The effects of different bends on performances of wideband digital circuits.

The effects of different bends on performances of wideband digital circuits.

Bends presenting on a SERDES channel introduce discontinuity effects. Normally, such effects are caused by the extra capacitance at the corner of a bend. It will add reflections to the channel signal and can generate radiation at higher frequencies. Minimizing discontinuity effects of SERDES channels has become an issue channel designers need to consider.

In RF design applications, engineers have been using bends with chamfered corners to reduce radiation from right angle bends. Will such bend structure also behave better in digital applications and introduce less reflection on signals?

To answer this question, we studied different bend structures (see Figure 1 to Figure 4) and compared their behaviors with short traces without bends. What we found is: for a wide band signal, such as SERDES signals, a particular bend structure may not always provide better behavior than other bend structures over the entire frequency range. In our tests, we used chamfered bend and a bend with round corner. Two bends of each kind are connected to a pair of differential traces with total length of 500mil. The S-parameters of the two nets are extracted using IE3D. For comparison, the differential traces without bends are also modeled with the same 3D field solver.

The results (see attached table and images) show that chamfered bends introduce less reflection than straight differential traces do at low frequencies (see Figure 5). This is because the trace impedance is not maintained at 100-ohms at all frequency points (see Table 1). However, chamfered bend structure has the worst behavior around 10 GHz due to the resonance caused by discontinuities

In conclusion, certain bend structure may introduce less discontinuity effects at certain frequencies. Designers for SERDES channels need to examine entire frequency range of SERDES signals to determine if using one type of bend structure has more advantage than using others.

Table 1: Comparison on the equivalent Zc in different frequency ranges.

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