Noise frequency band

A novel optimization approach based on the Newton-Kantorovich iterative scheme applied to the Riccati equation describing the reflection from the inhomogeneous half-space was proposed recently [7]. The method works well with complicated highly contrasted dielectric profiles and retains stability with respect to the noise in the input data. However, this algorithm like others needs the measurement data to be given in a broad frequency band. In this work, the method is improved to be valid for the input data obtained in an essentially restricted frequency band, i.e. when both low and high frequency data are not available. This... 

In practice, since x(t) is a frequency band-limited signal, equation (11) shows that H(u) is known only on the finite interval wherein X(u) 0. There are also problems when the input signal is small, reduced to noise. 

The sound absorption of materials is frequency dependent most materials absorb more or less sound at some frequencies than at others. Sound absorption is usually measured in laboratories in 18 one-third octave frequency bands with center frequencies ranging from 100 to 5000 H2, but it is common practice to pubflsh only the data for the six octave band center frequencies from 125 to 4000 H2. SuppHers of acoustical products frequently report the noise reduction coefficient (NRC) for their materials. The NRC is the arithmetic mean of the absorption coefficients in the 250, 500, 1000, and 2000 H2 bands, rounded to the nearest multiple of 0.05. 

For off-resonance decoupling of 13C NMR spectra, frequency offsets of about 0.5 1 kHz are used. In order to avoid complete collapsing of multiplets by large, noise-modulated frequency bands, non-modulated decoupling fields are usually applied. The decoupling frequency offset can be adjusted until the multiplets are so narrow that no or only slight overlapping occurs. 

Dynamic contact between two bodies (e.g., one sliding over the other) generates noise that is characteristic of the hardness of these bodies and of the physical and chemical properties of their interface. Moreover, the velocity of the sliding movement modifies the frequency band of the emitted acoustic waves. The sliding displacement of a finger at the surface of the skin for assessing its smoothness... 

Johnson noise arises because the random thermal motion of electrons in an isolated resistor produces random fluctuations in voltage between its ends, covering a broad frequency band. It can be shown that... 

The lower detection limit is influenced by the noise frequency and amplitude distribution compared with the signal band width and height. The noise level is usually measured over a given time period which is a multiple of the signal width. The noise can be expressed as peak-to-peak (p-t-p) or root-mean-square (rms) values. The latter gives about 70-80% lower noise levels. Figure 15-7 summarizes the detection limits of some selected chromatographic detectors. 

Nyquist theorem - An expression for the mean square thermal noise voltage across a resistor, given by RkTAf where R is the resistance, k the Boltzmann constant, T the temperature, and /y the frequency band within which the voltage is measured. 

In some facilities, special electrically shielded rooms minimize environmental electrical noise, particularly 60 Hz alternating current (AC) hne noise. Since much of the information of interest in the EEG lies in frequency bands below 40 Hz, low-pass filters in the amplifier can be used to greatly reduce 60 Hz noise. For attenuating AC noise when the low-pass cutoff is above 60 Hz, many EEG amplifiers employ a notch filter specific only for frequencies in a narrow band centered around 60 Hz. 

Lock-in amplifiers provide a very narrow frequency band pass and thus achieve an excellent signal-to-noise ratio. With suitable resistance-capacitance filters, an effective bandpass width of 1 Hz can be obtained. The lock-in amplifier uses a chopper to modulate the energy source. A reference source, usually a small flashlight bulb, is modulated at exactly the same frequency. The two signals are combined in a synchronous detector to produce sum and difference frequencies the sum frequency will be twice the chopping frequency and the difference frequency will be zero. 

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