A High-Linearity Vernier Time-to-Digital Converter on FPGAs With Improved Resolution Using Bidirectional-Operating Vernier Delay Lines

Time-to-digital converters (TDCs) act as a core component in many timing-critical systems. Tapped delay line is the most widely used style for the field-programmable gate-array (FPGA)-based TDCs, but it suffers from several intractable problems such as poor nonlinearity and long delay-line occupation. Recently, a new ring-oscillator-based Vernier TDC circuit using carry chains was demonstrated to have much smaller nonlinearity with a much shorter delay line. Since the used delay line is not compensated, the timing jitter increases (or the precision degrades) remarkably when the oscillation number is large. This would incur undesirable lower resolution selection to maintain acceptable precision. In this article, a novel bidirectional-operating Vernier delay line structure is proposed to reduce the required oscillation number during the fine time measurement. Traditionally, for the Vernier delay line, the fast signal propagates along the slow delay line, while the slow signal propagates along the fast delay line, to which we call the normal Vernier delay line. The new structure proposes two parallelized Vernier delay line, consisting of one normal Vernier delay line and another abnormal Vernier delay line. The term "abnormal" refers to that the fast and slow signals are unusually fed to the fast and slow delay lines, respectively. A prototype TDC circuit based on this new method was implemented on a Stratix III FPGA. Test results demonstrate that by adopting the proposed novel circuit architecture, both the resolution and the root-mean-square error can be improved from the 30-40-ps level to the 20-30-ps level.