Abstract: One or more digital filters in the output of an echo detector in a radar or sonar system are periodically checked for correct operation by the application of a test signal to their input and the comparison of a resulting output signal with a predetermined response code stored in a read-only memory. If the comparison fails, a visual indicator is actuated; if the defective filter is one of a plurality, it is blocked and the gain of the echo detector is adjusted to compensate for the reduction in signal level. With two filters in respective circuit branches carrying detected echo signals in relative quadrature, these filters are tested alternately; failure of either of them may cause its automatic replacement by a standby filter.