3 Equalizer Placement within a SAS-3 Channel

The benefits of a linear equalizer can only be realized if it is placed some distance away from the SAS-3 transmitter. The physical distance may vary based on materials used in PCB fabrication, but not the attenuation. Since linear equalizers compensate for PCB or cable based attenuation, some attenuation must occur prior to the equalizer to result in any benefit. Figure 5 helps to show the minimum input loss required to optimize the possible channel extension. As the level of equalization in the DS125BR820 is increased, the device must be placed further away from the transmit source. Using an equalization setting which can drive the DS125BR820 beyond the 1.2 V dynamic range will result in non-linear operation. This will degrade the waveform and result in sub-optimal link training results.

Placement to maximize channel extension is not limited to a location roughly 10 dB removed from the transmit source. It is possible to achieve full extension when the DS125BR820 is placed at any point beyond the 10 dB minimum. If placement closer than 10 dB from the transmitter is required, a lower level of equalization is recommended.

Figure 5. Illustration of Channel Extension and DS125BR820 Placement