Frequency Response Calibration Research Articles

A Novel Method for Microphone Channel Frequency Response Calibration Based on Newton Algorithm

Microphone arrays are widely applied in practical applications. In an array, each channel conventionally contains an acoustic sensor and a following signal conditioning circuit (SCC). Frequency response mismatches among channels will adversely affect array’s overall performance. Existing calibration methods provide promising results; however, their accuracy varies considerably across full frequency band and the passband bandwidth shrinkage problem also arises. To address these problems, we propose a novel calibration method based on Newton algorithm. The proposed method cascades calibration filters after channel output signals allowing the frequency response calibration to be equivalent to the optimization of calibration filter coefficients. First, all uncalibrated channels’ bandwidths are analyzed using a chirp calibration signal, resulting in appropriate construction of the desired output. Next, a spectrum mean square error criterion between each channel output and the desired signal is utilized to formulate the frequency response mismatch. Finally, we employ Newton algorithm to design the calibration filter coefficients corresponding to each channel. Explicit mathematical derivation is provided. Since the cost function is quadratic, expressions of the constant Hessian matrix and its inverse are derived. Furthermore, the inverse Hessian can be efficiently computed using spectral coefficients of channel outputs. Simulation results reveal that the proposed method significantly outperforms the state-of-the-art calibration approaches in terms of calibration accuracy, calibrated passband bandwidth performance, and convergence rate. Real-world experiments also verify its effectiveness.

Fast and simple calibration of frequency response and IQ skew for a coherent optical transmitter using a low-bandwidth photodetector.

A fast, precise, and low-cost coherent optical transmitter calibration scheme is proposed that uses multi-tone signals of a novel, to the best of our knowledge, design with unequal frequency intervals. With a single measurement, the proposed scheme can simultaneously calibrate the frequency response and the IQ skew of the transmitter using only a low-bandwidth photodiode. Simulation and experimental results indicate that the measurement error in the frequency response and coherent transmitter (Tx) skew is less than 0.3 dB and 0.2 ps, respectively. The feasibility of the proposed scheme is verified by an experiment involving 25 Gbaud 16-quadrature amplitude modulation (QAM) signal transmission using a Kramers-Kronig (KK) receiver. With the help of this calibration method, the measured bit error ratio performance was increased from 1.77e-2 to 3.52e-3 when the received optical power was -8 dBm.

A Bayesian active-learning test for remote hearing-aid fitting

A major problem of remote or online hearing tests is the calibration of the equipment and control of the acoustic environment: For example, an audiogram requires calibrated headphones with a wide dynamic range and a silent room. Instead, a notched-noise test alleviates these problems and gives valuable information about auditory filters that can be used to fit a hearing aid. Although absolute levels still play a role, relative levels between signal and noise are more robust as the environment does not need to be quieter than the noise. Using traditional methods, determination of auditory-filter shapes across the whole frequency range would take hours. We developed a notched-noise test that uses Gaussian Processes to maintain a probabilistic estimate of the outcome, and to select stimulus parameters based on mutual information. This active-learning method gives an estimate of auditory-filter shapes in about 30 minutes across a wide range of frequency. We demonstrate that these estimates, even when obtained with headphones for which the frequency response and exact calibration is unknown, are sufficient to give a good initial fit of a hearing aid.

Distributed Frequency Response Calibration Based on Consensus Strategy in Microphone Array

Frequency response mismatches among microphones inevitably result in performance deterioration of signal processing algorithms for microphone arrays. A frequency response calibration method based on the consensus strategy is proposed in this article. Adaptive calibration filters concatenated with microphone channels are first introduced to compensate for the frequency response mismatches among microphones. Then, the frequency response mismatch problem is formulated as the minimization of a cost function measuring the spectrum mismatches among neighboring microphone channel outputs. Finally, the consensus strategy is used to design the calibration filter in each microphone channel. By using the consensus strategy, the proposed frequency response calibration method is robust to the room impulse response and noise differences among microphones. The proposed method outperforms gain calibration methods and the centralized frequency response calibration method. Experimental results reveal the validity of the proposed method.

