Measured Input Impedance Research Articles

Linear servo-controlled pressure generator for forced oscillation measurements.

In respiratory input impedance measurements, the low-frequency range contains important clinical and physiological information. However, the patient's spontaneous ventilation can contaminate the data in this range, leading to unreliable results. Unbiased estimators are a good alternative to overcome this problem, provided that the generator is considered linear. This condition is not fulfilled by most existing generators as they are based on loudspeakers, which have strong nonlinearities. The present work aims to contribute to the solution of this problem, and describes a pressure generator that minimises the nonlinearities by an optical sensor placed in a position feedback loop. The static evaluation shows a high linearity for the optical system. The well known frequency response of pressure transducers is used in the dynamic evaluation of the instrument. The analysis of the generator shows that the use of position feedback improved the frequency response. The total harmonic distortion (THD) measurement shows that closed loop resulted in an effective decrease in the nonlinearities. The reduction of THD achieved by the servo-controlled generator can contribute to the practical implementation of the unbiased estimators, increasing the reliability of the impedance data, especially in the low-frequency range. This system is compared with conventional generators and with another servo-controlled system.

Method for measuring the attenuation and phase constantsof a surface wave propagating along an infinite plane

A method is presented for determining the attenuation and phase constants of a surface wave propagating along an infinite planar layer of metal-backed material. Two input impedance measurements of a rectangular waveguide partially filled with the test material are used to infer the propagation characteristics of the plane surface wave. Measurements are presented which validate the method.

Impedance-matching system for a flexible surface-coil-type resonator

This article describes an impedance-matching system for a flexible surface-coil-type resonator (FSCR) used in electron paramagnetic resonance (EPR) experiments. To design the matching system, the input impedance of the FSCR was formulated using transmission line theory, and then the parameters of a matching circuit using varicap diodes were calculated. Experimental measurements of input impedance showed the validity of the formulation and the usefulness of the matching system. The matching circuit made by the varicap diodes 1SV186 offered the tunable bandwidth of 50 MHz for the prototype FSCR. Such a matching system also offers the possibility of remotely tuning EPR resonators electronically.

Measurement of the dynamic input impedance of a dc superconducting quantum interference device at audio frequencies

The coupling effects of a commercial dc superconducting quantum interference device (SQUID) to an electrical LC resonator which operates at audio frequencies (≈1 kHz) with quality factors Q≈106 are presented. The variations of the resonance frequency of the resonator as functions of the flux applied to the SQUID are due to the SQUID dynamic inductance in good agreement with the predictions of a model. The variations of the quality factor point to a feedback mechanism between the output of the SQUID and the input circuit.

Low-voltage standards in the 10 Hz to 1 MHz range

A step-down procedure is described for establishing voltage standards in the 2 mV to 200 mV range at frequencies between 10 Hz and 1 MHz. The step-down employs low-voltage thermal voltage converters and micropotentiometers. Techniques are given for measuring input impedance and calculating loading errors.

Measurement techniques for integrated-circuit slot antennas

Techniques for on-wafer integrated antenna measurement are presented. Two novel techniques and associated apparatus are developed specifically for the measurement of input impedance and radiation pattern of 20 GHz to 30 GHz slot antennas integrated onto GaAs wafers. Both draw upon equipment already used for nonradiating integrated circuits, i.e., a wafer-prober system, but involve significant modifications. The resulting setups are validated using a numerical electromagnetic analysis tool which shows the effect of the measurement fixtures on the experimental results. Finally, some examples of measured data are given to illustrate the usefulness of the techniques.

White noise approach for estimating the passive electrical properties of neurons.

1. The passive electrical properties of whole cell patched dentate granule cells were studied with the use of zero-mean Gaussian white noise current stimuli. Transmembrane voltage responses were used to compute the first-order Wiener kernels describing the current-voltage relationship at the soma for six cells. Frequency domain optimization techniques using a gradient method for function minimization were then employed to identify the optimal electrical parameter values. Low-power white noise stimuli are presented as a favorable alternative to the use of short-pulse current inputs for investigating neuronal passive electrical properties. 2. The optimization results demonstrated that the lumped resistive and capacitive properties of the recording electrode must be included in the analytic input impedance expression to optimally fit the measured cellular responses. The addition of the electrode resistance (Re) and capacitance (Ce) to the original parameters (somatic conductance, somatic capacitance, axial resistance, dendritic conductance, and dendritic capacitance) results in a seven-parameter model. The mean Ce value from the six cells was 5.4 +/- 0.3 (SE) pF, whereas Re following formation of the patch was found to be 20 +/- 2 M omega. 3. The six dentate granule cells were found to have an input resistance of 600 +/- 20 M omega and a dendritic to somatic conductance ratio of 6.3 +/- 1.1. The electronic length of the equivalent dendritic cylinder was found to be 0.42 +/- 0.03. The membrane time constant in the soma was found to be 13 +/- 3 ms, whereas the membrane time constant of the dendrites was 58 +/- 5 ms. Incorporation of morphological estimations led to the following distributed electrical parameters: somatic membrane resistance = 25 +/- 4 k omega cm2, somatic membrane capacitance = 0.48 +/- 0.05 microF/cm2, Ri (input resistance) = 72 +/- 5 omega cm, dendritic membrane resistance = 59 +/- 4 k omega cm2, and dendritic membrane capacitance = 0.97 +/- 0.06 microF/cm2. On the basis of capacitive measurements, the ratio of dendritic surface area to somatic surface area was found to be 34 +/- 2. 4. For comparative purposes, hyperpolarizing short pulses were also injected into each cell. The short-pulse input impedance measurements were found to underestimate the input resistance of the cell and to overestimate both the somatic conductance and the membrane time constants relative to the white noise input impedance measurements.

