The method of correlation is a technique well known from radar technology. Part of the Doppler signal is delayed by a known time t: this is equivalent to adding a phase equal to the frequency multiplied by t. The two signals, the original and the delayed one are compared in a cross-correlator which measures the phase difference from which, dividing by the known delay, the frequency is obtained. Correlators are simple to use and, because of their low cost, are the preferred choice in industrial applications. Period counters

When the number of signals per unit time is low, a high speed analog – digital converter can be used. The output signal of the photomultiplier is amplified, sent to a band-pass filter, digitized by the analog-digital converter and sent to a computer.

If the signal has only one frequency, the time between two zeros, which represent crossings of dark fringes (Figure 4.12), is the period. In order to minimize measurement error, the meter is programmed to count a number of zeros, and the period is obtained by the measured time divided by the number of zeros.

The computer checks the validity of each signal by comparing the time intervals between successive zeros (typically time intervals of 5 and 8 zeros). Only if these times give equal periods within the limits chosen, and only if the global number of zeros is not much smaller than the number of fringes contained in the control volume, is the signal accepted as valid. With these controls it is possible to detect signals disturbed by noise or signals generated by highly oblique particles passing through the control volume or those produced by two particles crossing this volume simultaneously.

Note that periods are measured and not frequencies (number of zeros in a fixed time); Figure 4.22 clearly shows the kind of errors made when measuring the frequency: if the measurement time is too small compared to the duration of the signal, few oscillations and fractions are included in it and therefore the error is large; if the measuring time is larger than the duration of the signal, the error is made of dividing the number of zeros for the gate time, which also includes a part when the signal is absent. As the length of the signal depends on the speed of the particle and the direction in which it crosses the measuring volume, the fixed time counting technique could only be used in quasi-stationary streams.

The period counter was the second system used in commercial anemometers, after the frequency tracker. Both systems are currently used for simple educational applications.

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