Threshold circuit without hysteresis

摘要

This threshold circuit has two active operating states and one passive failure state. It can be used for determining the position of an alternating voltage to be evaluated, for example a track voltage in a track circuit, with respect to a reference voltage, any disturbances (for example due to component failure) being indicated by the passive state which can be distinguished from the two operating states. The invention makes use of the finding that the instantaneous frequency of an aggregate voltage produced from the addition of two voltages of different frequencies is a function of the amplitude ratio of the added voltages. Accordingly, the threshold circuit according to the invention consists of an adding circuit (AD) in which a reference voltage (U2) with a frequency (f2) which is different from the frequency (f1) of the voltage (U1) to be evaluated is added to the voltage to be evaluated and of an evaluating circuit (AWS) for detecting the instantaneous frequency (f1) of the aggregate voltage (U1) and for comparing this instantaneous frequency with a nominal value. Various variants of the evaluating circuit are the subject-matter of the subclaims. <IMAGE>