Lock-in detection techniques

A phase-sensitive detector (PSD) constitutes the central part of instmments commonly known as lock-in amplifiers. Effectively, a PSD responds to signals that are of the same frequency and phase with respect to the reference waveform aU others are rejected. One 

Proper operation of the PSD requires the generation of a precision reference signal within the instrument. 

When the available reference is far from perfect or symmetrical, a well-designed reference channel circuit is very important. The internal, symmetric reference signal is usually generated using a so-called PLL circuit. In any real experimental set-up the relationship between the signal from the detector and the reference waveform will not always be exactly in phase. To allow perfect phase matching at the phase-sensitive demodulator, lock-in amplifiers include flexible phase-shifting circuitry for the reference waveform, which allows the introduction of a phase shift over a full period of 27t (360°). The phase controls on the lock-in usually take the form of a continuously variable 0-95° adjustment, plus three fixed increments of 90°, 180° and 270°. 

The signal and reference channels are mixed within the actual PSD unit. There are currently three common methods of implementing a PSD, i.e. using an analogue multiplier, a digital switch or a digital multiplier  

Output signal from a phase-sensitive detector 

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BRET [31, 32]), lock-in detection techniques exploiting optical switches [33], and schemes for alternating D/A excitation (ALEX [34]). The increased attention to quantitative FRET imaging encompasses the use of polarization [35-39], the perennial issue of calibration and standards [40-44], and practical guides to operational principles and protocols ([45, 46] and other references above). The fundamental distinctions between the requirements for live and fixed cell imaging cannot be overemphasized, as is exemplified in a report of erroneous FRET determinations with visible fluorescent proteins (VFPs) in fixed cells [47],... 

In order to employ a lock-in detection technique, as in EMIRS, the modulation frequency of the potential at the electrode would have to be at least an order of magnitude greater than F(v). Thus, the potential modulation would have to be c. 100 kHz too great to allow sufficient relaxation time for most electrochemical processes to respond. Instead, a slow modulation or single-step approach is employed, as follows ... 

Cova, S., et al. (1979). Versatile digital lock-in detection technique Application to spectro-fluorometry and other fields. Rev. Sci. Instrum. 50, 296. 

The Application of FTIR Spectrometers In essence, the routine use of FTIR spectrometers has meant that in situ infrared spectroscopic studies requiring high sensitivity, such as the study of adsorbed species, were no longer limited to the fast, reversible electrochemical systems dictated by lock-in detection techniques infrared spectra could be collected during a slow linear voltammetric sweep, during a series of potential steps to higher... 

This technique based on a hot wire thermal probe with AC excitation and 3 CO lock-in detection. Since the principle and procedures of the technique have been described in details previously [51] only a brief description is given here. We consider a thermal probe (ThP) consisting of a metallic wire of length 21 and radius r immersed in a liquid sample, acting simultaneously as a heater and as a thermometer. The sample and probe thermophysical properties are the volume specific heat pc and the thermal conductivity k, with the respective subscripts (5) and (p). The wire is excited by ac current... 

The SNIFTIRS approach is clearly related in some ways to the EMIRS technique, in that it does involve potential modulation, (but not lock-in detection), albeit at a much lower frequency ca. 0.01-to 0.02 Hz [83, 85,137], than the ca. 10 Hz typically employed in EMIRS experiments. Consequently, SNIFTIRS is also restricted to electrochemical systems that are essentially reversible over the timescale of the poterttial modulation, but has proved extremely sensitive, and is generally reported as being surface specific, only detecting potential-induced changes in adsorbed species [119, 138]. One specific exception to this generalization is where 1 and 2 are chosen such that the species of interest are fiiUy adsorbed at one potential, and fiiUy desorbed at the other potential [16, 85]. This led Weaver and Corrigan [139] to coin the general acronym PD IRS, for those approaches that involve multiple reference spectra as well as multiple sample spectra. Thus, Fig. 11(d) is an example of the PDIRS approach, as is Fig. 11(e), in which the sample potential is sequentially decreased... 

The detection sensitivity of the experimental arrangement is defined by the minimum absorbed power that can still be detected. In most cases it is limited by the detector noise and by intensity fluctuations of the radiation source. Generally, the limit of the detectable absorption is reached at relative absorptions AP/P > 10 -10 . This limit can be pushed further down only in favorable cases by using special sources and lock-in detection or signal-averaging techniques. 

If the laser power can be stabilized within lO- Po the power fluctuations are 1 pW and the corresponding signal fluctuation SS = 10 pV, i.e. about 25 times larger than the signal AS. One therefore needs lock-in detection or other special noise-suppressing detection techniques. 

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