Noise from power supply

Even if the receptor by itself has high accuracy, the sensor may be unable to execute the measurement in a defined place. Quality and total accuracy depend on the combination of receptor, the converter for measured values, and mechanical protection. Mechanical protection can take the form of pockets in water and fluid and also assembly boxes which protect against pollution, humidity, and temperature in the surroundings or against electromagnetic transmissions and noise from power-supplied pipes and cables. 

The linear power supply finds a very strong niehe within applieations where its ineffieieney is not important. These inelude wall-powered, ground-base equipment where foreed air eooling is not a problem and also those applieations in whieh the instrument is so sensitive to eleetrieal noise that it requires an eleetrieally quiet power supply—these produets might inelude audio and video amplifiers, RF reeeivers, and so forth. Linear regulators are also popular as loeal, board-level regulators. Here only a few watts are needed by the board, so the few watts of loss ean be aeeommodated by a simple heatsink. If dielee-trie isolation is desired from an ae input power souree it is provided by an ae transformer or bulk 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. 

Account must also be taken of small alternating currents which may be diverted from the sheath of a power supply cable by a bond connected to nearby buried structures. Such currents may be sustained for long periods and if they are diverted to the sheaths of telecommunication cables noise may be induced in the telephone circuits. 

The overall ability of a power supply to attenuate disturbances at its input is expressed as its PSRR (power supply rejection ratio). In graphs, PSRR is usually plotted as a function of frequency. We will invariably find that the rejection ratio is very low at higher frequencies. One reason for this is that the Bode plot cannot really help because the open-loop gain is very small at these frequencies. The other reason is, even a tiny stray parasitic capacitance (e.g., across the power switch and inductor) presents such a low impedance to noise frequencies (whatever their origin) that almost all the noise present at the input migrates to the output unimpeded. In other words, the power stage attenuation (which we had earlier declared to be Vo/Rin) is also nonexistent for noise (and maybe even ripple) frequencies. The only noise attenuation comes from the LC filter (hopefully). 

Question 9 Is there some interaction with nearby circuitry Yes, you could be picking up fields from nearby circuits, but that shouldn t affect a typical switcher, simply because it produces enough noise and fields of its own. However, it is a good idea to do the reverse-peel here. If I find the converter is on a larger system board, I immediately and carefully first cut off all the traces leading from its output and divert them to my predictable electronic load. I also cut the input traces and divert them to my bench power supply. If the problem is gone, it is an interaction problem. 

We have only covered the signal-to-noise problem several others must be solved simultaneously. Since space is a vacuum, one cannot cool the electronics or power supplies with a fan, but must ensure that thermal contact direct the heat to the spacecraft radiators. Solid state detectors (SSD) (see Section 2.3.5), uncommon in laboratory MS, are often used in space to get an additional energy signal from the ion impact, and these detectors must not go above 30°C. Likewise, fast electronics are often power hungry, and all that power must be dissipated as heat. More than one space MS has failed for thermal reasons. 

Polystyrene capacitors have exceptionally low tan S values (< 10 q, making them well suited for frequency-selective circuits in telecommunications equipment. Polymer capacitors are widely used for power-factor correction in fluorescent lighting units, and in start/run circuitry for medium-type electric motors used in washing machines, tumble-dryers and copying machines for example. They are also used in filter circuits to suppress radio frequencies transmitted along main leads. Such interference noise may originate from mechanical switches, furnace controllers and switch mode power supplies it not only spoils radio and television reception but can also cause serious faults in data-processing and computer equipment. 

A diagram of the counting sensor is shown in figure 5. From the counting sensor the residue passes to radioactive waste. The counting cell contains a coiled tube of a defined volume situated in a transparent case. On either side of the sensor case are situated two photomultipliers the outputs of which are processed by coincidence electronics. Much of the noise that produces the background count arises from the electronics and the power supplies. If two photo-... 

The choice between galvanostatic and potentiostatic measurements depends on circumstances. From the instrumentation point of view, galvanostats are much simpler than potentiostats. This is not only a matter of cost, but also a matter of performance. Thus, where it is desired to measure very low currents (e.g., on single microelectrodes), a battery with a variable resistor may be all that is needed to set up a low-noise galvanostat. At the other extreme, when large currents must be passed — for instance, in an industrial pilot plant for electrosynthesis - power supplies delivering controllable currents in the range of hundreds of amperes are readily available, whereas potentiostats of comparable output are either nonexistent or extremely expensive. 

A customer complains that his hard disk is making lots of noise. After examining the computer and hearing the noise for yourself, you notice that the high-pitched noise seems to he coming from the fan in the power supply. Which component(s) should he replaced. (Select all that apply.)... 

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