Improved mutual injection locking range for VCOs in a coupled oscillator system

Coupled oscillator arrays (COAs) have been introduced as a simple, low cost method to achieve spatial power combining with the flexibility to generate the phase shifts necessary for beam steering applications. One difficulty in fabricating large scale COAs is the need for tight fabrication tolerances in order for the oscillators to lock to a single frequency. When an oscillator array is used for beam steering, small variations in the free-running frequency among oscillator cells lead to large changes in phase shift. If the oscillator cells manifest wide oscillator locking range the tolerance to frequency difference is maximized. This paper describes the theory of coupled oscillator arrays and identifies the relationships that dictate the interaction between injection locking range, cell-to-cell frequency deviation, and phase error. A method is described for maximizing the locking range in oscillator cell design. The influence of the coupling network on oscillator interaction is discussed, and measured results for small fabricated arrays are presented. Measured results show oscillator cell designs that attain locking range as high as plusmn6%