AN82

AN82 HIGH-DENSITY MULTI CHANNEL OC-48 LAYOUT GUIDELINES S i 5 1 0 0 A N D S i 5 11 0 Introduction OC-48 small form-factor hot-pluggable modules (SFP) allow designers to place a large number of OC-48 links in a relatively small amount of space. As the density increases, adjacent channels are more likely to interfere with each other. When the Si5110 or Si5100 OC-48 transceivers are used to receive and transmit through the SFP module, it is necessary to minimize the adjacent channel coupling in order to meet the transmit jitter generation requirements. FOR THE In addition, adjacent devices can couple to surrounding devices if their TTL or high-speed traces can couple to each other. Since PCB fabrication does not allow for solid walls perpendicular to the plane of the board, a "wall" of vias can be used to isolate nearby traces as shown in Figure 1. Vias should be placed along the axis of propagation no farther than 100 mil apart. Diff Pair Via Wall Diff Pair Background The oscillator that generates the timing for the transmit data within the transceiver stores energy in electric and magnetic fields during oscillation. The magnetic fields extend several inches (centimeters) from the device but decrease in magnitude at a rate inversely proportional to the square of the distance. Consequently, distance is key to minimizing the adjacent channel coupling. In addition, energy from other sources can couple into the device through capacitive and mutually-inductive means. These paths ...