Corrective Measures for High Speed Links

The two main techniques for correcting the attenuation and other distortion encountered on a high-speed serial path are pre-emphasis and equalization. Both techniques have been around for decades having been incorporated in audio recording as well as FM radio and other communication techniques. In pre-emphasis, the transmitted signal is given a boost in its high frequency response since it is the higher order harmonics that are attenuated by the serial link. In the receiver, this type of correction is also called feedforward equalization (FFE). The technique works well but it increases the noise level. It is not the best solution for some designs. Another technique of receiver equalization is decision feedback equalization (DFE). This method uses a feedback signal from the receiver's comparator to adjust the level of correction at the input. This technique boosts the signal at the right place and time so it does not inject noise. DFE is the equalization of choice for most modern high-speed serial links.