Effects of Channel Length Fluctuation on the Performance of RF Oscillators

Abstract

<p>As the dimensions of the transistors keep decreasing, the effects of transistors' dimensions fluctuation will become increasingly important for differential radio-frequency (RF) integrated circuits (ICs). In most RFICs, the transistor's channel length is usually much smaller than its channel width. Therefore, small unintentional changes in the channel length due to unavoidable fabrication process variations can cause a large circuit performance deviation, and thus, the effects of channel length fluctuation are extremely important.</p> <p>This thesis investigates the effects of channel length fluctuation on the performance of RF oscillators. Two fully integrated, cross-coupled, differential LC oscillators have been designed in triple-well 0.18 J.lm CMOS technology and used as the test circuits. The measured results of the first VCO has a relatively large frequency tuning range of ~25% because two pairs of varactor are used and a power consumption <em>P</em>_con less than 1.5 m W with 1.5 V supply voltage. Also, it has an output power <em>P_out</em> of -13.3 dBm and a <em>P_con</em> of 1.31 mW at an oscillation frequency <em>f_o</em> of 5.5 GHz, which corresponds to the best phase noise performance of -121.2 dBclHz at 1 MHz offset frequency.</p> <p>The simulated results of the second oscillator has a <em>f_o</em> of 1.12 GHz, a <em>P_out</em> of 1.79 dBm and a <em>P_con</em> of 6.479 mW with 1.8 V supply voltage. Both the <em>f_o</em> and <em>P_out</em> are found to be decreased (increased) when the fabricated channel length is larger (smaller) than the one at the design and simulation stages. The maximum variation of <em>f_o</em> and <em>P_out</em> are 2.45 kHz and 0.08 dBm, respectively, with less than 20% of variation in the channel length. Formulae have been derived to predict these variations. Also, the use of body bias on the transistors is proposed as a means to compensate for the changes in the frequency performance characteristics due to the channel length fluctuations. In addition, it is found that the variation of <em>f_o</em> and <em>P_out</em> due to channel length fluctuations is more significant in oscillators with higher <em>f_o</em>. A recommendation is given to designers of how to design an oscillator with high <em>f_o</em> while minimizing the sensitivity of <em>f_o</em> and <em>P_out</em> to channel length fluctuations. While the phase noise is expected to be affected by the channel length fluctuations, simulation result does not show a significant dependence of the phase noise on channel length. This discrepancy is believed to be caused by the inaccuracy of the phase noise simulation.</p>