Bandwidth effect on pulse rise time measurement?

** 50MHz or whatever is the SIGNAL bandwidth of a scope, as defined in the spec sheet. Rise time is also given in the spec sheet, possibly subject to conditions like the height of the display on the screen.

If the input signal has a bandwidth greater than 50MHz or a rise time faster than 10nS, it will not be displayed accurately.

More precisely, if you SEE a sine wave on the screen at greater than 50MHz, its amplitude is in error by at least 3dB.

If you see a voltage step on the screen that takes 10nS, it is a near certainty the actual input step is faster.

.... Phil

I'm betting it's heavy.

One measures scope rise time by applying a very fast step to the input. If the step is way faster than the scope, just read the indicated 10/90 rise and that's the scope risetime.

People used to use tunnel diodes to make a 25 ps step, plenty fast to characterize 30 MHz toob scopes or even low-GHz samplers.

PSPL's first product was a "reference flat pulse generator", used for things like this. Tek recently bought PSPL.

It should be measured, to ensure that the step response is clean.

Old scopes used to do it right.

Yes, if the scope is reasonably gaussian.

Yes.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

According to the t = sqrt(a*a - s*s) formula, because 1 us >> 14 ns, we can neglect the s*s term, and go with t = sqrt(a*a) = a, taking t to be 1 us.

What is the error? If we earnestly plug in the 0.014 us for s:

sqrt(1 - .014^2) =~ 0.999902 us.

The error from ignoring the scope's response is only 1 part in 10000!

That is to say, the rise time of the signal is 71X slower than what the scope can do, and so in this situation, the effect of the scope rise time is insignificant; it tracks the signal almost perfectly.

The error is way below the two significant digit precision we have in the 0.014 us figure, so we would be silly not to ignore it. (And I'm assuming that the 1 us apparent rise time given as an example is really

1.0 us, otherwise we don't quite have two significant figures.) 15 us is an eternity for a 25 MHz scope.

A sine wave that rises from trough to peak in 15 us is oscillating at only 66.6 kHz.

Most scopes oversample pretty hard. A 60 MHz Rigol samples at 1 GHz. So does my (claimed) 200 MHz Tek.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

One can also do deconvolution in Fourier space, where the problem is clearer. The only input you need is the response to a very short pulse, but the procedure calls for divisions (and lots of those divisors are very close to zero).

It isn't a hard problem. That's because, there aren't any hard ways to divide by zero. Or easy ones.

OT: by the way John, where are you getting these bizarre reply subjects from? Reader bug?

Tim

-- Seven Transistor Labs, LLC Electrical Eng>John Lark>

I'm betting it's heavy.

One measures scope rise time by applying a very fast step to the input. If the step is way faster than the scope, just read the indicated 10/90 rise and that's the scope risetime.

People used to use tunnel diodes to make a 25 ps step, plenty fast to characterize 30 MHz toob scopes or even low-GHz samplers.

PSPL's first product was a "reference flat pulse generator", used for things like this. Tek recently bought PSPL.

It should be measured, to ensure that the step response is clean.

Old scopes used to do it right.

Yes, if the scope is reasonably gaussian.

Yes.

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

Beats me. I use Agent and hit "reply." Usenet sometimes does weird things.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

The classic deconvolution procedure is to FFT the device impulse response, FFT the ideal response, and do the complex divide to get the compensation filter FFT, then reverse transform. The classic problem is all the divides by zero or close to zero. There are lots of papers on tweaks to avoid the more radical divides.

It's like a graphic equalizer; you can't gain up a zillion dBs to make up for a deep hole in the system response, or all you'll get is noise.

I invented a non-transform based deconvolution algorithm that is remarkably simple and has no divides.

I don't actually understand it in any deep sense; I was just staring at the problem and thought that it would work. Haven't found a use for it yet; it might make a bad TDR into a good TDR or something.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

You don't need divides if you do it in the time domain. (But you've always been much more pessimistic about deconvolution than I am. Deconvolving smooth instrument functions is hard, but not all of them are smooth.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

No, I'm a great fan of FIR filters and deconvolution and stuff.

My decon algorithm wasn't hard, a couple hundred lines of PowerBasic. It is fun to keep iterating, making the filter better and better, and watching the noise explode.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

In message , Terry Pinnell writes

There's a useful note here:-

One point made in it is the use of scope probes. I think compensating a scope probe can partially reduce the effect of scope rise-time .

Brian

--
Brian Howie

You've just been too fast and the Agent has taken the beginning of your message and replaced the subject with it.

--

-TV

No, it can only reduce the effect of a poorly compensated scope probe. It can't help the risetime of the scope itself (without causing other, even more undesirable, problems).

Probe compensation is in the microseconds time domain, and rise times are down in the nanoseconds.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

It's only a newsgroup. It doesn't matter.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Context does matter, that is, if you want others to understand you.

--
Then why are you wasting your precious time here? 

John Fields.