1. Field of the Invention
This invention generally relates to phase-locked loop timing and delay circuitry and, more particularly, to a system and method for providing a timing signal that can be selected from any phase of a voltage controlled oscillator (VCO) output signal.
2. Description of the Related Art
FIG. 1 is a schematic block diagram of a timing circuit using a VCO with a four-phase output (prior art). Conventionally, the four phase outputs are 90 degrees apart. In one aspect, the VCO supplies two differential phase outputs, 180 degrees. One of the phase outputs of the VCO can be used as a system clock or loop acquisition, for example. The phase outputs can also be used for other system timing events that are related to the system clock or VCO output signal. However, if the timing event does not occur at 0, 90, 180, or 270 degrees, with respect to the VCO signal, delay elements must be added to achieve the desired phase relationship. For example (as shown), the 90 degree phase output of the VCO may be directed though logic circuits, such as an inverting buffer. The propagation delay associated with logic circuits results in an additional phase shift, so that a resultant 100 degree phase output is created.
However, the propagation delay associated with logic circuits typically varies with respect to lot and temperature. Thus, the phase of the resultant output signal changes. Even if time invariant delays can be added to a system to generate phase shifts, such delays create a phase shift that is completely dependent upon the VCO frequency. If the VCO is operated at a different frequency, the phase shift associated with the time invariant delay necessarily changes, and the output signal phase shifts.
It would be advantageous if a VCO output signal could be used as a phase-selectable timing signal.
It would be advantageous if the phase of the above-mentioned timing signal could be adjusted to account for changes in the VCO frequency.
The present invention system and method provide a timing signal that is derived from a precisely selected phase of a VCO signal. A multiplexer initially selects one of the VCO outputs, which are differentiated by 90 degrees. Then, the phase of the selected signal is finely adjusted through the use of parallel differential amplifiers. As a result, a timing signal can be generated that varies from approximately zero degrees, to approximately 360 degrees of the VCO output signal.
Accordingly, a method is provided for controlling the phase of a voltage controlled oscillator output. The method comprises: accepting a plurality of VCO outputs coarsely differentiated by phase; selecting one of the plurality of VCO outputs; finely modifying the phase of the selected VCO output; and, supplying the phase modified VCO output.
As mentioned above, it is typical that each of the plurality of VCO outputs be differentiated by 90 degrees. That is, accepting a plurality of VCO outputs coarsely differentiated by phase includes: accepting a first VCO output (I0); accepting a second VCO output (I1), differentiated approximately 90 degrees from the first VCO output; accepting a third VCO output (Q0), differentiated approximately 90 degrees from the second VCO output; and, accepting a fourth VCO output (Q1), differentiated approximately 90 degrees from the third VCO output.
Finely modifying the phase of the selected VCO output includes modifying the selected VCO output in the range between +45 and xe2x88x9245 degrees. In one aspect, the output is modified in 16 discrete steps.
Additional details of the above-described method, and a system for controlling the phase of a VCO output are provided below.