Prime-Rational Frequency Synthesis Method and Frequency Synthesizers
Frequency synthesizers are an essential element to modern navigation and communications systems, as they provide the means to align and synchronize distributed transmitters and receivers with high signal purity. The degree of synchronization and signal purity is directly proportional to the quality of service offered by optical data networks and wireless communications. When the quality of service is compromised, the economic return of the transmission infrastructure cannot be optimized. Frequency misalignment is directly proportional to positional accuracy determined by a remote navigation system. Inaccuracy in position limits the usefulness of a navigation system and usually incurs higher costs and complexity through the compensation of frequency alignment error in dynamic environments, e.g., GPS acqusition. Secure communications systems that incorporate frequency hopping and PN-code encryption require both accurate and fast (agile) frequency synthesizers. The agility and accuracy of a frequency hopping system directly improves its robustness to jamming, spoofing, and interception. Previous technologies required undesirable trade-offs among four esstential aspects: Range--the magnitude of frequency change derived from the signal reference. Resolution--the accuracy to which the synthesized frequency can be aligned to the desired channel Agility--the spped required for the synthesizer to change from one frequency to the next Spectral Purity -- the fidelity of the output frequency in both noise and unwanted spurious tones.
Using the JHU/APL approach to DFS alleviates these compromises in performance for frequency synthesizer design without taxing system complexity or resources. APL has demonstrated this potential benefit through ultrastable signal source research. DFS offers a significant advantage to frequency synthesizer architecture designes because the output frequency step can be made very small without large "divide by N" prescallers or small PLL reference frequencies. The resulting phase-comparator loop bandwidth frequencies used in DFS are easily filtered (spectral purity) and allow fast frequency change (agility). DFS distributes the frequency resolution among the constituent PLLs and allows for high flexibility in the choice of output frequency step (resolution) and frequency range. Typical DFS structures offer ratios of range over resolution of 4 to 5 orders of magnitude with using only three to four individual PLLs. These individual PLLs can be created from any common selection of commercially available integrated circuits, making DFS architecture conductive to microminiature structures including RF MEMS technology. An exemplary DFS architecture using only four COTS PLLs could operate on a 10 MHz reference signal and provide consecutive frequency step resolution of 100 µHz with a selectable range of at least ±10 kHz. The spectral purity of the synthesizer RF output would maintain spurious signal content below -100 dBc in a range of 1 kHz to 1 MHz from the carrier with no degradation in the spectral purity of the 10 MHz reference at frequency offsets less than 10 Hz.
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