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Show us your square wave
Folnia:
--- Quote from: tggzzz on October 12, 2024, 10:53:52 am ---
--- Quote from: Folnia on October 12, 2024, 08:18:14 am ---
--- Quote from: tggzzz on October 08, 2024, 04:08:52 pm ---
--- Quote from: Folnia on October 08, 2024, 02:21:55 pm ---Hi Tggzzz,
I tested with my board, the rising edge measured is 900ps. And the bandwidth of the scope is 525MHz.
So the calculation of the the pulse rising edge might be sqrt( 900² - (0.35/525M)² ) , around 604ps, much less than 256ps mentioned in your post. Did I miss something? (I don't find the spec of flip time in the datasheet of SN74LVC1G14DBVR so I guess it's just because of the different component...)
--- End quote ---
In your position I would want to understand exactly what the scope is doing. I would play around with whatever settings I could find to try to eliminate that pre-transition "ripple".
Scopes are designed to have good time-domain fidelity, and there are various definitions of such fidelity. That leads to the correspondence with a frequency domain spec (the 525MHz) being complex and potentially misleading. A neat illustration of that is one of my old scopes (Philips PM3410 from 1970), where the front-panel headline performance spec is "200ns risetime", not a frequency.
The "0.35" is a magic number that was derived from non-sampling scopes which have Bessell response input filters. Digitising scopes don't necessarily have that, and some claim that "0.45" is a better number.
--- End quote ---
I tried the test on another 2G BW scope. The rising is faster but there is still ripples before the rising (either before falling edge) . I guess it's because of some front filter integrated in the scope that limits the bandwidth? Do you have any ideas/tests to find out the cause?
And IMHO the major difference between latched inverter and none-lactched one is only the trigger window to avoid multiple flips. And I did a quick test and there seems no obvious changes.
--- End quote ---
I'm not sure what you mean by "latched inverter", but the only thing that should affect the risetime is the output buffers, resistors, decoupling.
I cannot comment on your scope's behaviour. All I can suggest is that you RTFM to see if there are alternative capture modes, e.g. for repetitive waveforms amd non-realtime sampling.
In your traces I cannot see the points where the input waveform is sampled. It is conceivable that the trace you see is a result of software interpolating between points, i.e. a post-sampling reconstruction. In some cases such things can be very misleading. (In general I prefer a scope to present the points on the display, and let my brain do the interpolatation).
I presume you have connected your device directly to the scope input, without a cable.
I presume your scope has a "proper" 50ohm input, not 50ohm//12pF.
--- End quote ---
Yes I connected the the board to the scope directly as shown in the attached photo(I also compared with using the 50Ω cable and it shows no difference).
The latched inverter is regarding the suggestion from TurboTom's post " the inverters (74LVC1G04 / 74LVC2G04) are better suited for outputting high-speed slopes. " so I did a quick comparison test.
I'll look into the setting about the capture and 50ohm input of the scope. Many thanks!
tggzzz:
Those scope inputs will be fine, as is the connection to the scope.
TurboTom didn't reply when I asked him what he meant.
*1G* should be better than *2G* due to the lower lead inductance per gate.
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