Input Impedance of Revascularized Skeletal Muscle, Renal, and Mesenteric Vascular Beds

Input impedance describes the relationship between pressure and flow in a vascular system and, hence, characterizes the outflow bed. The purpose of this investigation was to measure input impedance spectra in vascular reconstructions of skeletal muscle, renal, and mesenteric beds. Input impedance was measured in 107 vascular reconstructions in 96 patients. Reconstructions were performed at the aortofemoral/aortoiliac (AF, n = 20), femoropopliteal (FP, n=18), femorodistal (FD, n=41), infrapopliteal-inframalleolar (IM, n = 6), renal (REN, n = 16), or mesenteric (MES, n= 6) level. Grafts were constructed from autologous vein in all cases except AF bypasses in which bifurcated woven Dacron grafts were employed. Input impedance was measured intraoperatively after reperfusion. For impedance calculation, simultaneously acquired intraluminal pressure (transducer-tipped pressure catheter) and blood flow (electromagnetic probe) waveforms of ten-second duration were digitized at 200 Hz and subjected to Fourier transformation in near real-time. AF grafts exhibited the highest blood flow (443 ± 72.8 mL/minute) followed by MES (300 ± 30.4), REN (172 ± 43.9), FP (91.6 ± 20.0), FD (59.3 ± 5.09), and IM grafts (22.4 ± 5.44 mL/minute). A similar (inverse) trend was observed with respect to resistance (Rin), ie, MES≈AF<REN<<FP<FD<<IM(15.7 ± 2.2=23.7 ± 6.5<69.2 ± 12.2 < < 120 ± 31.9 < 137 ± 13.0 < < 337 ± 75.8 x 103 dyne•s•cm-5). As expected, AF grafts exhibited the lowest characteristic impedance (Z0 3.5 ± 0.8 x 103 dyne•s•cm-5). However, the Z0 of REN (20.2 ± 3.7 x 103 dyne•s•cm-5) grafts exceeded that of FP and MES grafts (10.5 ± 1.1 and 12.4 ± 4.0 x103 dyne•s•cm-5) and more closely approximated that of FD (22.2 ± 2.2 x 103 dyne•s•cm-5). The highest Z0 was seen in IM grafts (42.1 ± 15.8 x 103 dyne•s•cm-5). Therefore, the highest ratios of Z0/Rin were observed with MES (0.75 ± 0.23) and REN (0.33 ± 0.04) grafts as compared with the other groups (AF 0.23 ± 0.03, FP 0.12 ± 0.13, FD 0.19 ± 0.02, IM 0.16 ± 0.06). As expected, Rin of vascular reconstructions follows the general trend of MES ≈ AF < REN < < FP < FD < < IM. However, examination of the high-frequency components of the impedance spectra reveals that Z0 follows a different pattern, AF < FP MES < FD ≈ REN < < IM and the ratio of ZO/Rin is highest in REN and MES vascular beds compared with skeletal muscle beds. Thus, although the REN and MES beds are “privileged” (ie, maximally dilatated with low arteriolar tone), the intrinsic properties of the graft and larger blood vessels are no different than those of a femorodistal bypass.

