AN 958: Board Design Guidelines

ID 683073
Date 1/28/2022
Public
Document Table of Contents

2.5. Transmitter Pre-Emphasis and Receiver Equalization

Typical transmission media like copper trace and coaxial cable have low pass characteristics, so they attenuate higher frequencies more than lower frequencies. A typical digital signal that approximates a square wave contains high frequencies near the switching region and low frequencies in the constant region. When this signal travels through low pass media, its higher frequencies are attenuated more than the lower frequencies, resulting in increased signal rise times. Consequently, the eye opening narrows and the probability of error increases.

The high-frequency content of a signal is also degraded by what is called "skin effect". The cause of skin effect is the high-frequency current that flows primarily on the surface (skin) of a conductor. The changing current distribution causes the resistance to increase as a function of frequency.

You can use pre-emphasis to compensate for the skin effect. By Fourier analysis, a square wave signal contains an infinite number of frequencies. The high frequencies are located in the low-to-high and high-to-low transition regions and the low frequencies are located in the flat (constant) regions. Increasing the signal's amplitude near the transition region emphasizes higher frequencies more than the lower frequencies. When this pre-emphasized signal passes through low pass media, it comes out with minimal distortion, if you apply the correct amount of pre-emphasis. Refer to Figure 12 for a graphical illustration of this concept.

Figure 12. Input and Output Signals with and without Pre-Emphasis

Stratix® II GX and Stratix® IV GX devices provide programmable pre-emphasis to compensate for variable lengths of transmission media. You can set the pre-emphasis to be between 5 percent and 25 percent, depending on the value of the output differential voltage (VOD). Please refer to the Stratix® II GX and Stratix® IV GX user guides.

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