I am trying to juggle the competing priorities in the design and layout of my first differential pair.
The circuit is an Ethernet daughterboard. It has a socket (Pulse J3011G21DNL) and is a replication of the Microchip Ethernet Starter Kit II. Each of the 4 comms lines has a capacitor to ground (12 pF) and also a pull-up resistor of 49R9 to 3V3. These comms lines go to the LAN8740A.
So I have 2 differential pairs to route.
So many ‘helpful’ websites conveniently show what to do with a simple case of a differential pair, ie. one without the pull-up or caps. This is obviously more complex than the simple case.
Where are my priorities for physical layout ?
a. Do I maintain a constant spacing and length for each trace no matter what ?
b. Do I place the caps as close as possible to the socket no matter what ?
c. Do I place the pull-ups as close to one end as a priority ? (Where do they go ?)
My progress is slow because I am learning the new program and also how to do the differential pairs. I am having issues with the program as I go along. I’m losing my netlisting on components if I edit them. They come up as ‘unconnected’.
Rgds,
Steve
ps. I’m on Linux using the latest Kicad I believe.
i can’t comment on where the caps and resistors should be
i’m also not a specialist in board layout, but you can technically just use the differential pair routing mode (press 6) then just route the tracks
after that, if you wanna know or tune the full length of the tracks, but the resistors/caps prevent you from that - you can temporarily just flip them to the other side (press F on them), and remember to flip them back later
Thanks antto. I am afraid that the insertion of the components will change the impedances depending on placement. It is really difficult to find good information on the situation.