B.S.T.J. Briefs: A Microprocessor-Based Automatic Frequency Controller

This paper describes a microprocessor-based (MAC-8/MAC-Tutor) automatic frequency controller (AFC) that has applications to satellite communications systems. 1 Since satellite orientation to the sun varies over a 24-hour period, the frequency of a source on a satellite is susceptible to diurnal thermal effects. These effects result in a characteristic variation in frequency which is both periodic and repeatable. Loss of signal on satellite links occurs primarily because of severe attenuation resulting from heavy rains along the satellite-to-earth path. When a loss of signal occurs the satellite source can drift outside of the receiver pull-in range. When this happens, the overall outage time is extended by the time required to search for and relocate the satellite frequency. If the AFC has some knowledge of the frequency variation, the time to reacquire the lost signal can be minimized and the overall outage time reduced. The AFC described here reduces the uncertainty in the knowledge of the frequency of a source after a loss of signal. This uncertainty reduction is accomplished by using a microprocessor to estimate a current frequency from the last-known frequency and the rate of frequency change over a fixed time period from a previous day. Reducing the frequency uncertainty minimizes the time to reacquire the source after the lost signal reappears. The algorithms also search in frequency, track a varying frequency, and verify the acquisition of a valid signal. 2007