Miscellaneous SI Topics

Pre-emphasis

When the frequency of the signal goes above 1 GHz, you will notice that the signal attenuation becomes significant. The attenuation of the signal is not uniform across all the frequency ranges. The higher frequency components suffer more attenuation as compared to the low frequency signals. As a result the signal looks distorted at the receiver end. The digital signals contain random data patterns. A 1010101 like pattern creates a high frequency component. A 1111000011110000 like pattern creates a low frequency component. This creates an inter symbol interference. Observed in eye diagram of signal at receiver end, the eye opening is observed to be substantially closed.

One way to overcome it is by using pre-emphasis. In pre-emphasis, the higher frequency signal in amplified before the signal is launched. The amount of the pre-emphasis is matched to the amount of the path loss. This ensures that the frequency response is flat at the receiver end. Modern day high speed ICs contain pre-emphasis capability built in within the ICs with ability to program the amount of the pre-emphasis to match the losses on the PCB traces. If you are a PCB designer, you will have to work with the software person to match the setting of the pre-emphasis to match the losses at the PCB level.
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