A double-oven OCXO (oven-controlled crystal oscillator), the FDE/FDD505AH, is said to offer the best ratio of stability to size and power consumption on the market.
It uses stress-compensated cut resonators for stability and low-power operation. Power rating is a maximum of 4 W at turn on and just 1 W during use at 25°C while providing ±0.5 PPB stability. Aging is better than ±0.2 PPB/day. The device produces rectangular output and is available in standard Euro (FDE) or 25-mm (FDD) packages.
Fox Electronics, 5570 Enterprise Pkwy., Ft. Myers, FL 33905, (888) 438-2369, foxonline.com