Wideband mechanically tunable oscillators, having greater than 20 percent bandwidth, have been routinely constructed. The second or third harmonic output from an inductive radial mode resonant disc, or from a high Q cavity, tunable at both fundamental and harmonic frequencies, results in substantial power levels suitable for most local oscillator applications in the millimeter range extending up to approximately 100 GHz. When low noise performance is required the active device is a Gunn diode. In most applications packaged GaAs Gunn elements are employed. The use of InP Gunn devices promises to extend the fundamental operating range well beyond the upper limit of the W-Band (75-110 GHz) region. Correspondingly useful harmonic power output into the 300 GHz region is predicted from circuits utilizing this more recently developed device. On the other hand, electronic tuning of these sources has been limited to, at best, a few percent. Several configurations of series arranged varactor and Gunn diodes, along with novel applications of radial mode disc capacitors in full and reduced height rectangular waveguides, have led to greater than 10 percent electronic tuning range through the upper end of the V-Band (50-75 GHz) region. Another arrangement of series connected varactor and Gunn devices having quarter wave cylindrical resonators attached to the diodes, and constructed in coaxial cavities, have similarly produced wideband electronic tuning characteristics. Theoretical calculations have been carried out, for circuits using cascoded varactor and Gunn elements in conjunction with a non-linear radial mode disc capacitive transmission line, and compare favorably with measured results up through V-Band frequencies. Similar performance is predicted, for sources arranged in series geometries, well into the millimeter region.