7.3.6 FIR Filter (Limiting Mode)

The DS280BR820 has an optional limiting mode with a fixed-delay 3-tap finite impulse response (FIR) filter to provide transmit equalization. This FIR can be configured to apply pre-cursor and post-cursor boost to the high speed signal. The FIR filter also allows for main cursor amplitude control. The tap polarities in the FIR filter are fixed to allow for pre-cursor or post-cursor boost to be applied to the signal.

Figure 4. 3-Tap FIR Filter Block Diagram

Linear mode is recommended for the majority of applications, especially those which require Link Training. Common protocols such as 100 GbE and 40 GbE CR4/KR4, 50 GbE and 25 GbE CR, 10 GbE KR, InfiniBand EDR, and others require Link Training. Linear mode is required for Link Training so that the ASIC transmitter pre-cursor and post-cursor coefficients can propagate through the DS280BR820 in a transparent fashion. For applications which do not utilize Link Training, limiting mode may be used to provide output pre-cursor and post-cursor equalization for the purpose of improving the far-end eye opening. If the downstream receiver SerDes uses a decision feedback equalizer (DFE) to equalize the signal, the linear mode may be preferable to the limiting mode. DFE circuits often perform best when operating on a linear signal.