As Dave Dunham of Molex Corp. likes to say, "When designing high-speed serial links beyond 10 GB/s, everything matters." And part of that everything is accurate modeling of transmission line losses.
Failure to account for conductor roughness can ruin your day, especially if you are trying to push 28 GBaud/s (56 GB/s) PAM-4 signaling down your channel. To ensure first-time success at these speeds, using the right parameters for dielectric and conductor roughness to feed into modern EDA tools is a prerequisite. This is especially true for long backplane channels.
Many EDA tools include the latest and greatest models for conductor surface roughness and wide-band dielectric properties. But obtaining the right parameters to feed the models is always a challenge. So how do we get these parameters?
One way is to follow the design feedback method which involves designing, building and measuring a test coupon. After modeling and tuning various parameters to best fit measured data, Dk, Df and roughness parameters can be extracted. They are then used in channel modeling software to design the final product.
The benefits of this method are that it is practical and accurate—if you use the exact same material, glass style and copper foil in your final board stack-up. On the down side, a significant amount of expertise and equipment is required to design, build and measure the test coupon, which takes significant amount of time and money.
To read this entire article, which appeared in the October 2017 issue of The PCB Design Magazine, click here.