Progress Report of the IEEE Instrumentation and Measurement Group Technical Subcommittee on Precision Coaxial Connectors

The accuracy of RF and microwave measurements made in coaxial systems is often limited at higher frequencies by the mismatch errors from the connectors. The need for higher accuracies in coaxial impedance measurements and for improved coaxial transfer standards of attenuation and power has led to several independent designs of coaxial precision connectors within the United States and abroad. The Committee is in the process of establishing "recommended practices" to standardize definitions, testing procedures, and mechanical and electrical characteristics for precision connectors. The Recommended Practicesfor Precision Coaxial Connectors is divided into four parts. Part I describes "General Requirements and Definitions" for Laboratory Precision Connectors (LPC), General Precision Connectors (GPC), and mechanical and electrical properties. Part II describes "Parameters to be Specified" for environmental conditions and mechanical and electrical characteristics. Parts I and II were completed and approved by the Subcommittee and the Parent Committee in 1963. A draft of Part III concerning "Parameter Limits" for coaxial line sizes, axial and angular forces, critical dimensions and tolerances for mating surfaces, VSWR limits, insertion loss, leakage, contact resistance, and other characterstics has been completed. Part IV concerning "Test Procedures" is now being processed. In March, 1964, Parts III and IV will be presented to the Subcommittee for approval. When all four parts of the Recommended Practices for Precision Coaxial Connectors are approved, this document will then be submitted to the American Standards Association for consideration as a standard.     

The Realization of High-Frequency Impedance Standards Using Air-Spaced Coaxial Lines

Impedances of devices with coaxial inlets, having small reflection coefficients relative to a given characteristic resistance, can be determined accurately (± 0.1 per cent) by comparison with the nominally calculable characteristic impedance of a quarter-wavelength, air-spaced coaxial line in which the inner conductor is supported by the inner conductors of the apparatus connected at each end. In carrying out such comparisons, errors may be introduced by the presence of small series impedances at the connections between the rigid coaxial lines. Such connections cannot be eliminated because they must be made and broken during the process of making comparisons. Errors may also arise from the distributed resistance and the "internal" reactance of the line conductors, as well as from departure of the lines from uniformity in diameter. This paper describes experimental techniques for measuring these residuals and discusses their effects. The design requirements of suitable air-spaced coaxial lines as standards are stated, together with a brief mention of techniques for making comparisons with the standards.     

A Semi-Automatic Method for the Precision Measurement of Microwave Impedance

A system is described for the measurement of the magnitude and phase of the reflection coefficient over wide frequency bands. The method consists of recording the voltage at several points along the standing-wave pattern in a coaxial line. A fixed probe is used and the distance between the probe and the load is altered by successively inserting several lengths of precision coaxial line. Knowing these lengths and the frequency of the signal, a curve representing the standing-wave pattern is fitted to the experimental points by means of a computer program performing a least-squares minimization. To cover a given frequency band, the frequency is changed in discrete steps and the corresponding probe voltage recorded automatically. The measured impedance is referred directly to the characteristic impedance of air-dielectric coaxial lines.     

Generalised equation of soil critical electric field EC based on impulse tests and measured soil electrical parameters

The effective radius of the grounding electrode under high-impulse current becomes higher than the outer radius of the electrode because of soil ionisation. Because of this reason, the ground-impulse impedance is different from the 60-Hz grounding resistance, which is normally measured at low current. The impulse impedance for any given impulse current can be estimated if the critical electric field of the soil is known. It is therefore necessary to have a generalised soil critical electric field equation for grounding systems design as well as lightning performance study. A generalised equation for soil critical electric field Ec is proposed here as a function of soil dielectric constant kg and conductivity ^sigmag. Extensive impulse tests and electrical measurements were performed with different soils under different moisture contents. Dielectric constant kg was measured by the wave propagation technique using a soil-filled coaxial transmission line. Effect of waveshape on critical electric field Ec was statistically analysed for different soils.     

