Voltage-fluctuation ratio

If the backward impedance, that is, the voltage-fluctuation ratio of the substation, is 9% (= Zpu), the internal voltage of the substation model becomes... 

The essential point is to measure the fluctuations of the current flowing between two identical electrodes kept at the same potential by means of a zero resistance ammeter (ZRA), and at the same time measure their voltage fluctuations with respect to a reference electrode (RE). The voltage measurement can be made between the two electrodes connected by the ZRA and a third electrode, which may be identical to the others or be a RE, (see configuration b in Fig. 7-21). The noise resistance is then calculated as the ratio of a second-order statistics of the voltage fluctuations divided by the same quantity relative to the current fluctuations. Often, the quantities chosen are the standard deviations measured over a fixed period of time. 

CCD detector designers try to increase the signal-to-noise ratio of an amplifier in two ways (1) increase the responsivity, or (2) decrease the random current fluctuation between source and drain. The responsivity can be increased by decreasing the amplifier size. Decreasing the amplifier size decreases the capacitance of the MOSFET. The responsivity of a MOSFET obeys the capacitor equation which relates voltage, V, to the charge Q on capacitance C V = QIC. 

If the voltage is high enough, the noise of isolated contacts can be considered as white at frequencies at which the distribution function / fluctuates. This allows us to consider the contacts as independent generators of white noise, whose intensity is determined by the instantaneous distribution function of electrons in the cavity. Based on this time-scale separation, we perform a recursive expansion of higher cumulants of current in terms of its lower cumulants. In the low-frequency limit, the expressions for the third and fourth cumulants coincide with those obtained by quantum-mechanical methods for arbitrary ratio of conductances Gl/Gr and transparencies Pl,r [9]. Very recently, the same recursive relations were obtained as a saddle-point expansion of a stochastic path integral [10]. 

Figure 7.31 demonstrates the very good rectifying behavior of such a Pd Schottky diode on undoped ZnO thin film. The current density ratio determined for bias voltages of +0.6 V and -3V is about 104 as shown in the inset of Fig. 7.31. The ideality factor n is about 1.5. The temperature-dependent current-voltage (IV, see Fig. 7.31) and capacitance-voltage (CV) measurements from 210 to 300 K explain the reason for the slight deviation of the ideality factor from unity and the dependence of the reverse current on the reverse bias. The barrier heights of the diode of Fig. 7.31 jy and

Another approach for ENA is to transform the recorded potential and current fluctuations into the frequency domain using the East Eourier Transform (EFT) method [37]. The spectral noise resistance, Rsn, is given by the ratio of the voltage and current EETs at each frequency and the limiting value, can be used as a measure of corrosion resistance ... 

An example of direct measurements based on the frequency representation of fluctuation is the study of the association-dissociation reaction of BeS04 in a 0.03 M solution, with conductance measurements (Feher Weismann, 1973). It is particularly interesting that they could increase the ratio of the reaction noise to Johnson noise of the circuit, since the former is a quadratic function of the applied direct voltage, while the latter is independent of the voltage. 

The photomultiplier, as shown in Fig. 6, is almost universally used as a photon counter, that is, the internal electron multiplication produces an output electrical pulse whose voltage is large compared with the output electric circuit noise. Each pulse in turn is the result of an individual photoexcited electron. The numbered electrodes, 1-8, called dynodes, are each successively biased about 100 V positive with respect to the preceding electrode, and an accelerated electron typically produces about 5 secondary electrons as it impacts the dynode. The final current pulse collected at the output electrode, the anode, would in this case contain 5 400,000 electrons. The secondary emission multiplication process is random, the value of the dynode multiplication factor is close to Poisson distributed from electron to electron. The output pulse amplitude thus fluctuates. For a secondary emission ratio of = 5, the rms fractional pulse height fluctuation is 1 /V<5 — 1 = 0.5. Since the mean pulse height can be well above the output circuit noise, the threshold for a pulse count may be... 

The current, in general, is composed of a steady state or dc part determined by the mean dc potential E and the mean dc concentrations at the interface, Co and c , and an ac part, A/, determined by the ac perturbing potential A and the fluctuating concentrations Ac,. The faradic impedance is given by the ratio of the Laplace transforms of the ac parts of the voltage and current... 

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