High speed Designs - Part 3

Transmission lines is the first term when you hear while starting to work on high speed designs. Transmission line In PCB terminology, transmission line is a trace that connects various chips on the board. The transmission circuit is generally visualized as RLC circuit. The frequency response of any circuit depends on the R-L-C elements and they become predominant when used at high frequencies. 

Traditionally, engineers used interfaces like SPI, I2C, UART which are low speed interfaces. These interfaces didn't have issues with transmission line effects of the PCB traces. Unless the signals are routed over permitted lengths, there is no issue with maintaining the integrity of the signal. As signal frequencies increased, beyond 100MHz, with interfaces like Gigabit Ethernet, DDR, PCIe, etc the transmission line effects have to be considered. We generally read in design guidelines that the signal can't be routed for example, more than 1 inch and also the signal should have a recommended routing in the PCB, etc in the case of high speed signals. So, for a high speed signal even if you route shortest but don't take care of the recommended PCB routing guideline, you might end up with signal having signal integrity issues. Some of the major issues that we come across while working with high speed designs are:

Impedance mismatch

Reflections - overshoot, undershoot, ringing

cross-talk

Radiation

The basis of signal transmission comes from Maximum power transfer theorem of electrical circuit. This theorem states that the maximum power will be transferred from source to load when the source impedance matches the load impedance. If there is a mismatch, then power transferred to load gets reduced which meant there is more loss. In high speed design, when such scenario comes up, the source tries to send the signal to load and load will not be able to absorb the complete signal. Some part of the signal is reflected back to the source. This is what causes the reflections on the PCB. the signal when reflected back travels to source is reflected as there is a mismatch again. The signal hence forth travels between source and load while degrading over time. When these occur, the reflected signal adds/negates with the original signal causing the actual signal amplitude to increase/decrease. This is what we call undershoot/overshoot in high speed domain.