tee trace

W/o the copper, I'd call that "dielectric loading." Not sure if it is a rigorous term. It is a good compromise when it is impractical to get the other copper layer in place, for whatever reason.

That makes the path to the right longer!

I'm driving a Mach-Zehnder modulator, a 50 ohm load. The actual laser is lots of kilojoules.

Why is the track to the right of C28 so narrow, if you care about 25 Ohms impedance to the left of C28, or is that bit just a placeholder and not done yet?

Is this a high-volume or cost-sensitive product? If not, and if you have competent technicians who can solder, I'd probably solder a piece of semi-rigid micro-coax onto the board instead of using a microstrip, if I had to achieve 25 Ohms. Then you can make it much longer if you like. You can get it in very small diameters, and various impedances. I expect you could order it pre-cut and stripped ready to hand-place onto the board much like a SMT component, or thread it through a grounded through-hole pad if you like, etc.

For example you could buy UT-034-25-SP from

This is about it:

I want a 25 ohm run to span the distance from the driver to the cap and schottky, but then all I have is really tiny parts with minimal spacings.

The entire fast part of this circuit board is on a little wing, about a square inch.

I want low pad capacitances elsewhere on the board, so there's a compromise on dielectric thickness. I did one test board with an Isola microwave lam, and it's didn't seem to be any better than FR4, given that I keep everything short. The CPW grounds on my fat trace only help a tiny bit, not really worth it.

I've tried scraping away solder mask on microstrips and couldn't see any difference on 30 ps TDR.

I have another board coming up, with a long TDR test trace. I can add some official mask keepouts and see what happens.

The John Doe troll stated the following in message-id <sdhn7c$pkp$ snipped-for-privacy@dont-email.me:

And the John Doe troll stated the following in message-id <sg3kr7$qt5$ snipped-for-privacy@dont-email.me:

And yet, the clueless John Doe troll has itself posted yet another incorrectly formatted USENET posting on Wed, 15 Dec 2021 18:56:13 -0000 (UTC) in message-id <spddoc$g7q$ snipped-for-privacy@dont-email.me.

SaEcO4pH/7TI

The John Doe troll stated the following in message-id <sdhn7c$pkp$ snipped-for-privacy@dont-email.me:

And the John Doe troll stated the following in message-id <sg3kr7$qt5$ snipped-for-privacy@dont-email.me:

And yet, the clueless John Doe troll has itself posted yet another incorrectly formatted USENET posting on Thu, 16 Dec 2021 03:13:28 -0000 (UTC) in message-id <speasn$61i$ snipped-for-privacy@dont-email.me.

This posting is a public service announcement for any google groups readers who happen by to point out that the John Doe troll does not even follow it's own rules that it uses to troll other posters.

q13rtDDuJTy8

Nobody likes John Doe's contentless spam. For him to complain about somebody else going after him in the same way that he stalks others does have a certain ironic content.