A time-pickoff method using automatic gain control for a fast count rate

The response time of a comparator involved in a time-pickoff circuit depends on the overdrive and the slope of its input signal, which is the main cause of a timing error. For eliminating the dependency, a time pickoff method employing an automatic gain control (AGC) circuit renders the amplitude of the input signal to the comparator to be uniform. However, its application is limited to the low count rate measurement due to relatively long delay of the nonlinear bias circuit for controlling the gain in the AGC circuit. In this study, a time pickoff method, which employs an automatic gain control circuit with a shorter delay, is proposed for fast count rate measurements. According to the amplitude, the input signal is divided into 4 steps. Then, the gain in the AGC circuit is set to be same for the input signals of the same step, which is basically inversely proportional to the amplitude. This gain setting is performed at one time by amplitude comparison, which is similar to a flash type analog-to-digital converter (ADC) structure, and has a smaller propagation delay. The performance of the proposed method is analyzed through the experiments for input signals with a dynamic range of 40dB.