Wireless transmitters and receivers have become ubiquitous in modern society. While radios and televisions have been commonplace for many years, the explosion of cellular telephones and other mobile terminals has created an overwhelming demand for wireless transceivers. Most such transceivers have a need to switch between frequencies. For example, in a cellular system, the base station may assign the cellular telephone to a particular frequency channel and time slot on which to communicate. The frequency channel lies within the total range of frequencies available for communication within the applicable standard. At a later time, the base station may assign the cellular telephone to a second, different frequency on which to communicate.
To effectuate changing the frequency channel such as when a base station assigns a cellular telephone to a new channel, most such transceivers include a frequency synthesizer. The frequency synthesizer is used to create a carrier frequency that is mixed with the baseband signal so that the signal may be transmitted at the appropriate frequency. Likewise, the frequency synthesizer creates a signal, which, when mixed with a received signal, downconverts the received signal to baseband or an intermediate frequency where further processing may be performed. Frequency synthesizers rely on variable frequency oscillators to help create the desired frequencies.
Variable frequency oscillators have myriad other uses besides their use in a mobile terminal frequency synthesizer. Regardless of usage, most conventional variable frequency oscillators are integrated resonant inducto-capacitor based (LC) oscillators with analog varactors. A varactor is a capacitor whose capacitance changes in a controlled manner in response to a control signal, typically an analog voltage. The varactors are controlled to vary the capacitance of the LC oscillator, and thus control the frequency of oscillation. However, these analog varactors alone do not achieve the tuning ranges required for most transceiver systems. Some conventional oscillators supplement the analog varactors by using a bank of binary weighted capacitors to control coarse tuning. However, this supplemental bank of capacitors requires a state machine to run a coarse tuning algorithm. Further, as integrated circuit geometries and supply voltages shrink, especially in mobile terminal environments, the analog voltage over which the frequency synthesizer operates also shrinks, requiring complicated architectural changes.
In short, the miniaturization of integrated circuits and the gradual erosion of supply voltage levels necessitate alternate strategies for providing controllable oscillators for use in transceivers.