Accurate impedance model for antiresonant monopoleson finite ground planes

A simple and accurate procedure based upon reaction theory is presented for computing the input impedance of a monopole antenna backed by a finite ground plane. This procedure is used to finally verify theoretically some of the controversial antiresonant measured input impedances presented by Meier and Summers in their classic 1949 work

Arterial impedance in patients furing intraaortic balloon counterpulsation

Symptomatic improvement of a patient's hemodynamic condition during intraaortic balloon counterpulsation (IABC) is considered to result largely from a reduction in afterload. Afterload can be accurately quantified by arterial input impedance measurements. Here we report the effect of IABC on arterial impedance in humans. To characterize the effects of IABC on arterial input impedance, impedance measurements were obtained using aortic annulus Doppler flow and pressure from the aortic balloon catheter. Impedance spectra were compared between the cardiac cycles preceding and following the cycle with IABC in 25 patients. Intraaortic balloon counterpulsation increased stroke volume (23%; p = 0.001), reduced myocardial oxygen demand (11%; p = 0.02), and decreased the aortic pressure at the onset of systole (16%; p = 0.001). There was also a decrease in systemic vascular resistance (24%; p = 0.001), characteristic arterial impedance (21%; p = 0.002), and pulse wave reflection (20%; p = 0.006). Linear regression analysis showed that an increase in stroke volume was predicted only by the decrease in systemic vascular resistance (r = -0.81; p = 0.001). The reduction in systemic vascular resistance appeared to be the major mechanism by which IABC improved cardiac pumping efficiency. This effect may result from the passive distention of the peripheral vascular bed due to the propagation of the balloon-augmented diastolic pressure through the arterial system.

Millimeter-wave dual-band microstrip patch antennas using multilayer GaAs technology

This paper describes the performance and construction of several multiband integrated microstrip antennas around 35 GHz on in-house semi-insulating multilayer GaAs. The simulated and measured input impedances and radiation patterns at both the lower and upper resonant frequencies are given and some experimental results are provided to check the accuracy of the electromagnetic simulations.

Measured input impedance and mutual coupling of rectangular microstrip antennas on a cylindrical surface

Measured input impedance and mutual coupling of rectangular microstrip antennas on a cylindrical surface

Input impedance of dielectric resonator antennas top loaded with high permittivity and conducting disks

AbstractA numerical investigation verified by measurements is performed to study the difference between loading the dielectric resonator antenna with conducting or high‐dielectric‐constant disks. The basic objectives of this study are to suggest ways to eliminate losses by replacing conducting antenna parts with dielectric parts, to design antennas with dual resonances, and to broaden impedance bandwidth. Input impedances are computed and measured when the ground‐plane‐backed dielectric resonator is excited by a coaxial probe. Excellent agreement is obtained between the numerical and the measured input impedances. © 1995 John Wiley & Sons, Inc.

Some aspects of tuning and clean intonation in reed instruments

The influence of the first and second resonance frequencies on tuning, timbre (or tone colour) and ease of playing is investigated for reed instruments, such as the clarinet, alto saxophone and oboe. Theoretical analyses of the effects of the reed and the player embouchure (i.e. lip position and pressure on the reed) are reviewed, as well as the consequences of inharmonicity in the resonance frequencies. This review allows us to present interesting interpretations of the numerous experiments reported here. Three kinds of results are given: (1) comparison of playing frequencies and first resonance frequencies, for several fingerings with or without open register hole, leading to the definition of a frequency independent length correction for the embouchure; (2) examination of the effect of inharmonicity of the two first resonance frequencies on both tone colour and ease of playing, the causes coming from either the player embouchure or the instrument construction; (3) comparison between theory and experiment for the inharmonicity produced by the changes in conicity in oboes, leading to an interpretation of the maker 's choices. The results show how judicious use of simple tools, such as calculations or measurements of input impedance or playing data obtained using an artificial mouth, can be of help to the understanding of instrument construction and to the instrument designer.

An efficient and accurate model of the coax cable feeding structure for FEM simulations

An efficient and accurate coax cable feed model is proposed for microstrip or cavity-backed patch antennas in the context of a hybrid finite element method (FEM). A TEM mode at the cavity-cable junction is assumed for the FEM truncation and system excitation. Of importance in this implementation is that the cavity unknowns are related to the model fields by enforcing an equipotential condition rather than field continuity. This scheme proved quite accurate and may be applied to other decomposed systems as a connectivity constraint, comparisons of our predictions with input impedance measurements are presented, and these demonstrate the substantially improved accuracy of the proposed model.

Measurement of aortic input impedance in rats.

