Can a scope test it's own bandwidth?

[MrW0lf]

In fact OPs Pico 2405A can also do "it", as any other Pico and of course Analog Discovery 2 which has basically every feature one can imagine because it has in GUI scripting

With Picos it's not strictly "official" feature also because FFT is mostly broken on regular derivative(x) function. But it usually works when do own custom derivative and sin(x)/x is on. Sadly it cannot do it on ETS traces because they have decided to disable FFT on ETS. Also I cannot fully understand where I need to place trigger, mostly it works ok on 20% or 25% position, but not always. So in general this is all a bit too experimental to call a feature but interesting extra stuff to play with still:

PicoScope 6404D with Leo's pulser:


Some TDR with custom derivative instructions on 2408B (too slow edge for BW judgement):


PicoScope 6 hack to rise Sin(x)/x threshold for better resolution (by default can get only 512bin FFT):


And here math functions script I have written for AD2, from which getting TDR/TDT is trivial because now there is derivative trace and it can do FFT on any trace:


Maybe someday if get time do proper examples with all scopes and single fast source.

[macboy]

I'll add another way to measure not only the bandwidth, but the actual frequency response curve.

That works also with a suitable calibrated source but very few oscilloscopes include the needed differentiation and FFT functions.  I do not know of any DSOs with bandwidths below 500MHz which do.

If the FFT returns phase results, then aligning the step response with the FFT will also allow the phase response of the oscilloscope to be measured revealing even more about the frequency response.  This can also be used for real time VNA measurements albeit with limited dynamic range.

The scope can give FFT phase in addition to magnitude. You may be amazed at the waveform processing in these Lecroys, especially with the advanced options. It's mind boggling. Can you elaborate on "aligning the step response with the FFT"?

I had very recently been very interested in setting up an ad-hoc 2 GHz VNA using the scope and a 2 GHz signal generator (HP 8657B); it hadn't occurred to me to use the pulse response instead. I have some power dividers, directional couplers, terminators, and the pile of cables needed to connect everything. The processing of the result, in particular the calibration, has stopped me as I just haven't any idea how to implement that. Any pointers would be much appreciated.

[David Hess]

The old Tektronix application notes about their early DSOs which supported FFT magnitude and phase results show how to do it.

My recollection is that the impulse (or edge) needs to be aligned exactly in the center of the FFT window which may be the 50% trigger position but some DSOs make this difficult.