To measure a signal’s noise level, phase noise or jitter can be used. The choice depends largely on the application, although jitter can be calculated from phase noise if the phase-noise characteristic is known. In a five-page white paper titled, “Designing for Exceptional Phase Noise and Jitter Performance in Precision GPS Frequency Standards,” Precision Time and Frequency, Inc. provides a concise overview of the steps and considerations needed to design a high-performance, low-noise, precision-frequency standard.
The first step is to select the right crystal oscillator. A “good” oven-controlled crystal oscillator (OCXO) will have a phase-noise characteristic of at least −125 dBc/Hz at 10 Hz offset from a 10-MHz carrier. At higher offsets, phase noise will generally fall linearly to a noise floor of less than −160 dBc/Hz at 10 kHz offset from a 10-MHz carrier. In evaluating phase-noise specifications, it is critical to keep in mind the carrier frequency at which the measurement is quoted, as there is a direct mathematical relationship between carrier frequency and measured phase noise.
The next task is to preserve the attained low phase noise when buffering and distributing. In addition to filtering the power supply at the input to the printed-circuit board (PCB), filtering should be placed at the supply pins to the buffer amplifiers. When it comes to amplifying and distributing low-phase-noise RF signals, current feedback amplifiers are sufficient. However, the layout must be correct to ensure desired performance. For this application, bandwidth limiting is required. Capacitance to ground at the inputs and outputs also must be minimized as much as possible to avoid instability. With these tips, the white paper provides a solid foundation for designers working in this segment.
Precise Time and Frequency, Inc., 50L Audubon Rd., Wakefield, MA 01880; (781) 245-9090, FAX: (781) 245-9099, www.ptfinc.com.
