Digital Pre-emphasis

Digital Pre-emphasis: Overcome Transmission Channel Nonlinearities with a Digital Pre-emphasis Amplifier

A data signal with the appropriate amount of applied digital pre-emphasis (or de-emphasis) is able to counteract the nonlinear gain effects of the transmission medium in is traveling in. There gain nonlinearities lead to the various frequency components of the data signal experiencing non-equal delays. As a result, a non-conditioned signal at the end of a said channel will be distorted leading to possible errors by the receiver decision circuitry.

To make the channel’s nonlinear effects negligible, pre-emphasis can be applied to the signal on the transmitter side. The amount of pre-emphasis at each frequency should be equal, but opposite to that of the channel effects. In this way, a much more open (in both the vertical (amplitude) and horizontal (time)) data eye will be present at the receiver side that should significantly reduce any errors.

For data signal rates up to 32.5Gbps, ADSANTEC Inc offers the ASNT6119-KMF, which is a digital pre-emphasis 4-tap amplifier designed for the transmitter side. Each tap is spaced at an integer n multiple of the width of the associated full clock rate or the highest frequency data bit (T) starting at n=1 to n=4. Adjusting each of the four taps will either increase or decrease the gain of the amplifier at that specific frequency.

To give an example of how the amplifier could be used, let’s assume the transmission medium can be modeled as a low pass filter (LPF). In this simple case, more of the total gain of the amplifier would be given to the higher frequencies (i.e. pre-emphasis) of the data signal while less would be provided to the lower frequencies (i.e. de-emphasis). Thus the nonlinear gain aspects of the channel are overcome creating a wide open receiver side data eye.

The ASNT6119-KMF also contains other ways to make adjustments on the transmitter side to make the receiver side output look “better”. These include broad band amplitude control, the ability to adjust the eye crossing point to center it on the receive side, and higher frequency gain control (i.e. peaking adjustment). The taps can also be set in a way to produce a highly adjustable pseudo PAM4 data signal.

Application areas for ADSANTEC’s ASNT6119-KMF digital pre-emphasis amplifier include the test and measurement (T&M) sector and R&D laboratories.

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