Fast low-cost digital lock-in.

A fast low-cost digital lock-in has been developed for photon counting systems. Rates up to 5 MHz will be accepted by the input. The number of counts for positive and negative modulation are subtracted, and the result is emitted as a constant chain of pulses with a frequency of 5 MHz. The modulation frequency for gating the countup-countdown function is limited only by the TTL devices. For the developer's purpose 36 Hz has been used.