LVDS length tuning and a pi filter on the power input
Went back over the LVDS pairs to make sure the accordion tuning sections hold the target impedance, then brought power onto the board with a pi filter to keep switching noise out in both directions.
Back on the LVDS signals today.
I evaluated them again to make sure the impedance holds over the accordion length tuning sections, not just on the straight runs. The accordion is the zigzag you add to a pair to make up length so everything arrives together.
Turns out the accordion doesn’t add much impedance to the lines. So it’s safe to use, and I’m comfortable calling those sections on target along with the rest of the pair.
Good. One less thing to worry about.
Then power. I added power into the board and put a pi network filter on it. The job there is switching frequencies, and it cuts both ways: noise coming in from the supply, and noise the FPGA board itself would otherwise push back out onto the line.
Filter on the way in, filter on the way back out. Same network does both.
If you want to do this yourself
- Length Tuning in Altium Designer by Altium Academy, 19:27
- Differential Pairs - PCB Design Basics - Phil’s Lab #83 by Phil’s Lab, 21:52
- EMI Filters on Power Supplies: Design & Application Guide by Altium Academy, 15:06