1. Field of the Invention
This invention relates generally to voltage-controlled oscillators (VCOs), and more particularly, to delay-based VCOs which are resistant to noise fluctuations in the supply voltages and may be used in phase-locked loop (PLL) clock generator.
2. Description of the Related Art
Voltage-controlled oscillators (VCOs) generate an oscillating signal, in which the frequency is determined by a control voltage received by the VCO. VCOs are one of the major components in phase-locked loops (PLLs), among other basic circuits. PLLs, in turn, are a staple building block for many other circuits, including synchronization circuits, circuits for generating local oscillators, and circuits for modulating and/or demodulating signals. As the requirements on these circuits become more stringent, the corresponding requirements on VCOs also become more demanding. For example, a high performance PLL may require very low clock jitter at its operating frequency.
A common VCO design, which we shall refer to as delay-based VCOs, generates the clock by repeatedly waiting for a half-period delay (180.degree.) and then toggling the clock output (going from 0 to 1 or 1 to 0). Delay-based VCOs typically comprise a delay-line oscillator (including multiple delay cells) and a conditioning circuit. Delay-based VCOs are commonly used in part because circuit loops (such as PLLs) that include such a VCO are typically easy to stabilize.
Noise fluctuations in the supply voltage (supply noise), however, can introduce fluctuations in the oscillating signal produced by a conventional delay-based VCO. In particular, such supply noise typically causes fluctuations in the current from a current source in each conventional delay cell and hence in the delay time of each conventional delay cell. This results in frequency fluctuations in the oscillating signal.
Furthermore, such supply noise typically causes variations in the bias current provided by a conventional reference current generator for the conditioning circuit. This results in phase variations in the oscillating signal.
Conventional delay-based VCOs do not generally meet the stringent requirements of recent high-performance systems that operate at very high frequencies. This is in part because of their lack of sufficient immunity from supply noise. Thus, there is a need for a delay-based VCO which generates an oscillating signal that is substantially unaffected (in terms of frequency, phase, and/or amplitude) by supply noise.