The Boxcar Integrator

The boxcar integrator is a device which is able to recover the waveform of a repetitive signal or measure the amplitude of a repetitive pulse buried in noise. The major advantage of using a boxcar integrator to measure lifetimes is the relatively low initial cost of the system. The boxcar is basically a sample-and-hold system with two modes of operation, single point and scan, which are used respectively for pulse measurement and waveform retrieval. The time of sampling is determined by a reference pulse that is 

There are a number of potential pitfalls in the operation of a boxcar integrator and therefore a brief discussion of some of the operating principles is appropriate. A commonly used boxcar is the PARC Model 162 which, together with its associated plug-in modules, will be used as an example in the following discussion. 

Since the off time is normally much larger than the on time, the source frequency f = 1/(T 4- AD) is closely approximated by f= 1/T. Therefore, eqn. (8) can be written 

From eqn. (10), it can be seen that the narrower the aperture the better the SNIR will be. However, the experiment time in single point analysis varies inversely with aperture time and in scanned operation it varies inversely with the square of the aperture time. Thus, for example, in order 

If a narrow aperture is gradually scanned over a repetitive input signal, with the scanning rate sufficiently slow to allow the integrator to respond fully to every point of the input waveform, the entire input signal will be reproduced at the output with a resolution dependent on the aperture width. The processed output will be a replica of the averaged real time waveform slowed down by the scan rate and can be displayed on an oscilloscope or X—Y recorder, etc. 

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With aperture times available from lOOps to 550 ps, the boxcar integrator can be applied to the measurement of a wide range of lifetimes. However, there can be problems associated with the use of a boxcar to measure lifetimes. Some of these, together with some solutions, are discussed below. 

The boxcar integrator needs repetitive waveforms because it samples each time only a small time interval Ar of the input pulse and composes the whole period of the repetitive waveform by adding many sampling points with different delays. For many spectroscopic applications, however, only single-shot... 

The boxcar integrator measures the amplitudes and shapes of signals with a constant repetition rate integrated over a specific sampling interval At. It records these... 

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