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Much of the recent research in millimeter-wave quasi-optical systems has been directed at developing reliable solid-state power combiners. The grid oscillator developed by Popovic et al. (1991), is based on integrating active devices directly into a planar grid and has been used to generate powers as high as 10 Watts at 10 GHz. Essentially, the grid structure is a frequency selective surface whose period is only a fraction of a wavelength. Because the grid structure is a frequency selective surface, the impedance presented to devices is a strong function of frequency. This means the frequency tuning bandwidths of these arrays can be quite narrow, a distinct disadvantage for systems that require a broad tuning bandwidth. We present a new grid oscillator configuration in which varactor diodes are integrated directly into a planar array of FETs. The varactors are used to electronically tune the embedding impedance presented to the devices and, thus, the oscillation frequency. We investigate the frequency tuning of an X-band common gate grid oscillator using packaged varactor diodes. Simulations with the grid equivalent circuit model show good agreement with measurement and indicate that the tuning bandwidth can be increased considerably by reducing the minimum capacitance of the varactors. Such a reduction in capacitance can be realized through the use of chip devices and by using a single varactor tuner for each FET. In addition, the varactors could potentially be used to control the relative phase distribution across the grid, resulting in a beam-steerable quasi-optical source.