Fast Time-Constant Circuit

A fast time constant (or FTC) circuit produces time constants of extremely short duration compared with those of conventional circuits. FTCs can be implemented using a low product of resistance and capacitance (RC) or a low ratio of inductance to resistance (L/R). If these circuit elements are of proper magnitude and are properly connected in relation to input and output, the resulting circuit output will be the time derivative of its input.

In radar systems, fast time constants are used in signal processing to reduce the effects of certain types of undesired signals, emphasize signals of short duration to produce discrimination against the low-frequency components of clutter, and protect against interference of either frequency-modulated or amplitude modulated signals. FTCs are also used as anti-jamming device in radar voice amplifier circuits. They differentiate incoming pulses so that only the leading edge of the pulses are used.

Circuits with fast time constants are also used in other applications including electronic computer to obtain fast operating rates in their arithmetic-logic units, and in the electronic portions of fibre optic systems to obtain a better match between electronic and optical bandwidths. The differentiating circuit, i.e., the differentiator, inserted between the detector and video amplifier, will pass individual pulses but not the modulation frequencies.