US Patent 7173463 Generating multi-phase clock signals using hierarchical delays

Circuits and methods for generating multi-phase clock signals using digitally-controlled hierarchical delay units (HDs) are provided. A plurality of serially-coupled HDs outputs clock signals that are phase-shifted relative to a reference clock signal. Each HD includes either one or two variable delay lines that provide coarse phase adjustment of an associated input signal. Each HD also includes one or more phase mixers that provide fine phase adjustment of the input signal.