Circuitry is provided for detecting threshold crossings of an input signal (V1), which is generally sinusoidal or otherwise periodic, but which is subject to either high-frequency noise which would cause erroneous multiple threshold crossings, or low-frequency noise which would cause erroneous failure to detect a threshold crossing. Two detection elements (U1, U2) detect the crossing of two different threshold values and produce detection signals reflecting whether the input signal is above or below their thresholds. For noisy input signals, these detection signals may be bouncy. High- (A, C) and low-true (B, D) versions of the detection signals are logically combined (U3, U4) to produce a first signal (E) which is active only while the input signal (V1) moves in one direction, and is masked from the active state while the input signal (V1) moves in the other direction. Likewise, a second signal (F) is produced which is active the opposite way. The first and second signals set and reset a latch (V5), whose output state represents the threshold crossing. Because of the logic masking of activity on the second signal due to noise on the input signal, once the first signal (E) puts the latch (V5) in one state, the latch (V5) is not immediately put back to the other state by the second signal (F). Rather, the latch (V5) remains in the one state until the input signal (V1) changes direction, at which time the first signal (E) is masked but the second signal (F) is enabled. Hysteresis circuitry (R1, R2, C1, C2, L1) allows the two detection elements to operate on different threshold values. Also, low-frequency noise is reduced by making the hysteresis circuitry frequency dependent. Preferably, the circuitry is low-pass (C2, L1), so that the hysteresis effect is reduced for higher-frequency input signals affected by lower frequency noise. <IMAGE>
Anti-glitch system and method for laser interferometers using frequency dependent hysteresis