Detectors frequency response

If we use this assumption, it is always wise to plot the assumed V(t) behavior along with the measured V(t) data, and note any differences. When the detector frequency response does not obey Equation 10.9a and 10.12, determination of t or/ using the methods described will yield inconsistent results, and more rigorous definitions are required. 

Photomultipliers are used as detectors in the single-channel instruments. GaAs cathode tubes give a flat frequency response over the visible spectrum to 800 nm in the near IR. Contemporary Raman spectrometers use computers for instrument control, and data collection and storage, and permit versatile displays. 

Treffer s model system consists of signal power P (in watts) falling on the input aperture of the modulator (i.e., the spectrophotometer) that modulates the input power as a function of time by a factor M(t) such that 0 < M(t) < 1. The modulation function M(t) is not the modulation of the signal due to chopping but modulation of the signal due to scanning. The modulated signal falls on a detector with responsivity R (in volts per watt) (Kruse et al., 1962 Stewart, 1970) and flat frequency response. The idealized instantaneous... 

The modulated beam is directed through either the sample or reference side of the sample compartment and is finally focused on the detector. For most mid-infrared work, a triglycine sulfate (TGS) pyroelectric bolometer is used as the detector because of its very high frequency response (> 1 MHz). 

A silicon dioxide layer 3 is formed on an insulating CdTe substrate 1. A photo-resist coating 5 is formed over the silicon dioxide layer. The photo-resist layer is patterned and the silicon layer is partly etched away. The photo-resist layer is removed and a film of HgCdTe 9 of a first mercury to cadmium ratio is deposited by liquid phase epitaxial deposition over the entire surface of the substrate. The HgCdTe film is only formed at regions where the CdTe substrate is exposed and does not adhere to the silicon dioxide. Next, the silicon dioxide layer is removed. In order to increase the window of frequency response of the detectors, the process is repeated using a second mercury to cadmium ratio different from the first ratio. 

Time-resolved fluorometry fahs into one of two categories, depending on how the fluorescence emission response is measured (1) pulse fluorometry, in which the sample is illuminated with an intense brief pulse of light and the intensity of the resulting fluorescence emission is measured as a function of time with a fast detector system, or (2) phase fluorometry, in which a continuous-wave laser illuminates the sample, and the fluorescence emission response is monitored for impulse and frequency response. ... 

A radiation detector is a device in which the photons absorbed are transformed ultimately into electrical energy. The efficiency with which the photons are transformed into electrical power is described by the responsivity of the detector, and it is expressed as the voltage generated by one watt of incident radiant power. The average time required for the incident power to be transformed and dissipated by the detector characterizes the response time, or time constant of the detector. To be specific, the electrical response of a detector to a radiation beam time-modulated at frequency / is similar to the frequency response R(/) of a low-pass electrical filter with time constant r ... 

Since the optic fiber probe has time response characteristic of the nanosecond level, limited by the response of the light detector, the response of signals will have essentially no time lag. Thus another method for describing optic output signals for particle concentration is to analyze the power spectrum. The power spectral density function of random signals can express the frequency structure of signals by the mean square value. The power spectral density function Sx(/) can be defined as ... 

This is a good method for accomplishing our goal and we would be perfectly happy with it as long as we did not have to measure the frequency response of a large number of amplifiers. If we were the impatient sort, however, we could think of several faster methods. One would be to get several tone generators (frequency sources) which put out different tones and use them with tuned detectors which can separately but simultaneously detect those tones. You could perform the required measurement all at once but it is simply not practical to assemble the required number of separate tone generators and detectors if any kind of frequency resolution were required. ... 

T. Morimune, H. Kajii, and Y. Ohmori, Frequency response properties of organic photo-detectors as opto-electrical conversion devices, IEEE. Display Technol, 2(2), 170-174 (2006). 

The standard detector in routine FT-IR instruments is the pyroelectric DTGS (deut-erated triglycine sulfate) detector, whose response in the MIR range is wavelength independent. The detector operates at ambient temperature and shows good linearity across the whole transmittance scale. The DTGS detector responds to signal frequencies of up to several thousand Hz, hence the time needed to scan one spectrum at a resolution of 4 cm is of the order of 1 s. 

The IR chemical imaging system measures chemically-specific IR spectra using a mercury cadmium telluride (HgCdTe) FRA detector which provides broad frequency response (out to 18 i.m), high sensitivity (2 X 10 cm Hz 2/Watt), and an operating temperature of 40-60 K. 

Not all semiconductors can be prepared in both n- and p-types. Schottky barrier photodiodes are of special interest in those materials in which p — n junctions cannot be formed. They also find application as UV and visible radiation detectors, especially for laser receivers where their high frequency response (in the gigahertz range in many cases) is of particular usefulness. See Ahlstrom and Gartner [2.41], Schneider [2.42] and Sharpless [2.43] for more detailed descriptions. 

In many cases it is useful to determine how the detector noise depends upon frequency. A plot of noise voltage (or current) as a function of frequency is known as the noise spectrum. Because the various types of detector noise exhibit different dependencies upon frequency, measurement of the noise spectmm is useful in determining the mechanism giving rise to the noise. Furthermore, the noise spectrum and frequency response (i.e., signal spectrum) can be used to calculate the dependence of signal-to-noise ratio on frequency, thereby determining the dependence of D on frequency. 

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