The present invention relates generally to feedback control systems and more particularly to a circuit arrangement for use in controlling the operating mades of the feedback controlled system.  The invention is particularly suitable for application to automatic frequency control systems. 
The use of automatic closed-loop control is widespread in various sectors of modern technologies.  For purposes of monitoring the controlled variable of the system to control such variable to within a predetermined limited range, or other purposes, level detectors have heretofore been used in the detection of an error signal which indicatesthe deviation of the controlled variable from the desired value.  However, the amount of such deviation which is generated during the closed mode operation is much smaller than the amount of deviation generated when the system is in an open mode or disabled condition.  Therefore, the usual practice is to employ a first level detector having a small detectable range for operation in the closed loop mode and a second detector having a larger detectable range for operation in the open loop mode.  This entails the use of associated amplifiers and switching circuits which are manually operated in response to the operating mode of the system.