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www.thinkSRS.com Signal Recovery with PMTs Application Note #4 Photon Counting, Lock-In Detection, or Signal Recovery Methods Boxcar Averaging? Which instrument is best suited for detecting signals from a photomultiplier tube? The answer is based on many factors Gated Lock-in Integrator Amplifier including the signal intensity, the signal's time and frequency distribution, and the various noise sources and their time 7 dependence and frequency distribution. 10 Log Count Rate In general, the choice between boxcar averaging (gated integration) and lock-in detection (phase sensitive detection) is based on the time behavior of the signal. If the signal is Photon Counter fixed in frequency and has a 50 % duty cycle, lock-in detection is best suited. This type of experiment commonly uses an optical chopper to modulate the signal at some low .01 frequency. Signal photons occur at random times during the -9 'open' phase of the chopper. The lock-in detects the average 10 Log Duty Cycle 1 difference between the signal during the 'open' phase and the background during the 'closed' phase. the statistical noise of the Poisson counting distribution. The analog instruments degrade the SNR due to input noise. To use a boxcar averager in the same experiment would require the use of very long (50 % duty cycle) gates, since the This applications note begins by discussing photomultiplier photons can arrive anywhere during the 'open' phase. Since tubes and how to optimize their performance. The following the gated integrator is collecting noise during this entire gate, sections discuss the SNR of the various signal recovery the signal is easily swamped by the noise. To correct for this, methods. Techniques are described which can extend the active baseline subtraction can be used, where an equal gate analog instruments into the 'photon counting' regime. measures the background during the 'closed' phase of the However, these techniques have limits beyond which photon chopper and is subtracted from the 'open' signal. This is counting is preferred. Experimental data illustrating the identical to lock-in detection. However, lock-in amplifiers are achievable signal-to-noise ratios is presented. much better suited to t