AbstractA frequency‐locked RC active ladder filter is described, which is frequency‐locked to a sinusoidal control tone. The tunable ladder filter is used as a frequency discriminator by using two of the filter branch voltages to generate a control voltage. The control voltage is then used to tune or frequency‐scale the ladder filter transfer function to lock the filter to the frequency of the control tone. A simple linear frequency‐signal control system is developed to describe the frequency‐locked performance of the adaptive filter. The frequency‐locked filter can be designed so that the sinusoidal control tone is rejected at the filter output, making it possible to adapt the filter transfer function during transmission of a message. A practical frequency‐locked filter is given in which a frequency‐locked third order elliptic lowpass filter is designed. The performance of the practical filter is analysed using a simple linear contr