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Oscilloscope Rise Time Selection

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harioyyoth
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Oscilloscope Rise Time Selection

Post by harioyyoth » April 10th, 2020, 3:10 am

Dear Team,

In one of the Tek application notes, I have seen that "oscilloscope’s rise time should be < 1/5 x fastest rise time of the signal".May I know how you arrived this , from where this 1/5 came.

Regards
Hari

w2aew
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Re: Oscilloscope Rise Time Selection

Post by w2aew » April 10th, 2020, 6:11 am

The reason for the 5x rule is to ensure that the measured risetime is within 2% of the actual risetime. If the scope's risetime is too slow, it will slow down the measured risetime too much.

The measured risetime can be approximated by the square root of the sum of the squares of the actual risetime and scope risetime:

rise_meas = sqrt(rise_act^2 + rise_scope^2)

...to determine how much faster the rise_scope has to be in order to have 2% error, we can normalize and solve for N, where N is the fractional multiplier to apply to the rise_scope:

1.02 = sqrt(1^2 + (1*N)^2)
1.04 = 1 + N^2
0.04 = N^2
N = 0.2, which is the same as dividing rise_scope by 5.

tvellc
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Re: Oscilloscope Rise Time Selection

Post by tvellc » April 10th, 2020, 11:32 am

Is the rule of thumb that, the rise time of an oscilloscope is = to 0.35/BW, a good way to determine the scopes rise time?

w2aew
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Re: Oscilloscope Rise Time Selection

Post by w2aew » April 10th, 2020, 11:40 am

In general yes, as this approximates a single-pole rolloff. However, keep in mind that with today's digital scopes, especially those with very high bandwidth, employ a lot of DSP to flatten the frequency response and often the rolloff is much steeper beyond the 3dB point. In these cases, the approximation is closer to 0.4/BW to 0.45/BW.

tvellc
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Re: Oscilloscope Rise Time Selection

Post by tvellc » April 10th, 2020, 12:26 pm

Owning a digital and several analog scopes (all Tektronix), this was very helpful. Is there a method for measuring the scope's actual rise time? I have a pulse generator with a 5ns rise time.

Thanks a fellow ham K7MLG

w2aew
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Re: Oscilloscope Rise Time Selection

Post by w2aew » April 10th, 2020, 12:52 pm

If you *know* the true risetime of the signal, you could approximate the scope risetime using:

rise_scope = SQRT(rise_measured^2 - rise_actual^2)

but this will suffer with accuracy if there is a big difference between the scope risetime and the actual risetime. If would be best to use a very fast pulse and measure the scope directly (5x faster than the scope).

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