EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Chalky on September 03, 2013, 08:19:30 am
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Let's say I'm testing a modern DSO with a stated rise time in the order of 5ns or 2ns, and I want to confirm that rise time.
Do I need a special signal generator to get a signal with a rise time that short or shorter? Or, is there some other commonly available household or lab source that has such a fast rise time? ???
Thanks!
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https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/ (https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/)
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A pulse generator with a pulse that is faster as the Tr of your scope and with a period time that is long enough. If your scope is 5ns and the testpulse is 1 ns and stays 1 ns high. Then the pulse is over befor the scope had time to follow the pulse. But if the pulse duratuion is 10 ns the scope will have enough time to reach the top of the amplitude pulse. And only then you can measure the risetime. If the amplitude measured on the scope is lower as the amplitude of the pulse itself you know nothing other then that the pulse was to short.
So you need a pulse with a Tr faster as the Tr of the scope and that pulse must stay longer high as the Tr of the scope.
You can also measure bandwidth without calculations by using a sinewave and increase frequency unitil you reach the 3dB point. If you put the amplitudes over a range in a graph you see a bricklike shape for a DSO (that is, you should see that ) and a gussian shape if it is an analog scope. The best is to measure it using ther 50 Ohm input or use an inline teminator. On the other hand you can measure the response of the 1 M input but the problem is the 1M input is only 1M at DC and audiofrequency. I have measured the output impedance of a 100 MHz Rigol. That was around 30 Ohm at 100MHz. So Tr in 50 Ohm will be different from Tr in 1M
Here some examples of measurements: http://www.pa4tim.nl/?p=434 (http://www.pa4tim.nl/?p=434)
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A decent pulse generator (going up to 250 MHz+ or so) should also have sufficiently fast rise times for scopes up to 150 MHz or so. It depends on your lab whether you consider this a common instrument ;). I paid $150 for one. Of course there are also the dedicated scope calibrators that often feature a fast pulse.
Note the important point made by Fred: it's the rise time, not repetition rate that's important. Even though you might use a 250 MHz pulse generator to get the fast rise times, you would probably set the repetition rate to a low number like 1 MHz. Terminating the signal as close to the scope as possible is essential for good signal integrity.
If you put the amplitudes over a range in a graph you see a bricklike shape for a DSO (that is, you should see that ) and a gussian shape if it is an analog scope.
Low bandwidth (up to a few hundred MHz or so) digital scopes also have a Gaussian response. Just look at the ratio of rise time and bandwidth, it's generally 0.35.
One problem with using an unknown signal source to test a scope with an unknown bandwidth: how do you know that the rise time of the pulse is indeed much faster than that of the scope?
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Modern (fast) logic chips can provide sub 1ns risetimes.
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..... Or, is there some other commonly available household or lab source that has such a fast rise time? ???
Thanks!
If you have a signal generator with a 'sync' or 'trigger out,' it may have a fast rise time logic pulse at those outputs.
At a low enough signal generator frequencies, say 1MHz, you would then have the required step input as PA4TIM described. (That is, the pulse has to stay high/low long enough for the scope to fully respond to the rising/falling edge.
My RIGOL signal generator (160MHz for sine output) has a sync output that I have estimated to have a 600ps rise time into a 50 ohm load. The trigger out from the WAVGEN on my Agilent DSO has a similarly fast rise time (I can't recall the actual number but I did measure it) even though the rise time for the WAVGEN signal output is much slower. The RIGOL signal generator itself is limited to a 5ns rise time square wave at its signal output terminals. 10x slower than the sync output's rise time.
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http://www.ebay.com/itm/TEKTRONIX-671-3507-00-Training-Evaluating-PCB-For-Digital-Oscilloscopes-/380543739262 (http://www.ebay.com/itm/TEKTRONIX-671-3507-00-Training-Evaluating-PCB-For-Digital-Oscilloscopes-/380543739262)