Method of and apparatus for recovering a reference clock

摘要

An apparatus (21) is provided for recovering a reference clock generated by a master clock (22) in a sender (20). The sender (20) sends timing packets over a network (3). The apparatus comprises a controllable slave clock (27) and a control circuit (25, 26, 28) which determines the slave clock error and controls the slave clock (27) so as to reduce the error. The error is determined as a function of (mxN)-Ca(n), where 1 $C_a\left(n\right)=\left(\sum _{i=0}^{q-1}C\left(n-i\right)\right)/q.$ N is the number of cycles of the master clock between the sending of consecutive timing information items, C(r) is the number of slave clock cycles between receipt of the (r-m)th and rth timing information items from the network, m is an integer greater than 0, and q is an integer greater than 1. The control circuit (25,26,28) may alternatively or additionally be arranged to apply a correction Vadj(t) to the slave clock at regular intervals Tadj, and be arranged to apply a gain parameter dependent on the frequency difference between the master clock (22) and the slave clock (27) to each correction.