20.08.2015   Handbook Nuclear Terms

FREQUENCY ANALYZERS

6- 6.1 Usage

In the preceding section analyzers suitable for handling data in the time domain were considered. These analyzers

are used mostly for correlations or for obtaining transfer functions from single-frequency excitation However, there is an alternative approach, used primarily foi data having a continuum of frequencies, in which the various frequencies in the signal are separated and analyzed in the frequency domain Correlators seek the amount of correlation at various time intervals, whether within one signal or between two signals Frequency analyzers, on the other hand, seek the relative amounts of various frequencies in one signal or existing in a related fashion in two signals In both approaches Table 6.1 is used to compute the corresponding functions in the time or frequency domain from those functions which are obtained by experiment

The analyzers discussed in this section are most often used for directly obtaining the spectral power density, Px(f), and the cross spectral power density, Pxy(f) This implies that, in the system being analyzed, a continuum of excitation frequencies exists and has been generated by either internal or external excitation sources In manv instances the spectral power density contains the required information about the dynamics of the reactor system In other instances both Pxy(f) and Px(f) are determined, and then, by using Eq 6 20, the transfer function between x and у is obtained

 

SIGNAL TO BE ANALYZED x(t)

 

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P(f,) THE TIME-AVERAGED (OVER A TIME T) VALUE OF THE D-C COMPONENT OF THE INTEGRATOR INPUT

 
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Fig 6 15—Operation of the three essential elements (filter, detector, and averager) in a spectrum analyzer

 

resolution band, Af, then it is desirable that B(f) resemble the ideal filter of Fig 6 16 as closely as possible (This is discussed further in Sec 6-6 4 )

Spectrum analyzers that have been used in reactor experiments can be classified as follows specially designed and constructed,62 based on commercially available analog-

 

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DEVIATION OF FREQUENCY FROM BAND CENTER

Fig. 6.16—Shapes of some spectral windows used in spectral analysis Here rm is the maximum lag used in the correlation function associated with the spectra

 
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(6 37)

 

where B(f) is the spectral-window function of the analj/er If P(f) is a function varying substantially within the

Подпись: Fig. 6.17—Operation of the four essential elements in a cross-spectrum analyzer

computer components,48 and a commercially available spectrum analyzer 11 7 Some spectrum analyzers use elec­trical filters resonant at the desired frequency, fj, others use a heterodyne technique in which the signal modulates a high-frequency carrier, fc, that is subsequently filtered in a narrow band. Since all these approaches perform the same P(f) determination, such factors as cost, convenience, and availability are usually the deciding factors for ex­perimenters