Adaptive Frequency Response Calibration Method for Microphone Arrays

Frequency response mismatch among microphones inevitably leads to performance deterioration of signal processing algorithms of microphone arrays, especially for distributed microphone networks. An adaptive frequency response calibration method for microphone arrays based on frequency domain least mean square (FLMS) algorithm is proposed in this paper. Adaptive calibration filters are first introduced to calibrate the frequency response mismatch among microphones, which are concatenated with microphone channels. Then, the frequency response mismatch problem is formulated with a combined spectrum mean square error criterion of pairwise microphone outputs, and an unit-norm constraint is imposed to avoid undesired solutions. Finally, FLMS algorithm is used to design the calibration filter in each microphone channel. This method achieves good frequency response calibration performance even under severe ambient noise and reverberation conditions. Experimental results reveal the validity of the proposed method.

Frequency Response Calibration Using Multi-Channel Wiener Filters for Microphone Arrays

Frequency response differences among microphones will result in performance degradation of microphone arrays. To remedy this problem, a frequency response calibration method based on multi-channel Wiener filters (MCWFs) is proposed in this paper. The geometric relationship of a single sound source and uncalibrated microphones is first formulated via maximum allowable errors of gain and phase. Next, the system model of the frequency response calibration is constructed, and the microphone signals are approximated to each other using the MCWF. Finally, a least-mean-square (LMS) algorithm is presented to obtain the Wiener solutions rapidly. The proposed method can effectively compensate both the gain and phase errors caused by different sensitivities among microphones in noisy and reverberant environments. The simulation and real-word experimental results reveal the validity of the proposed method.

Первые результаты абсолютных измерений потока солнечного излучения на Иркутском радаре некогерентного рассеяния (ИРНР)

The Irkutsk Incoherent Scatter Radar (IISR) allows us to carry out passive radio observations of the Sun and other powerful radio sources. We describe a method for absolute measurements of spectral flux density of solar radiation at IISR. Absolute measurements are meant to determine the flux density in physical units [W·m–2·Hz–1]. The IISR antenna is a horn with frequency beam steering, therefore radio sources can be observed at different frequencies. Also there is a polarization filter in the antenna aperture, which passes only single (horizontal) polarization. To acquire flux density absolute values, the IISR receiver is calibrated by the Cygnus-A radiation. Since the Sun’s position in the IISR antenna pattern is determined by a frequency differing from the Cygnus-A observation frequency, we perform an additional calibration of the antenna overall frequency response in the 154–162 MHz operation frequency range, using the background sky noise. The solar disk size is comparable with the main beam width in the north—south direction, hence the need to take into account the shape of the brightness distribution in the operation frequency range. The average flux density of the quiet-Sun radiation was ~5 sfu (solar flux units, 10–22 W·m–2·Hz–1) at the 161 MHz frequency.

Power Calibration of Electrosurgical Analyzers Up to 1 MHz

For clinical applications, according to IEC 60601-2-2, power calibration up to 1 MHz is required for electrosurgical units (also referred to as electrosurgical generators), electrosurgical analyzers and their accessories. This paper discusses a reference system which is built for the power calibration of electrosurgical analyzers in the frequency range of 100 kHz to 1 MHz, in combination with powers up to 400 W, voltages up to 400 V, and load impedances between 200 and 500 $\Omega$ . The principle behind the system is based on a calibration of the analyzer input impedance and voltage. Measurements of the individual components required for calibrating analyzers in this range are described. A high-frequency (HF) power source was developed to generate high power for the electrosurgical analyzer. A low-dissipation capacitive voltage divider was developed for HF voltage calibration. With the frequency response calibration of the low range and nonlinearity calibration of the high range of the voltmeter, calibration of the HF voltmeter up to 400 V was realized. The impedance of the analyzer load is measured with impedance meter and the power stability of the load impedance was tested. A summary of the uncertainty components at 1 MHz is presented. The expanded uncertainty ( $k = 2$ ) of the power calibration at 1 MHz is approximately 1.3%, which satisfies the requirements for commercial electrosurgical analyzers.

