Chopper stabilization

Two different schemes are used for modulation chopper stabilization, which works similarly to an AM radio, and correlated double sampling (CDS), which performs two measurements of the noise only and the noise plus signal [19]. Fig. 6.1.11 shows conceptual diagrams of the two techniques. Although the additional circuitry looks complex, actual implementations are surprisingly simple [20, 21]. 

Fig. 6.1.11 Modulation techniques a) chopper stabilization, b) correlated double sampling...

CDS is somewhat simpler since no demodulator and filter are needed. However, since the signal is sampled, CDS suffers from noise aliasing. Unless eliminated with appropriate circuit techniques [22, 23], this will result in an elevated noise floor. Chopper stabilization is preferred in printed circuit board implementations since all necessary components are readily available. 

Many other integrated solutions are documented in the literature. These include single-ended and differential solutions employing chopper stabilization [1, 20, 24, 31], circuits using correlated double sampling [2, 3, 32] and applications for capacitive readout circuits and corresponding implementations [15]. 

Alternating-Current, Direct-Current, and Chopper-Stabilized Preamplifiers... 

Alternating-, Direct-Current and Chopper-Stabilized Preamplifiers 159... 

Denison T, Consoer K, Santa W, Avestruz AT, Cooley J, Kelly A (2007) A 2 p.W 100 nV/rtHz chopper-stabilized instrumentation amplifier for chronic measurement of neural field potentials. IEEE J Sohd-State Circ 42 2934-2945... 

Smaller values of are obtained for interferometers operated in a double-beam mode, since the moveable mirror must be left stationary for a fraction of the cycle time to allow the detector to stabilize each time the beam is switched from the sample to the reference position. With an optical null grating spectrometer the chopper is used not only to modulate the beam but also to alternate the beam between sample and reference channels. Thus, it takes approximately the same time to measure a transmittance spectrum using a double beam optical null spectrometer as it takes to measure a single-beam spectrum with the same S/R. Hence, for this type of spectrometer may be assigned a value of 2. 

Figure 9.14. Brillouin spectrometer using fibre optics to increase the signal-to-noise ratio. (1) Light source consisting of a master laser (1a) a slave with matched frequency (1b) and control unit (1c) for sensitive stabilization of the difference frequency Sv. (2) Signal splitter. (3) Fibre coupler. (4) Polarizer. (5) Chopper. (6) Lens. (7) Cuvette placed on a goniometer. (8) Termination. (9) Slit. (10) Broad-band (10 GHz) APD. (11) Photodiode with a smaller bandwidth (1 GHz). (12) Spectrum analyser (10 GHz) for controlling the intermediate frequency Sv. (13) Spectrum analyser (1 GHz) for the measurement of the half-power bandwidth, Av, of the Brillouin peak. (14) Amplifier system. (15) Process control computer. (Reproduced with permission of Elsevier, Ref [96].)...

Illuminate the WE at AM 1.5 G and measure the current at zero-bias as a function of time. Depending on the material, a few seconds to a few minutes may be needed for the current to stabilize. Dark current can be determined by intermittently blocking the light source with a shutter or chopper. 

Amplifiers may be broadbanded, that is, the amplifier may respond to a wide frequency range. If the amplifier is constructed to respond to a narrow band of frequencies, it is called a tuned ac amplifier. Successive stages of amplification, all tuned to the same frequency, will narrow the bandpass width. The modulation of the source or the mechanical chopper used in the optical path must maintain the frequency to which the amplifier responds thus a narrow-bandpass ac amplifier imposes more stringent requirements on the stability of the modulation devices. 

In laser pyrolysis, a precursor in the gaseous form is mixed with an inert gas and heated with CO2 infrared laser (continuous or pulsed), whose energy is either absorbed by the precursor or by an inert photosensitizer such as SFs. Swihart [84], Ledoux et al. [116,117], and Ehbrecht and Huisken [118] prepared Si nanoparticles by laser pyrolysis of silane. By using a fast-spinning molecular beam chopper, Si nanoparticles in the size range of 2.5-8 nm were deposited on quartz substrates to study quantum confinement effects [116]. Li et al. [119] improved the stability of the Si nanoparticles ( 5 nm) by surface functionalization and obtained persistent bright visible photoluminescence. Hofmeister et al. [120] have studied lattice contraction in nanosized Si particles produced by laser pyrolysis. The method has been used to synthesize metal nanoparticles as well (see Table 2.1). Zhao et al. [121] obtained Co nanoparticles by laser pyrolysis of Co2(CO)s vapor at a relatively low temperature of 44° C. Ethylene was used as a photosensitizer for CO2 laser emission. Nanoparticles... 

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