Whennoequalizationisappliedatthereceiverend,ahigh-frequency"0"pulsemaynotbe" />
Transmitter Preemphasis |
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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." ); margin-bottom: 15px; "> 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. ); margin-bottom: 15px; "> Figure 3. Preemphasis filtering in frequency domain. ); margin-bottom: 15px; "> Effective use of this equalization technique will affect three main system design parameters: ); margin-bottom: 15px; "> Cable length Cable type Maximum system data rate
); 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). ); margin-bottom: 15px; "> 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. ); margin-bottom: 15px; "> Table 1. Preemphasis and Deemphasis Levels 0x05 D[3:0] Preemphasis Level (dB) 1000 1.1 1001 2.2 1010 3.3 1011 4.4 1100 6.0 1101 8.0 1110 10.5 1111 14.0 Deemphasis Level (dB) 0000 Not used 0001 -1.2 0010 -2.5 0011 -4.1 0100 -6.0 0111 Not used
); margin-bottom: 25px; "> In the following sections, how to utilize both the transmitter and receiver equalizers will be discussed, along with tabular test data.
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