Transmitter Preemphasis

); margin-bottom: 15px; "> When no equalization is applied at the receiver end, a high-frequency "0" pulse may not be able to reach the midlevel of the signal swing after consecutive "1's", as shown in?Figure 2. The figure illustrates how frequency-dependent attenuation can be overcome by emphasizing transitions and deemphasizing "no transitions."

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Figure 2. Preemphasis filtering in time domain.

); margin-bottom: 15px; "> The cable has a lowpass transfer function due to the conductor and dielectric losses, as shown in?Figure 3. By utilizing equalization (a highpass transfer curve), a flat (uniform attenuation) system frequency response can be obtained within the?bandwidth?of the desired frequency range.

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Figure 3. Preemphasis filtering in frequency domain.

); margin-bottom: 15px; "> Effective use of this equalization technique will affect three main system design parameters:

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• Cable length
• Cable type
• Maximum system data rate

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); margin-bottom: 15px; "> For instance, the totally closed eye at the end of a 10m cable can be reasonably opened by 6dB preemphasis (Figure 4).

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More detailed image?(PDF, 1.3MB)
Figure 4. 3.125Gbps data after 10m cable: (a) none vs. (b) 6dB preemphasis.

); margin-bottom: 15px; "> As described in the?MAX9259?data sheet, the preemphasis level is set by register address 0x05, D[3:0]. The user can program the preemphasis level based on?Table 1. The negative preemphasis levels correspond to where high-frequency terms are not emphasized, but only the low-frequency terms are deemphasized. It is also important to note that over-boosting will cause a slight increase in the timing jitter.

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); margin-bottom: 25px; "> In the following sections, how to utilize both the transmitter and receiver equalizers will be discussed, along with tabular test data.