Recommendation for a Jelly Bean High Frequency (>= 200MHz) Schmitt Trigger - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: November 14, 2023, 08:17:05 pm
content: I want to square up a Si5351 Frequency Generator at its top end. Data sheet attached.I can just search at Mouser, but your experience/knowledge will help find the right component for the job and avoid pitfalls and mistakes.The SN74LVC1G14MDBVREP appears to be fast enough at 5V. See DS/Pg3&4.Thanks for your help.

author: tggzzz
author_rank: Super Contributor
timestamp: November 14, 2023, 08:29:42 pm
content: What do you mean by "square up"? 50:50 duty cycle? Faster transition times?What problem are you trying to solve with that solution?

author: t1d
author_rank: Super Contributor
timestamp: November 14, 2023, 10:15:02 pm
content: Thanks, tggzzz,The Si5351 outputs a square wave. At the higher end of its frequency capabilities, the waveform integrity begins to decline. This is rather typical, I think. It changes from a nice square wave to a poor triangle wave. I don't think the duty cycle is changing so much, but I will give that a look. So, I am using this as an opportunity to learn about using Schmitt Triggers. Maybe there is a better/easier solution? I don't know much about fast signals...Thank you for your help.

author: Benta
author_rank: Super Contributor
timestamp: November 14, 2023, 10:36:28 pm
content: If you really need to "square up" a 200 MHz signal, you'll need something like >10 GHz bandwidth. I'd forget your 74xxx1G thing for that.A GaAs or SiGe limiting amplifier or something like that might do it, but why?What do you need to drive, that demands that kind of thing?

author: golden_labels
author_rank: Super Contributor
timestamp: November 15, 2023, 12:02:54 am
content: An ideal square wave consists of infinite number of sinusoidal signals and requires ∞ V/s slew rate. A practical one, fortunately, does not. But you still need a considerable number of harmonics.This chartshows the first four components, and for 200 MHz wave the last one is already 1.4 GHz. Peter Falstad created a greatFourier series appletin which you may play with this kind composition to get the right feeling. Pre-configured waveforms are set by buttons on the right.The other limitation is slew rate. Let’s say, as an example, you want your 200 MHz signal to go reasonably fast from 0 V to 5 V. Assuming that “reasonably fast” for us is: below 5% of the entire cycle width. A single cycle is 1 / (200 MHz) = 5 ns. 5% of 5 ns is 250 ps. (5 V) / (250 ps) = 20 kV/µs. Or 0.050 ns/V. Now look at specs of any discrete logic family chip you want.If that signal has to be synchronized with something: chips also have transition time. Possibly not impotant in your case. But, if you want two signals going through two different channels to be less than quarter a cycle away (likely for a clock…) and these signals are 200 MHz, propagation times must not differ by more than a nanosecond. Jellybean discrete logic? Nope.

author: t1d
author_rank: Super Contributor
timestamp: November 15, 2023, 12:38:24 am
content: Quote from: Benta on November 14, 2023, 10:36:28 pmThanks, Benta. Presently, I am just learning, as said. I was aware that some headroom is needed, but I thought that was about x2-2.5. Realistic uses, for me, would be a DIY 10MHz GPSDO, with a distribution amplifier system on the output, to use as a shop reference. But, I am just learning in general, not designing. I am also aware that a timing signal does not have to be a perfect square wave. But, a good shape couldn't hurt.

author: KE5FX
author_rank: Super Contributor
timestamp: November 15, 2023, 12:41:11 am
content: You don't want to distribute 10 MHz with a square wave. It'll get into everything.

author: t1d
author_rank: Super Contributor
timestamp: November 15, 2023, 12:43:50 am
content: Quote from: golden_labels on November 15, 2023, 12:02:54 amGood, solid information. Thank you.

author: t1d
author_rank: Super Contributor
timestamp: November 15, 2023, 12:45:06 am
content: Quote from: KE5FX on November 15, 2023, 12:41:11 amThat makes good sense. I imagine that is why references are generally sine waves?

author: KE5FX
author_rank: Super Contributor
timestamp: November 15, 2023, 03:21:35 am
content: Exactly. If a square wave is needed by the equipment you're connecting the reference to, it will be generated internally.

author: t1d
author_rank: Super Contributor
timestamp: November 15, 2023, 03:35:46 am
content: Quote from: KE5FX on November 15, 2023, 03:21:35 amYou made a really good point. My original question did not originate from a design perspective... I was playing with the Si5351 and noticed that the waveform degraded at higher frequencies. I thought, why not clean it up with a Schmitt Trigger, which I have not used before? Now, I am learning that it is not so easy.

author: golden_labels
author_rank: Super Contributor
timestamp: November 15, 2023, 08:06:22 am
content: Note that your oscilloscope may also be a limiting factor. It has limited bandwidth, higher frequencies are attenuated. It no longer displays the actual signal it receives.But yes, in practice high clock rate digital signals are very far from anything resembling an ideal square wave.