High-frequency noise

The noise is expressed as noise density in units of V/(Hz), or integrated over a frequency range and given as volts rms. Typically, photoconductors are characterized by a g-r noise plateau from 10 to 10 Hz. Photovoltaic detectors exhibit similar behavior, but the 1/f knee may be less than 100 Hz and the high frequency noise roU off is deterrnined by the p—n junction impedance—capacitance product or the amplifier (AMP) circuit when operated in a transimpedance mode. Bolometers exhibit an additional noise, associated with thermal conductance. 

Controlling high frequency noise generation and radiation is the blackest of the black box art in switching power supply and product-system design. It is a subject that warrants a book all to itself and it is the final area that will interfere with the release of your product into the market. This appendix cannot adequately cover the subject, but will overview the major considerations involved with product design. 

One subtle, but major noise source is the output rectifier. The shape of the reverse recovery characteristic of the rectifiers has a direct affect on the noise generated within the supply. The abruptness or sharpness of the reverse recovery current waveform is often a major source of high-frequency noise. An abrupt recovery diode may need a snubber placed in parallel with it in order to lower its high-frequency spectral characteristics. A snubber will cost the designer in efficiency. Finding a soft recovery rectifier will definitely be an advantage in the design. 

Once the component values have been calculated, the physical construction of the transformer and the PCB layout become critical for the effectiveness of the filter stage. Magnetic coupling due to stray inductive pick-up of high-frequency noise by the traces and components can circumvent the filter all together. Added to this is the fact that the common-mode filter choke looks more and more capacitive above its self-resonance frequency. The net result is the designer needs to be concerned about the high-frequency behavior of the filter typically above 20 to 40 MHz. 

About measurements in the presence of a high-frequency noise... 

We know that a 0.1 pF input capacitor takes care of the (high-frequency) noise. But it neither can nor do almost anything to smooth out the (low-frequency) ripple. However, we are now in a position to start calculating how much bulk capacitance we really need to ensure trouble-free performance (for typical ICs ). 

What are the reasons for clock instability High-frequency noise is always generated at turn-on and turnoff in any switcher. This noise can infiltrate into the IC via various pins. It can be very hard to filter out and control. You may need to ultimately simply avoid turning the Fet OFF too dramatically. In most switchers, the turn-on transition is traditionally delayed (or slowed) just a little, so as to allow the output/catch diodes to recover... 

Modern power supplies solve almost all the problems, but unluckily they usually produce high-frequency noise, a dramatic drawback in a low-temperature laboratory. 

To reduce the high frequency noise of Fig. 16.9, and also to minimize low frequency vibrations of the tower, the crystal copper frame has been mechanically decoupled from the cryostat. 

The square wave produced by the double-beam in space spectrometer is preferred since there is only one detector and the signal is a square-wave that is essentially an alternating current. Alternating currents are much easier to manipulate electronically. In particular they can be easily amplified and noise that is either direct current noise or high-frequency noise can be filtered from the signal. 

Fig. 11.3. The influence of the input capacitance on output noise. To make a simple estimation, the input noise of the op-amp is represented by an ac source at the noninverting input end. The smaller the input impedance, the larger the noise at the output end. Therefore, the input capacitance generates a large high-frequency noise.

The insertion of an RC circuit in the feedback electronics right before the high-voltage amplifier for the z piezo has some other advantages. First, much of the high-frequency noise is efficiently filtered out. Second, it facilitates the realization of the electronics for spectroscopic study, which we will discuss in the following section. 

At this point, we note that there is no mechanism presently built into the relaxation methods to prevent undesirable high-frequency noise from growing with each iteration. Any spurious solution 6(x) satisfies Eq. (1) (see also Chapter 1, Sections V.A and V.B) for co beyond the band limit. If we know that the object 6 is truly band limited, with frequency cutoff co = 2, we can band-limit both data i and first object estimate d(1). The relaxation methods cannot then propagate noise having frequencies greater than Q into an estimate o(k). (One possible exception involves computer roundoff error. Sufficient precision is usually available to avoid this problem.)... 

The demand that the solution 6 be consistent with the data i results in the improved resolution that we expect from a deconvolution method. As we have explained, however, it also results in the amplification of high-frequency noise. The smoothing of this noise to some extent defeats the purpose of deconvolution. The tradeoff between smoothness and consistency is explicit in the formulation of a method first described by Phillips (1962) and further developed by Twomey (1965). In this method, we minimize the quantity... 

For this work, the spectrometer function s(x) was determined by the method outlined in Section II.G.3 of Chapter 2. In digitizing the data, a sample density was chosen to accommodate about 70 samples taken across the full width at half maximum of s(x). A 25-point cubic polynomial smoothing filter was used in the deconvolution procedure to control high-frequency noise. Instead of the convolution in Eq. (13), the point-successive modification described in Section III.C.2 of Chapter 3 was employed. In Eq. (24) of Chapter 3, we replaced k with the expression... 

Because the most serious problem arising in the deconvolution of spectra is that of noise, detailed attention to smoothing in a fashion consistent with the uniform attenuation of high-frequency noise will result in the best possible deconvolution results. 

For a detector to be of use in quantitative analysis, the signal output should be linear with concentration for a concentration-sensitive detector and with mass for a mass-sensitive detector. Some detectors have an additional time constant purposely introduced to remove the high-frequency noise. This should always taken into consideration, since a slow detector response can significantly broaden and attenuate chromatographic peaks relative to those actually sensed. Moreover, a versatile detector should have a wide linear dynamic range so that major and trace components can be determined in a single analysis, over a wide concenua-tion range. 

The high-frequency noise can be diminished either by slowing the time constant of the amplification circuitry or by averaging the signals on the computer. The low-frequency noise is caused by thermal fluctuation in the instrument. If one observes the CD spectrum with extremely small CD magnitudes, a two or three hour warmup... 

Only about 1 in 10s carbon atoms produces an ion, but ion production is proportional to the number of susceptible carbon atoms entering the flame. In the absence of analyte, 10 14 A flows between the flame tip and the collector, which is held at +200 to 300 V with respect to the flame tip. Eluted analytes produce a current of 10 12 A, which is converted to voltage, amplified, filtered to remove high-frequency noise, and finally converted to a digital signal. 

By the use of frequency filters (RG circuits) which cut off the high frequency noise from a low frequency signal. Care must be taken to avoid distortion of the signal. An RC circuit as shown below is a low pass filter of time constant t = RG. This gives a rough value of the cut-off frequency. 

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