Questions about High Speed PCB impedance
When designing a high speed digital design including an FPGA and GSPS A/D interfaced with multi-gigabit/s serial ports. I have some questions like following:
1.When I use tools to calculate the impedance of a trace on a PCB. The impedance is irrelevant to the length of a trace in these tools. But as I know the impedance of a trace should depend on its length.So I am confusing about it.
2.In a serial port, is the impedance matched by using series termination resistors or by tuning the traces themselves to suitable width or distance?
3.Led light on what situation in general we should use resistors and on what situations the traces make the desired impedance?
I can only answer your third question. You need both the resistors and the controlled impedance traces. However, many LVDS drivers and receivers have the termination resistors built-in. If this is the case you do not need external termination resistors. Check the driver datasheets to determine whether you need to include them externally or not.
For your question 2, my answer is as following: Impedance matching of PCB traces is controlled by the traces, not by the termination resistors. For 2 reasons: 1. the receivers measure the voltage across it to determine the logic level of the signal; 2. the termination resistors match the driver impedance and receiver impedance. You need the termination resistors and you must "tune" the traces by adjusting their widths to keep the desired impedance.