Recomendation for Jelly Bean Comparator Fast Enough to Clean Up 10MHz - Page 1
Forum section: electronics / beginners

author: t1d
author_rank: Super Contributor
timestamp: March 08, 2023, 10:55:52 pm
content: Me: Hobbyist skills/knowledge with nice lab and soldering equipment. Self-taught, so there are holes in my knowledge base, such as in this instance.I am experimenting with DIYing a GPSDO. Presently, I am working on cleaning up the signal from the 10MHz OCXO. I do not have any Comparators in stock that are fast enough. (I played with a LM339 and got garbage.)The OCXO is 5VDC and single rail. Its output is HCMOS Square Wave; H=>4.5V, L=<0.5V. So, a single rail, low power Comparator would be good. Rise/Fall times are =<6nS. Data sheet attached.Thank you for your help.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 08, 2023, 11:25:58 pm
content: Since you've already got an HCMOS signal, just follow it with a 74AC04 inverter, or similar.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 08, 2023, 11:38:16 pm
content: May be an HC132 or similar Schmitt gate. Lately I've been using single gates to save board space. The 74LVC1G14GW is fast.

author: t1d
author_rank: Super Contributor
timestamp: March 09, 2023, 12:08:39 am
content: Quote from: edpalmer42 on March 08, 2023, 11:25:58 pmThanks, Ed. You are correct. And, the signal direct from the oscillator is fairly square. But, part of the point is to learn more about comparators. I apologize for not stating that.

author: t1d
author_rank: Super Contributor
timestamp: March 09, 2023, 12:40:33 am
content: Maybe the LM3501? Push-Pull Output. But, not cheap at $3.15USD. Data sheet attached.

author: langwadt
author_rank: Super Contributor
timestamp: March 09, 2023, 12:43:56 am
content: you could look at LVDS receivers

author: t1d
author_rank: Super Contributor
timestamp: March 09, 2023, 01:01:52 am
content: Quote from: langwadt on March 09, 2023, 12:43:56 amThank you for joining in. I did not know about LVDS receivers. I will enjoy giving them a look.

author: David Hess
author_rank: Super Contributor
timestamp: March 09, 2023, 02:43:13 am
content: Differential line receivers, like for RS-485, are just comparators with a logic output and can be ideal for applications like this. If you can put up with a SOT23-5 package, then the MAX3280E seems ideal.

author: BillyO
author_rank: Super Contributor
timestamp: March 09, 2023, 04:51:35 am
content: What's wrong with the signal? Do you have a scope capture we can see?These are as clean as they come. They do need to drive into a 50 ohm load. Are you doing that?What is your application?This is what they look like driving a 50 ohm load. Is that not clean enough?

author: BillyO
author_rank: Super Contributor
timestamp: March 09, 2023, 05:17:17 am
content: I saw your note about learning about comparators.If you look at the capture I attached you will see that the rise time and fall time are fast. That scope is good for about 620MHz, which means the actual rise and fall times of the oscillator are in the 900ps and 800ps times respectfully. In other words, you will need a low dispersion comparator that will be flat to at least 1GHz to do this oscillator justice (as in - not make it worse). Comparators like that are not easy to come by or work with and usually require differential drive on each input. Putting such a device into use will certainly give you lessons on using comparators. More than most have.

author: MIS42N
author_rank: Frequent Contributor
timestamp: March 12, 2023, 11:59:28 pm
content: Why? Comparators are for detecting levels in analog signals. This is a digital signal. The specification sheet of the OCXO looks like for an OSC5A2B02, I use one in my GPSDO feeding into one gate of a 74HC04, then use that gate to feed the other 5 gates to drive into 50 ohm. Output looks fine.So what's to learn about comparators? Give them the right load and get the right result. I don't have much design experience but I would venture to say most comparators are not designed to drive a significant load and if a load like 50 ohm was intended the signal would then be buffered by another device.In this application, a comparator takes a digital signal and converts it into ... a digital signal (with a small delay). Is there a reason to do that?