reference level say example some level what give 6div height signal and example with 10k to 1MHz. (usually reference is more low than 1MHz)
Okay, we have now this level out from generator and after generator fine adjust we see exactly 6div peak-peak sine.
What we do next... if we are looking -3dB point. After then we start rise freq and go up until first time we see signal level on screen drops perfectly -3dB under reference level (6div) or more low depending what we are looking.
But then, lets go bit more high freg (if there is enough room before Nyquist*0.8 or better if below Nyq*0.7). Lets think we find -12dB point. Signal is now attenuated 12dB...
Now, keep this frequency and rise generator output level now 12dB. Simple thing and of course we see now original reference level... 6div height signal.
Or... after then we scratch our head and think what hell there happen... and why.
It not enough clear effect you can test in -20dB point or even more high frequency... more high more weird.
Also it can of course do with all, -3, -6, -9, -12, and so on...
Some may think is it same. With 3dB mostly result is nearly same but not always...in some scopes difference may start even with -3dB point if reference level is least 6div. (this depends also what is scope display vertical and front end FS)
If you want find -10dB point you may find big difference... if you go to -30dB point. You may get "even "no result" situation... it is least good to try even once and then scratch head and know bit more about fact that all can see in scope screen is in some point unknown lies mixed with unknown truths.
Now, this other principle for test is:
Set signal level perfectly 6div height with some low freq just for reference point.
After then rise freq until -3dB drop. Rise generator level 3dB. Is level now 6div height. If not, drop frequency until signal is back to this 6div height.
Usually in 3dB point do not differ so much but if go higher frequencies difference is big or 6div height can not even reach how much ever you rise signal.
I do not remember if this method was in some old HP or Tek some oscilloscope service manual... or where... select reference level and freq, then rise test signal 3dB and rise freq until displayed signal drop back to this defined reference level.