Satellite Communication and GPS Frequency Reference for Earth Stations

Large earth stations host communication systems that provide mission-critical, high-bandwidth services to multiple customers. Within these earth stations are many frequency-based devices with independent local oscillators that generate and convert carrier frequencies, and modulate and demodulate data. Locking these oscillators to a common, stable and low-noise frequency reference distribution system is fundamental to minimizing frequency issues and supporting higher data rates. This application note focuses on the important role and benefits of a frequency reference distribution system, design considerations, and best practices to deploy a system that meets large earth station reliability and quality-of-service requirements.

GPS Time and Frequency Standard

A GPS time and frequency standard locks to the atomic-clock-based, GPS satellite signals to produce a frequency reference with excellent accuracy and stability. When equipped with a special, oven-controlled-crystal-oscillator (OCXO), reference frequencies are generated with ultra-low phase noise. The GPS time and frequency standard provides a common reference to the up-converters, down-converters, modems, and encryption devices to ensure they operate to earth station frequency specifications.

High Reliability Frequency Reference for Large Earth Stations

Large earth stations support require a higher level of reliability and redundancy. For a high-reliability frequency reference, redundant GPS time and frequency standards are combined with a distribution system. The distribution system monitors the status of the GPS references and automatically switches to the healthy system when a fault is detected. The distribution system supplies multiple "copies" of the selected frequency reference to earth station equipment to meet quality of service requirements.

The FDC can be monitored and controlled locally via a dedicated RS-232 serial port and/or remotely via an optional network interface port. The FDC monitors all the output ports as well as the inputs for signal presence and asserts an alarm if a fault is detected. The status of all ports is easily viewed via the communication interfaces, therefore system operators can monitor the status of all distributed frequency references from one central location. The status of the inputs, outputs, power supplies, and alarms are also displayed for easy viewing on the front panel LEDs.