Chopping Frequency Research Articles

Photocurrent characteristics of solution-processed HgTe nanoparticle thin films under the illumination of 1.3 µm wavelength light

The photocurrent characteristics of HgTe nanoparticle (NP) thin films fabricated on glass substrates were investigated under the illumination of 1.3 µm wavelength light in this work. The photocurrent obtained from an NP thin film with a vertical structure was 119 µA in magnitude at an applied voltage of 2 V. The magnitude of the photocurrent for an NP thin film with a lateral structure was 23.8 nA at 10 V. Nevertheless, the relative magnitude of the photocurrent measured as a function of the chopping frequency was largely the same for the thin films with both the lateral and the vertical structures. The frequency-dependent photocurrent mechanism of the NP thin films will be discussed in detail in this paper.

Thermal Effusivity Measurement of Virgin Coconut Oil-Methanol Mixtures using Photoacoustic Technique

Thermal effusivity of virgin coconut oil-methanol mixtures were measured using open photoacoustic cell technique. The samples were prepared by simply mixing virgin coconut oil and methanol using similar procedure applied for preparation of biodisel. Thermal effusivity of the sample was obtained by fitting the experimental data of photoacoustic amplitude signal to the expression of photoacoustic signal as a function of chopping frequency. Thermal effusivity of mixtures decrease between 0.0851 W s1/2 K−1 cm−2 (pure virgin coconut oil) and 0.0644 W s1/2 K−1 cm−2 (pure methanol) with the increasing of methanol in the mixture.

Comparison of photocurrent spectra measured by FTPS and CPM for amorphous silicon layers and solar cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently introduced as a fast and highly sensitive method for the evaluation of the optical absorption coefficient of photoconductive thin films such as microcrystalline silicon layers. This contribution represents the first study of FTPS utilization for amorphous silicon layers and cells. FTPS spectra are compared with results of Constant Photocurrent Method (CPM) and Dual Beam Photoconductivity (DBP) measured at different chopping frequencies. We will concentrate to highlight the appropriate measuring conditions and evaluation procedures for correct data interpretation. Moreover, we will present our novel approach for the interference free determination of absorption coefficients of thin films grown on transparent substrates, which is mainly important for very thin layers where broad interference fringes do not allow correct evaluation of parameters such as a slope of the Urbach tail and the defect density.

Pyroelectric Effect Induced by the Built-In Field of thep−nJunction in the Quantum Paraelectric PbTe: Experimental Study

We report here the first observation of a pyroelectric effect in a nonpolar semiconductor. This effect originates in the temperature-dependent electric dipole of the PbTe p-n junction. The junction was illuminated by a chopped CO(2) laser beam, and periodic and single-pulse pyroelectric signals were observed and measured as a function of temperature, reverse bias voltage, and chopper frequency. The measured pyroelectric coefficient is approximately 10(-3) microC/cm(2) K in the region of 40-80 K. The theoretical model describes quantitatively all experimental features. The time evolution of the temperature inside the junction region was reconstructed.

Detectivity of YBCO transition edge bolometer: modulation frequency, bias current and absorber effects

Bolometric response and noise characteristics of YBCO superconductor transition edge IR detectors with relatively sharp transition and its resulting detectivity are investigated both theoretically and experimentally. The magnitude of response of a fabricated device was obtained for different bias currents and modulation frequencies. Also dependency of intrinsic noise of the device on the bias current in the vicinity of critical temperature was studied for various values of chopping frequencies. Using the measured and calculated bolometric response and noise characteristics, we found and analyzed the device detectivity versus frequency for different bias currents. The detectivity versus chopping frequency of the device did not decrease following the response strongly, due to the decrease of the noise at higher frequencies up to 1KHz, resulting in maximum detectivity around the modulation frequency of 100Hz. We also improved the detectivity of the device through the increase of the surface absorption by using a novel infrared absorber, which is made of a Copper-Carbon composite, coated in a low temperature process. Within the modulation frequency range studied in this investigation, comparison of device responsivity before and after the coating is also presented.

이산화탄소 감지소자를 위한 마이크로볼로미터 구조 최적화 및 특성연구

In this work, we optimized a microbolometer for application of a detector by using MEMS technology. We fabricated a stable thermal isolation structure by varying the lengths of supporting legs which affect bolometer performance. We could fabricate more stable thermal isolation structure for the microbolometer through the results of ANSYS simulations, and minimize the fabrication processes by using bulk micromachining to use a detector. The microbolometer shows a detectivity of cmHz/W at a chopper frequency of 8 Hz and a bias current of with a vacuum package of about torr. Therefore, we put to conclusion that the microbolometer optimized in this experiment could be useful for the application of a detector.

