Continuous and switched-capacitor multiphase oscillators

This paper presents novel designs of multiphase oscillators. These oscillators generate
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symmetric signals, i.e., equal in amplitude and equally spaced in phase, which have a wide range of applications in communications and signal processing. First, multiphase oscillators based on active sequence discriminators (SD's) are proposed. Then multiphase oscillators based on the SD and two-integrator-loop circuits containing only capacitors, switches, and op amps are derived. In addition to the fact that these switched-capacitor oscillators (SCO's) are easily constructed in IC form using MOS technology, they are shown to possess other attractive features. The input to the switched-capacitor (SC) circuit is a clock and the output is a sampled-and-held sinusoid of a frequency that is related to the clock frequency by an arbitrarily chosen capacitor ratio. The oscillation frequency follows linearly a continuously varying clock. Thus there is no need to vary any of the oscillator components to change the oscillation frequency. Ultralow oscillation frequencies can be generated using practical component values. The harmonic content can be made very small by increasing the number of samples in each oscillation cycle. Also, the amplitude stabilization and control circuitry with excellent settling properties are presented. It is shown that the complexity of the amplitude stabilization circuitry does not increase with the number of oscillator phases in the SD oscillator design. Experimental results using both the continuous and SC realizations are given which are in agreement with the theory presented.