Power supply line 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. 

We had gotten roughly the same empirical level of improvement when we tried a ferrite sleeve as shown in the lower arrangement of Figure 11-4. But the sleeve works mainly by increasing the impedance on both lines to the common mode noise coming out of the power supply via conduction. Ferrite sleeves made specifically for EMI suppression purposes also... 

Electronic instruments are subject to instrumental systematic errors. These can have many sources. For example, errors may emerge as the voltage of a battery-operated power supply decreases with use. Errors can also occur if instruments are not calibrated frequently or calibrated incorrectly. The experimenter may also use an instrument under conditions in which errors are large. For example, a pH meter used in strongly acidic media is prone to an acid error, as discussed in Chapter 20. Temperature changes cause variation in many electronic components, which can lead to drifts and errors. Some instruments are susceptible to noise induced from the alternating current (ac) power lines, and this noise may influence precision and accuracy. In many cases, errors of these types are detectable and correctable. 

Common noise sources include vibration, pickup from 60-Hz lines, temperature variations, frequency or voltage fluctuations in the power supply, and the random arrival of photons at the detector. 

Note There is nothing special about the DM noise current direction indicated in Figure 9-1. It can well be the other way around — that is, going in through either L or N, and coming out of the other. In off-line power supplies, we will see that in fact, the direction reverses every ac half-cycle. 

But how different can the Vl and Vn scans be In fact, the above two equations have inspired a rather misleading statement often found in related literature — if the noise emission is predominantly DM, the Vl and Vn scans will look almost the same. The scans also look identical if the noise is predominantly CM. And if the Vl and Vn scans look very different, that implies that both CM and DM emissions are present. However, in the case of an off-line power supply, this statement is clearly not true. Because, that would imply that somehow the emissions on the L and N lines are different. However, we know that in any typical off-line power supply (with an input bridge rectifier), the L and N lines are... 

The power supply to the hollow cathode source is modulated and an ac detection system is used. This arrangement prevents any radiation from the flame or resonance detector from producing an output signal. Random noise is less troublesome than in a conventional spectrophotometer. The resonance detector must, of course, produce a cloud of atomic vapor of the same element being aspirated into the flame. The hollow cathode source also must emit resonance lines of the same element. Analytical calibration curves closely parallel those obtained with conventional atomic absorption systems and sensitivities and detection limits are similar. 

In addition to power-line coupled noise, computers and cathode-ray-tube monitors can be a troublesome source of interference in many clinical and laboratory situations. Digital logic electronics often involve fast switching of large currents, particularly in the use of switching-type solid-state power supplies. These produce and can radiate harmonics that spread over a wide frequency spectrum. Proximity of the amplifier or the monitored subject to these devices can sometimes result in pulselike biopotential interference. 

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