Systems for signal analysis

The various systems of analysis are based on the following logics:

■ direct measurement of the frequency through the use of narrow band filters;

■ measurement in the time domain (period counters);

■ measurement in the frequency domain (frequency spectrum).

4.3.7.1 Bank of filters

This is a very effective but a very expensive method: it needs a lot of filters connected in parallel, each corresponding to a different frequency. It is obvious that there will be an output signal only from the filter whose frequency is fD (Figure 4.20). Counting the number of signals passed by each filter, a histogram of the kind of Figure 4.9 can be drawn.

Since each filter requires a certain time (or a number of cycles) to decode the signal, it can immediately be seen that the great advantage of this system is speed: typically it can analyze in a given time a number of

signals 50 times higher than those processed by a tuner that reviews in passing the different frequencies one after the other. Because of the high cost, this system is used only when necessary, as for example in the analysis of supersonic turbulent flows.

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