An on-chip independent frequency reference circuit for audio applications
Mullins, Austin M.
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A low-cost circuit for generating a fixed frequency (nominally 100 MHz) in a CMOS process is proposed with a simulated accuracy of 0:4% across a very wide temperature range and worst-case phase noise less than -130 dBc/Hz at a 384kHz oset, making it a viable clock source for high-performance digital audio processors. The primary application of this device is to replace expensive crystal oscillators for audio processors that receive only marginal benefit from the extremely high quality of crystal-based references. A number of implementation options were considered and evaluated based on manufacturing cost, absolute accuracy, phase noise, and power consumption, taking into account the fact that automated testing facilities in industry allow manufacturers to calibrate each device after packaging. The topology chosen was a high-frequency LC-oscillator that uses an on-chip spiral inductor in conjunction with an on-chip capacitor bank and temperature compensator to produce a resonant frequency around 1.6 GHz. The output from this core LC-oscillator is then passed through a modulo-16 frequency divider chain, reducing the center frequency to 100 MHz. A circuit model was developed and this circuit was in turn laid out, veried, and extracted in software to determine the eects of anticipated parasitic RC elements. The nal post-layout simulation results suggest that it is possible to achieve 0:4% absolute accuracy after calibration, and that the circuit shows excellent phase noise performance at all simulated process corners.