1. Technical Field
The present invention relates to digital circuits in general, and in particular to voltage controlled oscillators. Still more particularly, the present invention relates to a voltage controlled oscillator having common-mode feedbacks and differential controls.
2. Description of Related Art
A voltage controlled oscillator (VCO) is an extremely sensitive, high-gain analog circuit. For silicon implementations, the common VCO architectures have been ring oscillators and inductor-capacitor (LC) oscillators.
Ring oscillators are relaxation oscillators that do not require resonant elements such as crystals. Although ring oscillators have many advantages such as wide tuning-ranges, small footprints, etc., they tend to have a relatively high phase noise or timing jitter, which is largely caused by the lack of a high-gain tuned element such as an LC tank.
LC oscillators tend to perform much better in wireless and radio-frequency communication systems that require a local oscillator to generate a pure sinusoid signal. However, for wired serial transceivers that depend on the availability of multiple phases of a serial clock to provide clock recovery/synchronization, ring oscillators are definitely preferred over LC oscillators. This is because a four-stage ring oscillator can, for example, readily generate eight phases of a high-quality, high-frequency clock, but the generation of different phases of a high-frequency clock is a much more difficult task for LC oscillators.
The present disclosure provides an improved ring oscillator based VCO with low jitter for wired serial transceiver applications.