Risetime of oscilloscopes, an sign of performance ?

[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.



 

[BravoV]

post removed

[Mechatrommer]

he tried to be helpful. and we "the educated" (not me) should understand whats his country's economy is facing right now.

[Kiriakos-GR]

his country's economy is facing right now.

What this haves to do with Risetime of oscilloscopes?



And by the way if some one does not care to extend his learning, he has my permission to play football instead.

[Mechatrommer]

economy got nothing to do with risetime, just the price.
i dont want to learn football. my legs are too small for that

[Kiriakos-GR]

You are one from the few, who have my permanent permission to tease me freely. 

Thanks for the fast tips people, It is a nice starting point.

[Mechatrommer]

You are one from the few, who have my permanent permission to tease me freely. 

hey i'm not racist! i went to another forum just to find threads with more replies from the same flag. well sorry OT, it got nothing to do with osc risetime
ps: dont mention the "permanent permission" thing. someone can start another flamewar with that term

[mission cat]

The Tektronix field engineers recommend that you use five times the bandwidth of the circuit under test.

When an oscilloscope has five times the bandwidth of the signal under test, the accuracy of the amplitude or rise-time of the displayed signal is -0.1%.

0.1% is the std accuracy for laboratory work. The NASA specs on the moon missions was +/-0.1%.

Basically when you have an instrument accuracy of 0.1%, you don't need to worry about the true signal amplitude or rise-time.

If you have SPICE, you may verify the amplitude of any signal viewed without doing math. Model the scope as a single pole R-C filter and the input as a step or sine-wave. Next adjust the input signal amplitude or rise time until the Spice output is the same as the oscilloscope display. Then the Spice input signal is the true signal.

For RF circuits, I insert a 50-ohm between the probe and the circuit under test. Above about 10MHz, the probe ground wire-length is critical. There are probe attachments which allow the probe coaxial path to be extended all the way to the probe tip.

Greenwood Industries sells little connectors which the probe tip may be plugged into. From this connector, create short wires to the BB of less than 1" in length. Now the scope display should ne OK up to about 50MHz, after correcting for the scope risetime.

It would be nice if oscilloscopes corrected for their rise time by enhancing the display to show true signal rise time and amplitude, automatically.

[hacklordsniper]

And what oscilloscope is discussed in this thread?

[Balaur]

post removed
« Last Edit: November 09, 2011, 03:16:59 AM by Simon »

Quick question: It is Bravo who removed his comments, or it is Simon (the moderator) who did that?