With the rapid development of video applications, data transmission traffic is growing exponentially, and higher data transmission rates are urgently needed. Therefore, low-cost twisted pair (TP) has also received special attention. The high-frequency attenuation of the TP cable is the main factor limiting its application performance. The high-frequency attenuation causes significant inter-symbol interference (ISI) of the received signal, which makes it difficult to recover the clock and data, resulting in an increase in the bit error rate (BER). The result of the attenuation of the transmitted signal by the cable before it reaches the receiver can be seen from Figure 1. The transmitter and receiver take some form of line equalization, which can greatly reduce ISI and restore severely degraded data to ensure reliable operation.
Figure 1. ISI at the receiver side
The 3.125Gbps high-speed transceiver in Maxim GMSL products allows system designers to dynamically adjust the equalization level of the actual cable to provide a reliable communication link. Both the transmitter and receiver have equalization adjustments that can be set independently or in combination to extend the data transmission distance. Flexible equalization adjustments allow the use of a variety of low cost, lossy cables.
This application note describes how to build a reliable communication link with Maxim GMSL products and lossy cables. This article briefly introduces the line equalization technique.
GMSL sends pre-emphasis and receive equalization
The GMSL link uses transmit pre-emphasis and receive equalization to compensate for transmission loss.
Send pre-emphasis
If no equalization is used at the receiver, the data line may not be restored to the middle of the signal swing when a series of "1"s appear consecutively, as shown in Figure 2. The figure explains how to overcome the high frequency attenuation problem by aggravating the edge of the transition and de-emphasizing the "non-jumping edge".
Figure 2. Time domain pre-emphasis filtering
The conductor and dielectric losses cause the cable's transfer function to appear as low-pass filtering, as shown in Figure 3. With equalization (high-pass transfer curve), a flat (uniform attenuation) system frequency response characteristic can be obtained in the corresponding frequency band.
Figure 3. Frequency domain pre-emphasis filtering
Rational use of equalization technology can improve system design in three ways:
cable length
Cable type
Maximum system data rate
For example, with a 6dB pre-emphasis, the eye diagram that is completely closed at the end of the 10m cable can be turned on (Figure 4).
Clear image (PDF, 1.3MB)
Figure 4. 3.125 Gbps data transmitted over a 10 m cable: (a) no pre-emphasis; (b) 6 dB pre-emphasis.
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