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Description
Achieving sub-picosecond clock phase alignment is a challenge in timing distribution systems. A common strategy is to implement a control system that measures the feedback phase, and shifts the distributed clock to compensate for variations. At sub-picosecond level, deterministic phase-shifting becomes a major bottleneck as the cost and complexity of solutions increase significantly with the required precision.
This paper presents a novel technique, called System-Agnostic Method for Biphasic Alignment (SAMBA), that improves the phase-shifting resolution of digitally controlled PLLs by orders of magnitude. It can be implemented using COTS PLLs as well as PLLs integrated from programmable devices such as FPGAs.
SAMBA works by cascading two PLLs and tuning the frequency of their oscillator in order to cause the phase-shift resolution of each PLL to be slightly different. This small difference can be exploited by using the Vernier Effect to produce ultra-fine phase steps: by shifting the phase of both PLLs in opposite directions, the net phase shift will be the difference between the resolution of both PLLs. With a linear combination of shifts, the resolution can be further improved to the greatest common divisor of their individual resolution.
Results show that SAMBA can provide phase-shift steps as low as 100 fs, which is the limit of our measurement. The mathematical model, however, predicts that it can be arbitrarily low. Indirect measurements by averaging multiple steps demonstrated that an average resolution as low as 1 fs can be achieved.
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