Back to the beginning: ADCMP606 w/ 120 ps edges

A project log for The Rise and Fall of Pulses

A project in which I try to go faster and faster

Ted YapoTed Yapo 04/17/2019 at 02:155 Comments

I started this project with a pulse generator based on a ADCMP606 CML-output comparator. It was faster than my scope at the time, so all I could really say was that the rise-time was less than 350 ps or so. Last night, I found some quality time with the new scope, and measured the rise time of that circuit at less than 120 ps.

At a longer timescale, the step looks pretty good, with minimal overshoot. It's a handsome pulse, attesting to the robustness of the comparator in the face of my inept board design :-) This comparator has a datasheet rise time of 160 ps, which implies typical Analog Devices understatement/over-performance.

Overall, for a $6 (single-quantity) part, it's not bad at all. I think I'm going to make another revision of this board and release it. With a 120 ps rise-time, it's certainly good enough for testing oscilloscopes up to around 2 GHz.


salec wrote 04/18/2019 at 09:35 point

If you had steeper edges, would your new oscilloscope allow you to measure them? I mean, the whole setup, not just oscilloscope, but also probes, fixture (PCB) ...

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Ted Yapo wrote 04/18/2019 at 10:43 point

Yes! The SD-24 heads I'm currently using (20 GHz) have a 17.5 ps rise time. There are no 20 GHz probes as far as I know; it's all substitution measurements at 50 Ohms. So, if I'm careful, I can get into the tens of ps. I do have another pulse generator I built, not fully debugged yet, that achieved a 63 ps step. And, I've measured the internal TDR step generated by one of the scope channels itself at a 28 ps rise time (pic below).

Tektronix also made 40 GHz and 50 GHz plugins for this scope. I keep scanning ebay for bargains, but they're typically out of my price range. A cheap one will turn up eventually.

EDIT: I found 20 GHz probes. At the prices they fetch, they might as well not exist.

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salec wrote 04/18/2019 at 11:37 point

Where are the limits currently, I mean, like, what's in Guinness' Book of Records for the shortest rise time?

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Ted Yapo wrote 04/18/2019 at 12:36 point

@salec I don't know what the record is.

Single-digit picosecond rise time steps are commercially available.

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Yann Guidon / YGDES wrote 04/17/2019 at 18:57 point

Awesome :-)

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