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Cheap Fast Rise Time Oscillator

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BillyO:
The boards for the latest prototype are in and I built up a couple for testing.

I got some good results!  The last prototype (mentioned in this thread: https://www.eevblog.com/forum/testgear/best-brand-name-for-an-oscilloscope/msg4554020/#msg4554020) was reading about 2.15ns on my SDS2504XP.  The latest one reads 1.47ns.  Given that the scope has a rise time of about 680ps that would give the oscillator a rise time of about 1.3ns actual.

The rise time is dependent on the choice of 74AC14 too.  1.47ns is from the slowest of the two I built.  The fastest was 1.37ns or about 1.2ns actual.  Unfortunately I don't have a faster scope readily available to get closer to actual rise time.  I have a few ideas for the next (and possibly final) version such as using a 4-layer board, slightly shorter trace to the BNC and using micro-USB for power, but I don't expect these will speed it up much.  The 74AC technology is just not going to wiggle much faster.


tggzzz:
If you want <300ps, have a look at https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

BillyO:
Nice!  I might try that next using one inverter for a relaxation oscillator.  You could build it for about $3!

BillyO:
Boards for the latest version  are in.  Below in purple.  Highlights of this version:
 
* Smaller board
* Shorter output trace
* Only 4 inverters on output to reduce PD skew
* Crystal replaced by relaxation oscillator to reduce size and cost to build
* 4-layer board with 2 full ground planes and a full Vcc plane
Will build one up tomorrow to test.

The 2nd device (black) is a TDR version in early stages.  Waiting on the proper parts to properly test the design so far.

magic:
You will never get good results with DIP or even SO package. Too long pins, too long traces, too much inductance in series with the output, too much inductance in series with the bypass cap. Guaranteed severe overshoot and ringing, easily visible even on 100MHz scopes.

For the sort of results shown by tggzzz, SOT-23 (or smaller) package and low inductance layout are essential.
And LVC logic appears to be faster than AC. There may or may not be differences between manufacturers too.
But nothing is as important as small package and physical construction details.

My own attempt, never tested on "serious" gear but you can see its 200MHz performance by scrolling a few posts up:
https://www.eevblog.com/forum/projects/compact-74ac14-pulse-generator-pcb/msg3475680/#msg3475680

It's an open question whether this or tggzzz's single-gate chips on a pair of power/ground planes would be better :-//


edit
And by the way, the chips you are using are not TI but Chinese mystery meat, just so you know.

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