Low power and low voltage chopper amplifier without LPF

A mobile sensor system for very low level biological signals such as neuron spikes is required to implement with a scaled CMOS technology. For a key circuit of these systems, a chopper amplifier (CA) which suppresses DC offset and 1/f noise of MOS devices is widely used. However, the conventional CA consumes large power because it requires a wide-band amplifier exceed a chopping frequency and a post Low Pass Filter (LPF) for eliminating modulation noise. In this paper, a new CA architecture for reducing power consumption is presented. In the architecture, the demodulator is placed at the input of 2nd stage amplifier and the 2nd stage has a narrow band determined with a 1st pole. Moreover the post LPF is not required. The proposed CA was designed and simulated with a 0.18µm CMOS process and a 1.2V supply. When the ratio of chopping frequency and signal band width is set to 100 (=10kHz/100Hz), the power consumption of the CA is reduced to 1/88 (=7µW/616µW) compared with the conventional CA.

DSP-Based Stepping Motor Drivers for the LHC Collimators

The control electronics of the LHC collimators stepping motors will be located in radiation safe zones up to 800 meters far from the motors. With such cable lengths the standard chopping drivers do not work properly because of the voltage losses on the cable and even more because of the high cable capacitance. The capacitance in fact produces a ringing phenomenon on the driver feedback current that limits the control chopping frequency to the point of being incompatible with the tight EM emissions requirements of the LHC tunnel. In some cases the feedback loop may even become unstable and the driver would fail. The problem was solved by accurately modeling the overall motor-cable system taking into account non-linearities due to hysteresis and eddy currents and by designing an adaptive digital controller, self-tuning to the real cable length. The controller will aim at increasing the chopping frequency to reduce the spectral density of the emissions and at damping the oscillations of the feedback current to avoid instability. In this paper the model of the motor-cable system as well as the digital controller are described. Particular attention is devoted to the hardware implementation based on a TI Cx2000 DSP.

Photomodulated voltammetry investigations on the benzyl radical

Benzyl radicals were generated photochemically directly in an electrochemical cell, using diphenylacetone as a precursor. The half-wave potentials and the plateau current of benzyl radical reduction and respectively oxidation were determined in acetonitrile, propylene carbonate, N, N-dimethylformamide and dimethylsulfoxide. While half-wave potentials and transfer coefficients were independent of chopper frequency and phase setting, the plateau current was markedly influenced by these parameters. The lifetime of the radical was obtained from the dependence of the phase shift of the current on the angular frequency. The influence of precursor concentration, light intensity and nature of the solvent on the radical lifetime was investigated.

Photoluminescence-free photoreflectance spectra using dual frequency modulation

Photoreflectance (PR) spectra are usually obtained by measuring the relative change on the reflectivity of a semiconducting sample induced by a chopped laser beam. The laser beam can also produce photoluminescence (PL) emission at the sample surface which, detected at its same frequency, could appear as an offset distorting the PR spectrum. This parasitic and intrinsically noisy PL signal, not easily discriminated electronically nor optically filtered, can become the dominant part of the PR spectrum at low sample temperatures, hiding spectrum features under its associated noise, or even avoiding data acquisition. An alternative method for PL signal discrimination is proposed in this work, using a different chopping frequency for each light beam: PL and reflected signals will appear each one at its own chopping frequency, while PR signal will be detected at its frequency sum, allowing signal separation by frequency. Both experimental setups are compared using a highly luminescent quantum well structure at low temperatures. While the standard setup suffers the PL limitation, the proposed method overcomes this constraint, allowing good quality spectra to be measured at temperatures as low as 12K.

Responsivity of ZnTe∕n-GaAs heterostructures formed by infrared nanosecond laser deposition

With nanosecond laser pulses at 1064nm, ZnTe was ablated and deposited onto n-GaAs. The intrinsic and extrinsic room temperature responsivities of the heteropairing was investigated with lock-in technique employing various optical chopping frequencies. The work brings to light two peculiar features: The photocurrent of the structure possesses a frequency independent (isosbestic) point and the dominating photocurrent, i.e., defect or bulk related, crossing the sample can be sensitively altered via the sign of the bias. The reported features here, which cannot be achieved with GaAs itself, might open useful technological calibrating applications for light detectors.

