The origin and suppression of limit cycles in digitally controlled RF PA loops

The origin of limit cycles in digitally controlled RF PA loops is analysed using the theory of Describing Functions and is shown to result from a code dependent gain caused by DAC quantisation. The effects of the limit cycle in terms of spec compliance for the particular case of the GSM protocol are examined. It is shown how compliance can be maintained even in the presence of limit cycles by carefully specifying the control slope of the PA. Finally, in the case where the PA control slope cannot be constrained, it is shown how the introduction of an intentional nonlinearity into the loop can be exploited to suppress the limit cycle.