Electronics > Projects, Designs, and Technical Stuff

Risetime of oscilloscopes, an sign of performance ?

(1/3) > >>

Kiriakos-GR:
Just from a quick look at the specs of an portable 5MHz digital scopemeter, it indicates an Risetime of 17.5ns.
I found the same specification in a 10 years old or more, 20MHz analogue oscilloscope.

Is this considered as sign of performance ?

And if it is so, are any rule of the thump about modern scopes, that should have an specific Risetime VS the MHz that they are capable to meet,
so to have an some sort of guide of what is considered acceptable for the range of 5MHZ up to the 50MHz ? 

   

metalphreak:
Risetime is an indicator of the bandwidth of the frontend usually. Like on the Rigol 1052E hack. Normally its the frontend bandwidth limited to 50MHz, but with the mod only changing the front-end its capable of 100MHz.

I wouldn't imagine it would be hard to make a decent 20MHz front-end input, but perhaps the rest of the hardware is only good for 5MHz?

ejeffrey:
For a scope with a 1st order low-pass filter (most low to mid range scopes), risetime and bandwidth are interchangable: f_3 = 0.35 / t_r.  0.35 / (17.5 ns) = 20 MHz.  There is no simple formula for a higher order filter, but usually it is slower, or the risetime is hard to define due to ringing.

If you see a rise time of 17.5 nanoseconds on a scope with a rated bandwidth of 5 MHz, one of three things (that I can think of) is happening.  Either one of the parameters is in error (copied from the data sheet of a different model?), the manufacturer is using a non-standard definition of bandwidth or rise-time, or your scope has a severe under-sampling problem, and they are listing the bandwidth as the nyquist frequency which is below the 3 dB point of the AFE.

If you post the model of the scope, we can provide more useful information.

jimmc:

For conventional 'scopes with a Gaussian frequency response there is a very well known relationship between rise-time and bandwidth.
Tr = 0.35/BW
eg 0.35/20MHz = 15.5nS

Digital 'scopes require a sharper filter to avoid aliasing at the expense of some ringing.
For these Tr = 0.5/BW is more appropriate.
eg 0.5/5MHz = 100nS

To me 5MHz digital scope with a 17.5nS rise-time suggests a poor design with little anti-alias filtering.

For more details see
 http://www.tek.com/Measurement/App_Notes/55_19248/eng/55W_19248_2.pdf

Kiriakos-GR:
Thanks people, your help to my question are invaluable.  :)

Here is the specifications in full.



 

Navigation

[0] Message Index

[#] Next page