LC-oscillator as a clock for data conversion

Following the ever-improving performance of data converters, the accuracy of the clock that is sampling the data converter, needs to improve at the same pace. The clock accuracy is especially a problem for high-resolution, wide bandwidth converters. The aim of this project is to provide for a next-generation high-performance clock as a library block for the Process & Library Technology (PLT) group of CTO (formerly AMoS). This clock should provide a maximum output frequency of at least 2GHz with only a few ps of jitter while operating in a mixed-signal environment. Since the design is intended as a library block, boundary conditions are flexibility (for example tunability of the output frequency), robustness with respect to interference, variations in PVT, etc. and portability to new technologies. Conclusions: In the existing PLL, the current-controlled oscillator (CCO) was identified as the major opportunity for lowering the interval jitter of the PLL (i.e. the 1/f part of the phase noise of the output clock). Hence, this TN analyzes the use of an alternative oscillator based on an LC-tank. This LC-oscillator is based on an existing design by Nenad Pavlovic. It is designed in a 90-nm CMOS technology. The analysis leads to the following conclusions: • The LC-oscillator is tunable from 4.9GHz to 6.6GHz. The high initial output frequency enables a sufficient tuning range at lower frequencies after division. By using only integer division ratios and making use of the tuning possibilities, any frequency below 1.68GHz can be synthesized. • The phase noise of the LC-oscillator is characterized by the number L=-189dBc/Hz. Assuming a PLL bandwidth of 1MHz, the LC© Koninklijke Philips Electronics N.V. 2006 iii PR-TN 2006/01062 Unclassified oscillator would contribute only 0.1ps of interval jitter. This is about 60 times less than that of the ringoscillator. Part of the improvement is due to a 10 times larger bias current; i.e. 2mA. The major part of the improvement is due to the quality factor of the LC-resonator. • The non-linear varactor characteristic causes up-conversion of noise and interference. Hence, the performance of the LC-oscillator is ultimately limited by the varactor quality. • The tail current of the oscillator determines the oscillator swing. The oscillator swing, in its turn, “averages” the varactor value. As a consequence, the oscillator frequency can be tuned by varying the tail current. This tuning mechnanism allows for a straightforward integration of the LC-oscillator in the existing PLL architecture. • The effect of supply noise and other interference is largest when they couple into the tune pin of the varactors. Since the oscillator is refererred to ground, these pins as well need to be decoupled to ground in order to keep the interference as a common-mode signal. For the same reason, the inverters that set the bank of digital capacitors need to be connected to the analog supplies as well. iv © Koninklijke Philips Electronics N.V. 2006 Unclassified PR-TN 2006/01062