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Light Detection
Published in Araz Yacoubian, Optics Essentials, 2018
One method of detecting a signal that is embedded in noise is to use a lock-in amplifier. An example is to detect an optical signal in the presence of high ambient light. A lock-in amplifier is an instrument that synchronizes a detected signal (e.g., a signal from a photodiode) with a modulated light source. The modulation can be achieved either using a mechanical chopper or by modulating the light source (see Figure 3.4). A good source for further understanding of the lock-in amplification process can be found in the manufacturer’s technical notes (see, for example, Stanford Research Instruments website for tutorials on lock-in amplifiers, www.thinksrs.com/downloads/PDFs/ApplicationNotes/AboutLIAs.pdf). Many of the old lock-in amplifiers consisted of analog electronics. With the advent of digital electronics, current lock-in amplifiers are digital, using digital signal processing as a means of synchronizing the modulated source with the detected signal. Using lock-in amplifiers enables detection of signals with signal level below signal-to-noise ratio (SNR) of less than 1.
Amplifiers and Signal Conditioners
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
A lock-in amplifier is based on the same principle as a carrier amplifier, but instead of driving the sensor, here the carrier signal drives the experiment, so that the measurand is frequency translated. Lock-in amplifiers are manufactured as equipment intended for recovering signals immersed in high (asynchronous) noise. These amplifiers provide a range of driving frequencies and bandwidths for the output filter. Some models are vectorial because they make it possible to recover the in-phase and quadrature (90° out-of-phase) components of the incoming signal, by using two demodulators whose reference signals are delayed by 90°. Still other models use bandpass filters for the modulated signal and two demodulating stages. Meade [9] analyzes the fundamentals and specifications of analog lock-in amplifiers. Signal recovery (a division of Ametek, Inc.) offers, upon registration in its website, several application notes about analog and digital lock-in amplifiers.
Electrical Properties of Semiconductor Nanocrystals
Published in Victor I. Klimov, Nanocrystal Quantum Dots, 2017
The charged states formed after charge transfer to nanocrystals often have lifetimes in the microsecond to millisecond range,29 so their population and decay can be studied without the need for pulsed lasers. In the quasi-steady-state technique, often known as photoinduced absorption (PIA), the sample is excited by a continuous wave (CW) laser beam modulated with a mechanical chopper at frequencies up to a few kilohertz. Absorption is measured at energies between 0.5 and 3 eV using monochromated light from a tungsten lamp together with an appropriate detector, as shown in Figure 7.4. A lock-in amplifier is used to measure the small change in absorption at the chopping frequency, allowing fractional changes in transmission as low as 10–6 to be measured. The lock-in amplifier measures the components of modulation that are in-phase and 90° out-of-phase with the excitation. Monitoring the signal as a function of chopping frequency and pump intensity provides information about the lifetime and recombination mechanism of the excited species.
Lock-in amplifier as an alternative for reading Radio-Frequency identification (RFID) tags in sensing applications
Published in Instrumentation Science & Technology, 2021
Guilherme R. De Lima, Giovani Gozzi, Lucas Fugikawa-Santos
Laboratory benchtop measurements of RFID signals are commonly performed using network analyzers or digital oscilloscopes equipped with the fast-Fourier transform (FFT) algorithm. Besides the cost of the equipment, network analyzers are extremely specific equipment used in telecommunications, whereas digital FFT-equipped oscilloscopes are still not broadly disseminated and, in some circumstances, are unavailable in research laboratories. Lock-in amplifiers, on the other hand, are extremely versatile equipment which can be used in a plethora of electrical, electronic, and optical measurements,[13,14] being easily available in research facilities. Moreover, the synchronous measurement principle in a lock-in amplifier allows measurements in a wide sensitivity range up to 120 dB.
Magnetoelectric effects in shear-mode magnetostrictive/piezoelectric composite with a Z-type structure
Published in Mechanics of Advanced Materials and Structures, 2023
Benjie Ding, Tingfeng Ma, Licheng Hua, Jianying Hu, Minghua Zhang, Jianke Du
The ME coupling performance of Z-type and S-type structures are tested to further demonstrate that the former exhibits a larger ME voltage. The structure is placed at the middle of the Helmholtz coil to ensure that the length direction of Terfenol-D is in parallel to the direction of the applied magnetic field. The current used in the Helmholtz coil to calibrate the AC magnetic field amplitude is set at 1 Oe. The frequency of sinusoidal AC signal produced by the lock-in amplifier ranges from 1 to 100 kHz. Under 600 Oe bias magnetic fields and 5 V (effective value) voltage, the ME coupling coefficient is plotted as a function of driven frequency for the Z-type and S-type structures (Figure 4). The maximum ME coefficients for Z-type and S-type structures under resonance frequency are and respectively. The peaks for the Z-type structure are adjacent to each other, which broadens the range of the ME frequency response. When the ME coupling coefficient exceeds several resonance peaks are observed from 22.7 kHz to 57.5 kHz () and from 61.5 kHz to 65.7 kHz () for the Z-type and S-type structures, respectively. The frequency range is about eight to nine times that of Moreover, resonance frequency of the Z-type structure is lower than that of the S-type structure, which implies that the proposed structure contributes to effectively reducing eddy current loss. The proposed Z-type ME cantilever exhibits a stronger ME response than that of traditional sandwich structures. In the Z-type structure, the piezoelectric plate generates a thickness-shear displacement. Furthermore, the shear piezoelectric coefficient (d15) of PZT-5A is several times larger than the forward piezoelectric coefficient (d31) [23]. For further enhancement of the ME coupling coefficient, after special cutting, the voltage generated by the in-plane shear strain of the piezoelectric material can be output from the thickness direction to make it operate in d15 shear-mode.