Constant Fraction Discriminator Involving Automatic Gain Control to Reduce Time Walk

A constant fraction discriminator (CFD) is a widely used time-pickoff circuit because it ideally produces no time walk for signals with equal rise times. However, a practical CFD produces some time walk because of the dependency of the response time in an involved comparator on the input signal size. In this paper, a CFD combined with an automatic gain control (AGC) circuit at the input stage is proposed to reduce this dependence. The AGC has four step gains that are inversely proportional to the amplitude of their input signal. Accordingly, the AGC reduces the dynamic range of the signal entering the CFD and consequently reduces the dependency of the response time on the input signal, i.e., time walk. For a short resolving time, this gain setting is performed at one time by amplitude comparison. The proposed CFD is evaluated by experiment for input signals with a rise time of 2 ns and a dynamic range of 40 dB, and the time walk is reduced to 277 ps, a decrease of more than 42%.