The Colpitts and Clapp oscillators are well known for having good properties in the communications field. However, at ultra high frequencies (above about 500 MHz) the tuning range, the tuning linearity, power output flatness, and noise performance become greatly degraded. The current state of the art has been to accept a limited tuning range and then use multiple oscillators to cover wide bands and to use filtered and amplified harmonics of lower frequency oscillators to achieve acceptable power flatness and phase noise performance.
The Colpitts oscillator is described in U.S. Pat. No. 1,624,537. Bell U.S. Pat. No. 2,225,897 discloses a parallel resonant tuned circuit between the grid and plate of a vacuum tube and a capacitor between the grid and one side of the tuned circuit but does not have an impedance in the feedback loop to control the phase shift. Blum U.S. Pat. No. 2,756,334 discloses a parallel resonant tuned circuit between the grid and plate of a vacuum tube and a capacitor between the grid and one side of the tank circuit but, like Bell, has no way to control the phase shift in the feedback loop for the tank circuit.