Acoustic metrology – an overview of calibration methods and their uncertainties

The paper gives a brief description of the principles and the uncertainty of the acoustic calibration methods that today are applied by National Metrology Institutes and calibration service centers. Even if some of the calibration principles have been applied over more than half a century, the methods and the instrumentation are still being refined in order to minimize their uncertainty, to extend their frequency ranges, to include extra parameters and to speed up slow processes. In addition to the traditional methods for microphone sensitivity and frequency response calibration, new development areas, like for example wind power, has created needs for low-frequency and infra-sound calibration, down to 0.1 Hz. Other high-tech areas have lead to the development of methods for phase response comparison calibration of microphones for large arrays, for sound intensity measurement and for verification of dynamic linearity of microphones at very high sound pressure levels, up to about 174 dB that corresponds to 10 kPa.

Permittivity determination of liquid materials using waveguide measurements for industrial applications

There is a need for a simple and relatively inexpensive microwave method for complex permittivity, e, determination of liquid materials in industrial applications. Measurement cables that connect the measurement cell to the measuring instrument (usually a vector network analyser) may be either subject to severe phase instability particularly if they are long or metrology-grade cables are not available, or subject to thermal variation. In these instances, calibration techniques based on complex scattering parameter measurements may drastically suffer from extension of the reference plane to the boundaries of the measurement cell or sample end surfaces. In such situations, amplitude-only methods based on simplified calibration techniques such as response calibration are appropriate and very feasible. These methods have applications in industry such as thickness measurement and disbonding and delamination detection. The motivation of this study is to investigate a suitable method for e determination of liquid materials using amplitude-only waveguide measurements. The analysis is restricted to lossy dielectric materials, which possess large enough attenuation such that the multiple reflections between their two end surfaces can be neglected. We derived an equation in terms of the loss tangent of these materials for selecting a suitable material thickness. We presented two approximations (frequency-independent and power-series representation) for e determination of these materials. For validation of the method, we measured the e of methanol and commercially available antifreeze solution by our method using a simple (frequency response) calibration and by another waveguide method using the thru-reflect-line calibration.

Wideband calibration of photodetector frequency response based on optical heterodyne measurement

AbstractIn this article, an accurate wideband calibration method for photodetector frequency response measurement by optical heterodyne measurement technique, which uses two single‐mode tunable lasers operating near the wavelength of 1.55 mm, has been proposed. The errors introduced by extra fixture as well as laser output fluctuations were considered and eliminated simultaneously. Experimental result indicates that significant improvement in measurement accuracy has been achieved in the frequency range from 0 to 48.65 GHz, and the calibrated photodetector frequency response is matched well with the standard data provided by the manufacturer. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 44–48, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23957

Infrasound calibration of measurement microphones

Increasing interest for traceable infrasound measurements has caused the Consultative Committee for Acoustics, Ultrasound and Vibration (CCAUV) of BIPM to initiate a key comparison calibration project (CCAUV.A-K2) on pressure reciprocity calibration down to 2 Hz. Ten national metrology institutes, including the Danish Primary Laboratory of Acoustics (DPLA), take part in this project. In addition DPLA has started its own infrasound calibration project, which is described in this paper. The purposes of this project are verification of the CCAUV results and development of methods for calibration of general types of measurement microphone between 0.1 and 250 Hz. The project includes the design of an active comparison coupler, an experimental low-frequency reference microphone, and new methods for its frequency response calibration. One method applies an electrostatic actuator and requires a low-pressure measurement tank, while the other requires an additional microphone, whose design is closely related to that of the reference microphone that is to be calibrated. The overall calibration uncertainty (k=2) for ordinary measurement microphones is estimated to less than 0.05 dB down to 1 Hz and less than 0.1 dB down to 0.1 Hz, if the reference is calibrated in the latter mentioned way, i.e., by the related microphones method.

Automatic calibration of channels frequency response in interferometric radiometers

The large field of view required in Earth observation interferometric radiometers does not enable the use of a variable delay to compensate for different transit times, as in radioastronomy. A technique is presented to characterise the influence of a channels frequency response automatically in the shape of the cross-correlation function.

