Simulating Lossy Transmission Lines; Cable; Connectors; Skew Between Conductors - Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual

Chapter 4: PCB Materials and Traces

Simulating Lossy Transmission Lines

Due to the different modeling implementations used by various circuit simulators
(frequency-domain versus time-domain techniques), it is important to check that the
models accurately reflect actual losses. One method is to compare the models against
known published configurations.

Cable

Cables are controlled-impedance transmission lines due to the constant physical
dimensions of conductor and dielectric along the length of the cable. The highest quality
cable shows little variation in these dimensions and also has a wide bandwidth with low
loss at high frequencies.

Connectors

The connectors attached to cables should exhibit low parasitic inductance, low-parasitic
capacitance, and low crosstalk for high bandwidth operation.

Skew Between Conductors

When selecting a cable, look for a specification of the skew between the conductors in a
cable. If the conductors are not length matched, the skew appears in the common mode
and directly reduces the eye height.
46 www.xilinx.com Spartan-6 FPGA PCB Design and Pin Planning
UG393 (v1.1) April 29, 2010