Signal conditioning becomes critical as data rates increase and when driving long backplanes or cables. As high-speed signals travel thru the transmission medium, the signal is distorted and the eye begins to close. The blue line in this chart is the approximate 3 dB point. At this data-rate, FR4 length signals have lost about 30% of their energy. Somewhere around this point, the eye pattern begins to deteriorate, jitter increases and the BER (bit-error-rate) begins to suffer. The red line is the 6 db curve where the signals have lost about 50% of their energy. The eye pattern begins to close and the jitter increases significantly at this point. These curves are approximations and actual system performance will depend on many factors such as impedance discontinuities and the amount of noise from other sources, such as crosstalk. In general, for most systems, somewhere between the blue and red line is where signal conditioning becomes necessary to avoid excessive bit errors. Note that for XAUI data-rates, somewhere less than 10 inches of FR4 is where signal integrity begins to suffer. This chart is an approximation of signal attenuation vs data-rate and Infiniband or PCIe cable length. The Blue line is the 3 dB point – this is where signal degradation becomes noticeable and jitter begins to increase. By 6 dB, the red line, there is significant distortion and eye closure. Somewhere between the Red and Blue line is generally where signal conditioning is used to compensate for the transmission loss in the cable.