Measurement of aortic input impedance in the rat is complicated by a high basal heart rate but is possible if appropriate compensation is made for frequency-dependent errors in modulus and phase resulting from analog filters in the equipment and from nonalignment of pressure and flow sensors. Because input impedance is a complex quantity, accurate values for both phase and modulus are required before meaningful interpretation of the data can be made. We measured aortic pressure and electromagnetic ascending aortic blood flow in mature, ether-anesthetized, open-chest male Wistar rats. Pressure and flow waveforms were averaged in the time domain and converted to Fourier series. Flow moduli were corrected for the measured frequency response of the flowmeter. Phase spectra were corrected by the classic frequency-domain and two new time-domain methods. Compensation for instrumentation errors was assessed at two different flowmeter filter settings in five animals. Reproducibility, variability, and the effects of vasoconstriction were assessed in 43 animals. Three methods of estimating characteristic impedance from the impedance spectra were evaluated and found to produce comparable results at baseline and following pharmacological elevation of blood pressure with graded methoxamine infusion. Physiologically equivalent values for phase, as assessed by comparing oscillatory power calculated from the impedance spectra, were obtained with each of the phase-correction techniques. The new time-domain methods facilitate the assessment of aortic input impedance in this small animal model because they do not require measurement of the spatial separation between pressure and flow transducers and pulse wave velocity in the proximal aorta.

Measurements of the acoustic input impedance of cat ears: 10 Hz to 20 kHz

The acoustic input impedence of the ear is a useful measure of the behavior of the middle ear and of its effect on the acoustics of the external ear. A high-impedance acoustic source with an integral microphone was designed for acoustic-impedance measurements. The source's Norton equivalent circuit was determined from measurements of the sound pressure it generated in known acoustic loads. Tests on simple acoustic configurations show errors in impedance measurements of less than 10% in magnitude and 7 degrees in angle over a frequency range from 10 Hz to 10 kHz with increasing errors at higher frequencies. Measurements at the tympanic membrane (TM) on five cat ears with widely opened middle-ear cavities show an impedance that is compliance-like below 0.3 kHz and approximately resistive above 2 kHz. With the cavities intact the impedance magnitude is somewhat larger for low frequencies, has a sharp maximum near 4 kHz, and at the highest frequencies is little affected by the state of the cavities. Impedance magnitude varies among ears by a factor of 3. The pressure reflection-coefficient that is determined from the impedance is frequency dependent with magnitude between 0.2 and 1. To characterize the motion transformation of the TM we calculate the ratio of tympanic-membrane volume velocity to the velocity of the mallear umbo, called here the kinematic area ATK. This complex quantity is constant with an angle of zero for frequencies below 0.6 kHz, but at higher frequencies both magnitude and angle of ATK vary with frequency.

The dynamic indentation of a finite elastic plate

The Shore hardness teat has been widely used and has become the standard for characterizing materials in the rubber and plastics industries. The obtained hardness scale values, ASTM or IRHD, are often related to the static elastic properties of rubber like-materials. Recently, progress on the problem of obtaining dynamic viscoelastic properties has been made through the use of an oscillating spherical indentor and modeling the measured input impedance by the radiation impedance of a radiating sphere. When the sphere radius is small, such that ka<<1 for longitudinal waves, then the measured shear modulus and loss factor is in good agreement with that found using other dynamic methods such as the DMTA. This model is appropriate only for relatively soft materials or very thick hard materials where the thickness of the material can be ignored. The object of the current work is to extend the methodology to layers of finite thickness. The problem is studied under the assumptions of linear lasticity and the condition that there is perfect adhesion between the indentor and the elastic layer. The method of solution that is adopted will be described and as such it constitutes the first serious attempt to solve an elastodynamic boundary value problem of this type

Analysis of a magnetic resonance imaging antenna inside an RF shield

AbstractOptimization of an antenna for magnetic resonance imaging (MRI) requires an accurate analysis of antennas located inside a conducting cylindrical RF shield. A Galerkin‐moment method analysis which can be used for an arbitrary‐shaped shield is used. The surface current on a conducting plane is expanded by means of surface patch segments based on Richmond's method. Numerical results are compared with measured input impedance, which confirms the validity of the proposed method. Furthermore, a relation between the dimension of shield and the sensitivity of the antenna is discussed theoretically.

Viscoelastic characterization of materials using a dynamic hardness tester

The Shore or IRHD hardness test has been widely used for many years and has become one of the standards for characterizing materials in the rubber and plastics industries. The obtained hardness scale values, ASTM or IRHD, are often related to the static elastic properties of rubber‐like materials. The classical Hertz equation has also been used to correlate the hardness scale values to the static elastic properties. Recently, progress has been made on the problem of obtaining dynamic viscoelastic properties through the use of a vibrating spherical indentor and modeling the measured input impedance by the radiation impedance of a sphere radiating into a half‐space. In the limiting case where the sphere radius is small, such that ka≪1 for longitudinal waves, it is shown that the measured shear modulus and loss factor are in good agreement with that found using other dynamic methods such as the DMTA or the resonance method. The model is most appropriate for relatively soft materials or very thick hard materials where the thickness of the material can be ignored. The problem of very hard materials and finite thickness where the terminating boundary is important will be discussed.