Photo-formation of gold nanoparticles: Photoacoustic studies on solid monoliths of Au(III)–chitosan–silica aerogels

Photo-formation of gold nanoparticles in the solid monoliths of Au(III)–chitosan–silica aerogels with different Au/NH 2 molar ratios has been investigated using photoacoustic spectroscopy. Upon exposing to 320 nm UV light, a new absorption feature in the visible region around 525 nm could be seen due to the surface plasmon resonance of gold particles that are generated as a result of UV-induced reduction of Au(III) to Au(0). The plasmon band becomes stronger and shows saturation effects upon increasing the UV exposure time. A blue shift of about 7 nm is also noticed on exposing the sample (Au/NH 2 = 1/5) for 6 h, indicating a slight decrease in the nanoparticle size due to light-induced annealing with increasing the UV exposure time. The PA signals monitored as a function of chopping frequency show ω −1 dependence, implying the thermally thin character of Au(III)–chitosan–silica aerogels.

Spectral spatial fluctuations of CMBR: Strategy and concept of the experiment

Spectral spatial fluctuations (SSF) of the cosmic microwave background radiation (CMBR) temperature are considered as a result of an interaction of primordial atoms and molecules with CMBR in proto-objects moving with peculiar velocities relative to the CMBR. Expected optimistic values of Δ T/ T are 2 × 10 −5–2 × 10 −6 for SSF caused by HeH + at z = 20–30 which are possible redshifts of early reionization scenario. The bandwidth of the lines is 0.1–2% depending on the scale of proto-objects and redshifts. For the SSF search CMBR maps in different spectral channels are to be observed and then processed by the Difference method. Simulation of the experiment is made for MSRT (Tuorla Observatory, Finland) equipped with a 7 × 4 beam cryo-microbolometer array with a chopping flat and frequency multiplexer providing up to seven spectral channels in each beam (88–100 GHz). Expected Δ T/ T limit in the experiment is 2 × 10 −5 with 6′–7′ angular and 2% frequency resolution. Simulation shows that SSF may be recognized in the angular power spectrum when S/N in single frequency CMBR maps is as small as 1.17 or even something less for white noise. Such an experiment gives us a possibility to set upper limit of SSF in MM band and prepare future SSF observations.

A CMOS Chopper Offset-Stabilized Opamp

In this paper, an offset-stabilized operational amplifier is described. The amplifier employs a chopper amplifier in a low frequency path to cancel the offset of a wide-bandwidth amplifier. A sample-and-hold circuit is used to reduce the chopper ripple, and the low frequency path is also offset-stabilized to further reduce the residual offset. The amplifier has less than 1.5 offset at a 16-kHz chopper frequency, a unity gain frequency of 1.3 MHz with a 50-pF load, and draws 700 from a 5-V supply. The amplifier was realized in a 0.7 CMOS process, and has an effective chip area of 3.6.

Manufacture and characterization of sol–gel V 1− x− yW xSi yO 2 films for uncooled thermal detectors

V 1− x− y W x Si y O 2 films for uncooled thermal detectors were coated on sodium-free glass slides with sol–gel process, followed by the calcination under a reducing atmosphere (Ar/H 2 5%). The V 1− x− y W x Si y O 2 films as prepared inherit various phase transition temperatures ranging from 20 to 70 °C depending on the dopant concentrations and the fabrication conditions. Compared to the hysteresis loop of plain VO 2 films, a rather steep loop was obtained with the addition of tungsten components, while a relaxed hysteresis loop with the tight bandwidth was contributed by Si dopants. Furthermore, the films with switching temperature close to room temperature were fabricated to one-element bolometers to characterize their figures of merit. Results showed that the V 0.905W 0.02Si 0.075O 2 film presented a satisfactory responsivity of 2600 V/W and detectivity of 9 × 10 6 cm Hz 1/2/W with chopper frequencies ranging from 30 to 60 Hz at room temperature. It was proposed that with appropriate amount of silicon and tungsten dopants mixed in the VO 2, the film would characterize both a relaxed hysteresis loop and a fair TCR value, which effectively reduced the magnitude of noise equivalent power without compromising its performance in detectivity and responsivity.

