A software receiver adaptive phase lock loop method

Abstract

The aim of this paper is to introduce a new and relatively simple algorithm which is capable to tune the parameters of the loop filter in a phase lock loop, like the one that can be found in a global navigation satellite system receiver. The parameters are chosen while the system is running, in order to make the bandwidth of the filter adaptive with respect to the signal dynamic and strength. A wider bandwidth is obtained to make the filter more reactive when searching the carrier, while the bandwidth gets narrower when the system is in the tracking state, in order to cut off as much noise as possible. The algorithm is based on the observation of the estimated variance of the phase lock loop discriminator output. This variance is computed by means of a circular buffer and it is compared with a threshold when the buffer is full, in order to keep the bandwidth constant or to reduce it. Nonetheless, the algorithm avoids carrier unlock events due to an excessive reduction of the bandwidth, because the frequency controls are compared with the current noise equivalent bandwidth of the system, and it provides to widen the bandwidth when the signal dynamics requires it. Simulations prove that the method fits well an automotive environment and it can also be used for higher dynamic scenarios, if the parameters of the system are set opportunely.