3 Electrical Performance

Why is redriver linearity and bandwidth so important?

PCI Express along with most modern communication standards uses the concept of link training and adaptive signal conditioning. Although the specifics and algorithms will vary, all incorporate methods that allow receivers (Rx) to feedback or recommend finite impulse response (FIR) coefficient changes to the transmit (Tx) device. Working through this process enables the system Rx/Tx pair to arrive at a total channel solution for signal compensation without external intervention. For the linear redriver to work robustly in an autonomous link training environment, it must maintain and preserve the linearity of the channel while providing sufficient high frequency gain to effectively turn a long channel into a shorter, less lossy channel. The linear equalizer must possess sufficient bandwidth and dynamic range to accomplish this task without distorting the original signal. Hence, maintaining linearity of the redriver across different EQ Indices and DC Gain is very important.

The redriver allows for tuning both the Continuous Time Linear Equalizer (CTLE), or EQ Index, and DC Gain of the transmitted signal. This section outlines the effects of both of these parameters on the time domain and eye diagram of the transmitted signal from the linear redriver.

For reference, the settings outlined in Table 3-1 were used in sections 3.1, 3.2, 3.3, and 3.4. For further information regarding other redriver EQ Index settings, please reference the DS320PR810 data sheet.