Noise power spectra commonly

Fig. 8. Noise power spectra commonly found in chemical instrumentation. (A) white"...

F. 8. Noise power spectra commonly found in chemical instrumentation. (A) "white noise (B) flicker (1//) noise and (C) interference noise. (Reprinted with permission from Ref [36].)... 

More detailed information can be obtained from noise data analyzed in the frequency domain. Both -> Fourier transformation (FFT) and the Maximum Entropy Method (MEM) have been used to obtain the power spectral density (PSD) of the current and potential noise data [iv]. An advantage of the MEM is that it gives smooth curves, rather than the noisy spectra obtained with the Fourier transform. Taking the square root of the ratio of the PSD of the potential noise to that of the current noise generates the noise impedance spectrum, ZN(f), equivalent to the impedance spectrum obtained by conventional - electrochemical impedance spectroscopy (EIS) for the same frequency bandwidth. The noise impedance can be interpreted using methods common to EIS. A critical comparison of the FFT and MEM methods has been published [iv]. 

Error correcting codes have been applied to a variety of communication systems. Digital data are commonly transmitted between computer terminals, between aircraft, and from spacecraft. Codes can be used to achieve reliable communication even when the received signal power is close to the thermal noise power. As the radio waves spectrum becomes ever more crowded, error-correction coding becomes an even more important subject because it allows communication finks to function reliably in the presence of interference. This is particularly true in military applications, where it is often essential to employ an error correcting code to protect against intentional enemy interference. 

If the spectra Zi(cu) and Z2(tu) of the two sensors outputs are contaminated with instrumental noise (a noncausal output), a simple ratio Z2/Z1 may yield a biased or unreliable estimate of the response. By using only the correlated part of the signals, i.e., the part due to the common input of both sensors, the ground noise in this case, a more reliable estimate, will be obtained. This may be done by using a known relation between input and output of linear and causal systems the input-output cross-spectrum is the product of the transfer function and the input power spectrum (see, e.g., Ljung 1999). Consider a linear system whose input is the output of seismometer 1 (Zi) and its output is the output of seismometer 2 (Z2). This system would have a transfer function A2M1 = P2i(oj)/Pii(ffl)>... 

Since a nuclear reactor is a statistical system, it will show fluctuations in neutron intensity. These fluctuations, or pile noise, are not commonly considered of interest in themselves, but only as interference to other experiments. However, since the nature of the pile noise depends strongly on important reactor parameters, its study can enable the determination of quantities less easily accessible by other means. In particular, Moore (f) points out that the noise spectrum of such a system, that is, the mean square noise amplitude per unit band width, is proportional to the square modulus of the transfer function or to the Fourier cosine transform of the autocorrelation function. Thus, observation of the noise spectrum of a reactor could yield information about the shape of its transfer function. To test this technique, pile noise analyses were done on various low-power experimental reactors at Argonne National J aboratory. Since these reactors operate at such a low level that power effects on reactivity do not appear, the shape of the low-frequency portions of their transfer functions would depend only on fairly well-known delayed neutron parameters, and thus would be of little interest. However, the high-frequency rolloff portion of the transfer function is strongly dependent on the quotient of the effective delayed neutron fraction over the prompt neutron... 

Cars, trucks, lawn mowers, leaf blowers, chain saws, power drills, television, radio, video games, computers... the list of noise makers in our modem life is almost endless, and our world keeps getting noisier. Noise— which can be defined as unwanted sound waves that were not present in the pre-modem electromagnetic spectrum—is one of the most common forms of pollution, one that can easily damage the hearing and general health of people and animals. 

The conventional TCD is configured with the filaments being connected to form a Wheatstone bridge. A property of the Wheatstone bridge is common mode rejection of the noise which is primarily due to the electronics (l.e. power supply stability and the amplifier circuit). The TCD noise spectrum resembles white (shot) noise rather than the 1/f (flicker) noise of ionization detectors. Modulation techniques for noise rejection of white noise is no better than a simple Wheatstone bridge. 

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