Conductance noise

Another major source of noise is the loop consisting of the output rectifiers, the output filter capacitor, and the transformer secondary windings. Once again, high-peak valued trapezoidal current waveforms flow between these components. The output Alter capacitor and rectifier also want to be located as physically close to the transformer as possible to minimize the radiated noise. This source also generates common-mode conducted noise mainly on the output lines of the power supply. 

The purpose of an input conducted EMI filter is to keep the high-frequency conducted noise inside the case. The main noise source is the switching power supply. Filtering on any of the input/output (I/O) lines is also important to keep noise from any internal circuit, like microprocessors, inside the case. 

The initial common-mode filter component values can be determined in a step-by-step process (like everything else in this book). To begin this process, either a baseline measurement of the unfiltered conducted noise spectrum is... 

A reasonable beginning is that one needs about 24 dB of attenuation at the switehing frequency of the switching power supply. This, of course, should be modified in response to the actual conducted noise spectral shape. One determines the corner frequency of the filter by... 

Figure 36 (a) Conduction noise amplitude versus frequency for various dc currents (b) nonlinear conductivity and SDW current in the same (TMTSF)2PF6 sample, T = 4.2 K as in (a). (After Ref. 111a.)... 

It is often colloquially stated that at frequencies below approximately 5 MHz the noise currents tend to be predominantly differential mode, whereas at frequencies above 5 MHz the noise currents tend to be predominantly common mode. But this may or may not be true always. Certainly at frequencies above 20 MHz, any conducted noise is most likely attributable to inductive pickup, for example from radiation spilling out from the cables. 

Finally, we observe that standard conducted EMI emission limits are typically only up to 30 MHz. We can ask — why weren t the limits set even higher The reason is that by 30 MHz, any conducted noise is expected to automatically suffer severe attenuation in the mains wiring, and therefore won t really be able to travel far enough to cause interference much down the road. However, since the cables can certainly still radiate (locally), typical EMI radiation limits cover the range from 30 MHz to 1 GHz. 

Most review papers on the subject of biological membrane noise have been devoted primarily to conductance noise that arises from channel switching between different conductance states. Several excellent reviews that vary in volume and in mathematical level of presentation of material are available (3-9). 

In biological ionic systems, conductance noise is induced by membrane structures such as ion channels and fluctuations in electrolyte conductance. Noise that originates from the switching of ion channels between different conductance states has been reviewed extensively (3-9). Therefore, we limit ourselves to a discussion of conductance fluctuations in electrolyte solutions (45) and new noise sources that have been identified recently for currents through open ion channels (46, 47). 

An excellent review of the early history of noise studies of different ionic systems, such as single pores in thin dielectric films, microelectrodes, and synthetic membranes, is reference 3. The review by Weissman (48) describes several state-of-the-art fluctuation spectroscopy methods that include (1) determination of chemical kinetics from conductivity fluctuations in salt solutions, (2) observation of conductivity noise that arises from enthalpy fluctuations in the electrolyte with high temperature coefficient of resistivity, and (3) detection of large conductivity fluctuations in a binary mixture near its critical point. 

Studies of single channels formed in lipid bilayers by Staphylococcus aureus alpha toxin showed that fluctuations in the open-channel current are pH-dependent (47). The phenomenon was attributed to conductance noise that arises from reversible ionization of residues in the channel-forming molecule. The pH-dependent spectral density of the noise, shown in Figure 6, is well described by a simple model based on a first-order ionization reaction that permits evaluation of the reaction parameters. This study demonstrates the use of noise analysis to measure the rate constants of rapid and reversible reactions that occur within the lumen of an ion channel. 

Figure 5. Propagation path of conducted noise in three-level inverter system...

Impedance coupling produced by ground sharing is a source of conducted noise that must be avoided. Guidelines for minimizing impedance coupling include the following ... 

Materials selected should be suitable for the purpose and be either inherently resistant to deterioration or adequately protected against deterioration by compatible coatings, especially in problem areas where corrosion can cause low conductivity, noise, short circuits, or broken leads, thus leading to degradation of performance. 

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