Uncertainty analysis of primary and secondary microphone sensitivity calibrations

Generally, measurement microphone calibrations consist of a relative frequency response calibration and a sensitivity calibration made at a reference frequency, 250 Hz or 1 kHz. As these frequencies are also the main calibrator operation frequencies, they are the most important calibration frequencies for microphones. The paper describes uncertainty of sensitivity calibrations at these frequencies. DPLA (Danish Primary Laboratory of Acoustics) makes primary calibrations by the pressure reciprocity technique in accordance with IEC 1094-2. The uncertainty of these calibrations influences the secondary calibrations performed by DPLA customers like the B&K Calibration Service Centres which perform comparison calibrations. Within recent years Primary Calibration Laboratories in Europe have taken several steps to reduce their calibration uncertainty. This uncertainty is related to the measurement of electrical transfer impedance of coupled microphones and other physical parameters, in addition to other factors such as calculation software and the operator. Also, the uncertainty of environmental sensitivity correction factors contributes significantly to the overall uncertainty of the measurement chain. Therefore, some coefficients are determined individually. The paper describes the weight of uncertainty components and resulting uncertainties. Today, ISO9000 and IEC/TC29 increase the need for uncertainty analysis.

An all-fiber RF modulation technique: frequency response calibration of optical detectors

Two all-fiber Mach-Zender interferometers have been designed to generate RF modulated light at 633 nm and 830 nm. The interferometers are scanned with a piezoelectric tube driven at its fundamental frequency of resonance. The actual experimental arrangement covers the frequency range 1 kHz to 1 GHz. The technique combines several interesting features such as the simplicity, the stability and reliability of all-fiber systems and the use of low-frequency electronics to control and generate the RF modulated light. >

Electrostatic actuator method for calibration of high- temperature fiber-optic microphones

A fiber-optic lever microphone has been developed for high-temperature applications. The sensitivity of the microphone is determined from a pistonphone calibration at room temperature. The electrostatic actuator method is used for the frequency response calibration of the microphone at room temperature and 1000 °F. The microphone is placed inside an oven, and is excited by the electrostatic force between the actuator electrode and the microphone membrane. The frequency response of the microphone, along with the background noise, is analyzed on an FFT analyzer. For a 0.072 in. diameter of the membrane, the typical sensitivity of the microphone is 1.3 μV/Pa, and the typical bandwidth is 47 kHz. The background noise includes contributions from the microphone and the photodetector, in which the shot noise is the dominant source of noise.

Comments on measurement of the electron cyclotron emission by Fabry-Perot interferometer and the experimental results on ECE in WT-III tokamak.

Improvement of the signal to noise ratio of the measurement of electron cyclotron emission (ECE) by scanning Fabry-Perot interferometer is reported. The most important point is the use of the glass beads filter in addition to the capacitive mesh low pass filter. The result of frequency response calibration of the total system is described. The time evolution of the high energy electrons (-100keV) produced in RF current driven tokamak WT-III due to the estimate from the ECE measurement is reported.

Method for fast sine-wave calibration of hot-wire frequency response

Direct measurement of the transfer function of hot-wire anemometers is needed whenever any turbulence frequencies approach the hot-wire frequency response. In particular, in high-speed applications it is not possible to obtain hot-wire frequency response sufficient to capture all of the turbulent kinetic energy. With knowledge of the hot-wire response, either an estimate of the turbulent integral scale or an approximation of the high-frequency part of the turbulent spectrum may be sufficient to correct measurements by post-detection frequency compensation. Described in this article are simple electronic circuits and a technique for obtaining sine-wave test frequency response of a hot-wire anemometer in a very short time without prohibitively high sampling rates.

Calibration of optical modulator frequency response with application to signal level control

A swept frequency technique for measuring the frequency response of an optical intensity modulator and its application to modulation level control are outlined. The technique is precise and suited for wideband measurements. Experiments have been performed up to 2-GHz frequency. The measured response confirms the theoretical prediction. In the experiment on level control, a modulation depth flatness of +/-0.3 dB was obtained for 0.1-2-GHz rf sweeping.