Novus Frequency Reference Comparisons
All frequency references start out with a primary source. That source could be a TCXO (temperature compensated crystal), OCXO (a crystal temperature stabilized in an oven), a GPS locked source, Rubidium source or an OCXO- locked to a Rubidium source. Key performance areas are temperature stability, long-term aging, phase noise and naturally-cost. Stability over time and temperature are self-explanatory. Phase noise deals with the short-term stability. Phase noise can be converted to jitter if desired. Jitter/Phase noise becomes a critical parameter in communications systems and some measurement applications.
TCXO– This is a special cut crystal that includes special circuitry to monitor temperature and corrects the crystal resonant frequency based upon temperature variations. This is based upon a temperature model of the crystal and affords temperature stability in the single digits ppm. Moderate phase noise performance. This is an economical choice if the performance demands are not too stringent and cost is a major consideration.
OCXO– By being held at a constant temperature, the temperature stability and aging stability improve dramatically (orders of magnitude). Phase noise improves as crystals with excellent phase noise but not as temperature stable can be used since the temperature is being held constant.
GPS- Locked Reference– The GPS transmitted information includes a timing signal PPS (one pulse per second) this signal is accurate in the range of 10 ns. A simple way of understanding the GPS locked product is that it has an internal frequency counter gated by the GPS timing signal to measure the crystal output and adjust the resonant frequency for errors. The algorithms for this process are very complex. This configuration provides outstanding long-term performance (10E-11). When the GPS signal is lost the unit will revert to an OCXO.
Rubidium sources are atomic clocks where core timing information is derived from an electron energy state transition. These products provide outstanding long-term stability (single digit ppb/year) with moderate phase noise performance.
Rubidium/OCXO -This configuration takes the best attributes of the OCXO (phase noise) and the long-term stability of the Rubidium. Frequently used in demanding applications such as satellite communications systems.