Ever wondered how the emission pattern from an impedance controlled trace using series terminations looked like? Ever asked what are the major factors that have an effect on emissions? Well, a while back i had the same questions and went on to investigate further. In this article, i have presented a brief about why terminations are used, what series terminations are and how they work, before going on to explain their emission pattern.
Terminations – What are they?
All of us are aware of “Maximum power transfer theorem”. It states that, in order to transfer maximum external power from a source with finite internal impedance, the impedance of the load must be made the same as that of the source. So, in a real high speed circuit, ideally to obtain maximum power transfer, we would want the source impedance to match the load impedance. That said, we need to keep in mind that a high speed trace on a PCB has finite capacitance and inductance associated with it and hence would act as a load to the driver. So in order to get maximum power transferred in a high speed circuit, we need to match the impedances of the driver with the trace and with the receiver. This combination allows for maximum power to flow from the source to the destination without any reflections. But many drivers are built for a better drive strength and tend to have impedances of around 20 ohms while the receivers are designed with high impedance of around 1 Mega ohms. So how do we match these two completely different parties and get as much power delivered? Well, we use terminations. Terminations are electronic components (generally passives) that are used to tweak the source and load impedances to the same value as the trace and allow maximum energy to be transferred. There are many different termination configurations that engineers use – the most common being, Series, Parallel, Thevenin, RC and Diode terminations. If you are interested to know in detail about these termination configurations, refer to the article “Termination techniques for high?speed buses” by Karthik Ethirajan and John Nemec.
Continue reading the complete article here:
