Spur-free, distortion-free reference clock. (News Briefs).

New generations of high-resolution Doppler radar rely on advanced digital-imaging and object-identification techniques. To test high data rate, high-fidelity digital signal processors in this application, the reference clock must have extremely low random noise and no spurious signals or half-period distortions.

NIST in Boulder has designed and built ultra-low-phase-noise reference clocks operating at 10 MHz and 100 MHz in which periodic and quasi-periodic spurious signals and half-period distortions are lower than any other reference source ever demonstrated. The performance of these clocks approaches the broadband phase-noise level set by thermal noise.

Jitter on a reference clock degrades conversion of radio signals to the digital-code domain. Small spurious signals and clock distortions cause spurious code responses and distortions in the conversion process, which can be far more problematic than random noise. We have recently shown how discrete lines due to external influences, such as electromagnetic interference, vibration, and power-supply ripple, translate to jitter. These low-frequency lines, rather than broadband phase noise, can be the dominant contributor to jitter, even for operations involving very high-rate (microwave-bandwidth) digital signals, where low-frequency modulation noise traditionally has been ignored as a concern.

We have demonstrated that jitter analyzers cannot sort out these lines, while phase-noise measurements can. Through a contract with industry, new filters were developed to reduce reference-frequency harmonic distortions, and these have been primarily responsible for the improved performance. However, further improvement was achieved through powering the systems from separate isolated batteries, from a more compact design that reduces pickup of external signals, and from the addition of electromagnetic interference shields.

CONTACT: David Howe, [phone]; [e-mail].