Comparisons between serpentine and flat spiral delay lines on transient reflection/transmission waveforms and eye diagrams

In contrast to the commonly employed single-ended delay lines, the employment of differential signaling may alleviate the occurrence of crosstalk and improve the signal integrity. This paper qualitatively investigates the time-domain reflection (TDR) and time-domain transmission (TDT) waveforms for the single-ended and differential delay lines with the serpentine and flat spiral routing schemes. A numerical formula is then proposed to quantitatively predict the voltage levels of the saturated near-end and far-end propagating crosstalk noises among the sections of differential delay lines. Signal waveforms and eye diagrams of the four basic routing schemes are obtained by HSPICE simulations, demonstrating that the combination of differential signaling and flat spiral layouts can exhibit the best delay-line performance. Furthermore, both the TDR and TDT measurements for differential delay lines are performed to validate the exactitude of proposed analyses.

I. Introduction

As the cycle time of computer systems falls into the subnanosecond regime, the fraction of cycle time to accommodate the clock skew for the synchronization of clock signal among the logic gates has risen. While several approaches have been proposed to minimize the clock skew, the delay lines are usually employed in the critical nets of a printed circuit board (PCB), for example, the serpentine or flat spiral routing schemes, as depicted in Fig. 1. Intuitively, the total time delay should be proportional to the total length of the delay line. However, the crosstalk noise induced by those closely packed transmission-line sections may cause a drastic deterioration in the total time delay and even result in the false switching of logic gates, especially for the serpentine delay line [1], [2]. Fig. 1.

Two typical routing schemes for the delay line. (a) Serpentine routing scheme. (b) Flat spiral routing scheme.