Study on new structure uncooled a-Si microbolometer for infrared detection

A new structure uncooled amorphous silicon (a-Si) microbolometer for infrared (IR) detection has been fabricated and characterized. New type of thermal isolation and IR absorption structures based on polyimide (PI) and bottom metal reflective layer are presented. The fabrication process is described in this paper. The as-prepared microbolometer has the advantage of low cost, high yield and moderate performance. The dependence of resistance with different aspect ratio on operating temperature has been investigated and the temperature coefficient of resistance (TCR) is achieved. Based on the measured responsivity and noise characteristics, the influence of detectivity on chopping frequency is discussed. According to the measurements and calculations results, the maximal TCR is −2.8% and at a bias voltage of 5V, the maximum detectivity of 1.7×108cmHz1/2W−1 is achieved at chopping frequency of 30Hz.

Cancellation of Amplifier Offset and <formula formulatype="inline"> <tex>$1/{f}$</tex></formula> Noise: An Improved Chopper Stabilized Technique

In this brief, a theoretical analysis on the limits of conventional chopper stabilization technique is presented and a solution conceived to improve the amplifier performance is proposed. The expected replicas of the 1/f noise at the chopping frequency and its multiples are attenuated by a modified chopping control. Simulations done by using records of real 1/f noise outputs showed that, by following the proposed approach, the spectrum of the signal is not affected while the 1/f replicas are reduced by more than 40 dB with respect to conventional techniques. The required circuit for the generation of the chopping signal is also described. The resulting overhead with respect to conventional techniques is negligible and fully acceptable.

Laser-based sensor for detection of hazardous gases in the air using waveguide CO2 laser

A spectrometer based on the principle of photoacoustic spectroscopy has been developed recently at our laboratory for the detection of hazardous gases such as O3, C2H4, SO2, NO2 and SF6. In most of our earlier works, we employed a mechanical chopper to modulate the laser beam and this chopper modulation has the crucial disadvantage of instability in the chopper frequency. Even a minor shift of about 1 Hz in the modulation frequency could significantly reduce the photoacoustic signal by an order of magnitude at the acoustic resonant mode of the photoacoustic cell. To overcome this problem, we developed a photoacoustic spectrometer where a wave guided CW CO2 laser beam is modulated electronically with the external frequency generator. Our preliminary results show that the electronic modulation of CO2 laser beam improved the sensitivity of our spectrometer by a factor of 6. The parametric dependence of photoacoustic signal on laser power, modulation frequency and trace gas concentration, was investigated and the comparison between the two modulation techniques is presented in this paper for detection of trace gases such as C2H4.

English

Pyroelectric sensors based on ceramic materials have found applications in IR detection in defence and civil systems. To develop suitable ceramic material for IR detector applications, a modified PZT system with compositional formula Pb 1-x Sm x (Zr 0.58 Fe 0.18 Mn 0.02 Nb 0.2 Ti 0.02 )O 3 where 0 ≤ x ≤ 0.025 was synthesised by conventional solid state reaction method. The materials were characterised for their properties like dielectric and pyroelectric coefficient. Hysteresis loop was recorded at room temperature. The sample with 2 mole per cent samarium (Sm) substitutions was found to be more promising for sensor application based on its high material's figure of merit (FOM). Sensors fabricated with this material were integrated with FET amplifier. The devices configured with compensating element were evaluated for different chopping frequencies. The value of material's FOM, F D , and detectivity, D*, were determined from measured parameters and were, 3.6 x 10 -5 Pa -1/2 and 2 x 10 8 cmHz 1/2 /W, respectively.

Study of thermal and optical properties of the semiconductor CdTe by photopyroelectric spectroscopy

In this study, photopyroelectric spectroscopy (PPES) was used to obtain thermal and optical properties of CdTe thin films in the 300–1200 nm wavelength range. The samples studied were grown on glass or BaF2 substrates using the hot wall epitaxy technique with varying thicknesses. The normalized photopyroelectric signal intensity and its phase were independently measured as a function of wavelength λ and chopping frequency f. Equations of both the intensity and the phase of the PPES signal, taking into account the thermal and the optical characteristics of the pyroelectric detector, were used to fit the experimental results. From the fittings, we have obtained the values of thermal diffusivity coefficient α, thermal conductivity k, optical absorption coefficient, and the optical gap of CdTe.