An Advanced Synthesized Ultra-Stable Oscillator for Spacecraft Applications
The current ultra-stable oscillator (USO) technology relies on quartz resonators that are highly precise. These resonators are selected based on the desired output frequency and stability, increasing the lead time and expense of each USO. At APL, we are focusing on a frequency-synthesized USO that is based on a standardized, fixed-frequency resonator, and we have developed a synthesized USO that provides a frequency reference for transponders and other instruments on space missions. This USO's frequency reference is stable enough for radio-science and navigation applications and is electronically adjustable to cover the entire deep-space communications band, allowing in-flight reassignment of the transponder frequencies. APL's synthesized USO also offers low mass and low-DC power consumption while maintaining world-class noise performance and frequency-stability performance.
A frequency source with a USO-quality output that is also frequency synthesized has several benefits. Because resonators must go through a lengthy (and costly) screening process to select the best-performing devices, it is impractical to screen and stockpile parts for every communication channel frequency. With a programmable frequency synthesizer, the resonator frequency can be standardized, making the parts readily available in specialized, high-performance configurations, and then it becomes practical to maintain inventory of these resonators and perhaps even a small inventory of the synthesized USO assemblies.
Another benefit of the synthesized output is a reduction in USO lead time. With the traditional system, the lengthy process includes mission design, frequency channel selection, resonator purchase, resonator screening, USO fabrication, and, finally, integration of the spacecraft. With a field-programmable USO output frequency, the resonator and USO tasks can occur in parallel with the mission design and frequency channel selection, greatly reducing the overall lead time.
Finally, with an in-flight programmable USO output frequency, there is greater flexibility in designing multiple missions in close proximity, such as NASA's series of missions to Mars. NASA has several current and future missions to Mars, and more will most likely be conceived. With a fixed turnaround ratio in the transponder and a common uplink frequency, the synthesized USO enables multiple downlink frequencies to be used for simultaneous tracking of multiple vehicles.
The design of the Synthesized Mars Technology Program (MTP) USO must provide an electronically adjustable output frequency with high spectral purity and minimal DC power. The design integrates an ultra-stable 5-MHz oscillator, a frequency multiplier, and a direct digital synthesizer (DDS). For the MTP USO, we have integrated and adapted APL flight heritage technology.
A synthesizer-based USO will enable in-flight reassignment of transponder channels. In addition, the Synthesized MTP USO will allow the use of a standardized frequency resonator and a common USO design for all channels, thus enabling shorter USO lead times for the missions. Technology advances in low-power ovens, lightweight packaging, and DDS architectures have been incorporated into the USO design that will benefit a wide range of civilian and military applications.