Hybrid multi/single layer array transducers for increased signal-to-noise ratio

In medical ultrasound imaging, two-dimensional (2-D) array transducers are necessary to implement dynamic focusing in two dimensions, phase correction in two dimensions and high speed volumetric imaging. However, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance, which decreases the transducer signal-to-noise ratio (SNR). We have previously shown that SNR is improved using transducers made from multi-layer PZT, due to their lower electrical impedance. In this work, we hypothesize that SNR is further increased using a hybrid array configuration: in the transmit mode, a 10 Omega electronic transmitter excites a 10 Omega multi-layer array element; in the receive mode, a single layer element drives a high impedance preamplifier located in the transducer handle. The preamplifier drives the coaxial cable connected to the ultrasound scanner. For comparison, the following control configuration was used: in the transmit mode, a 50 Omega source excites a single layer element, and in the receive mode, a single layer element drives a coaxial cable load. For a 5x102 hybrid array operating at 7.5 MHz, maximum transmit output power was obtained with 9 PZT layers according to the KLM transmission line model. In this case, the simulated pulse-echo SNR was improved by 23.7 dB for the hybrid configuration compared to the control. With such dramatic improvement in pulse-echo SNR, low voltage transmitters can be used. These can be fabricated on integrated circuits and incorporated into the transducer handle.     

Certification of normal samples of magnetic materials at high frequencies

Conclusions In measuring the magnetic parameters of samples, primarily in certifying normal samples by using sections of coaxial lines, it is necessary to select the optimum length of the line in relation to the available scales on the admittance (impedance) meter and the values of the measured parameters; introduce corrections for losses in the line either by calculations or graphically for materials with low losses; and allow for the electrical parameters of ferrites or magnetodielectrics (their complex permittivity), especially if the normal samples do not fulfill conditions (19).Translated from Izmeritel'naya Tekhnika, No. 12, pp. 42–45, December, 1970.     

Experimental Characterization of a Singly Loaded E-Plane Waveguide Post by Using a Tunable Coaxial Line

Equivalent-circuit parameters of a singly loaded E-plane waveguide post have been experimentally measured by loading one end of the post with a tunable coaxial line. The resulting structure shows a transmission minimum and a transmission maximum. The positions of the movable short for minimum and maximum transmission have been correlated with the parameters to be measured. It is seen that the errors due to uncertainty in the length measurement can be minimized through the use of a coaxial line with the highest feasible characteristic impedance. Agreement between theoretical and experimental results is satisfactory.     

A novel 24-GHz bandwidth coaxial probe

An electrical probe based on specially designed coaxial tips is shown to have 24-GHz bandwidth. It can be used to test high-speed signal propagation on planar or nonplanar chip or package interconnection structures with signal/ground pads as small as 50 μm. The detailed fabrication procedure, characterization, and use of the probe are presented. A variation of the design has 500-Ω input impedance and a bandwidth of 19 GHz     

一种端接负载的同轴传输线特性阻抗时域测量方法

针对作为微波阻抗及散射参数标准的同轴线特性阻抗准确测量的热点和难点问题,提出了一种端接负载的同轴传输线特性阻抗时域测量方法,其测量结果可溯源至几何量等物理参数,实验验证结果的有效性En＜1,该方法可以作为同轴传输线特性阻抗量值准确测量的参考方法。     

Thin-wire finite-difference time-domain model for insulated and resistively loaded cylindrical antennas driven by coaxial lines

A thin wire-based finite-difference time-domain (FDTD) model for a simple analysis of insulated and resistively loaded cylindrical antennas fed by coaxial lines is proposed. The resistive loading and the coaxial feed line are approximated to equivalent resistors along the antenna axis and the equivalent source over the infinitesimal feed gap, respectively. The effects of the insulation are corrected by employing thin-wire approximation and the boundary condition. A full coarse-grid FDTD algorithm is then implemented without additional grid refinements for the insulation, the resistive loading and the feed line. As a function of insulation properties and resistive loading profiles, the transient reflected feed voltage and the input impedance of antennas are calculated numerically. The validity of the proposed model is proved by comparing it with the results of the full fine-grid FDTD.     

Finite-difference time-domain model for the feeding gap of coaxial probe driven antennas

In the finite-difference time-domain (FDTD) method, a simple and realistic feed model for coaxial probe driven antennas is proposed here. The feed zone of the antenna may be considered as an equivalent source in view of the antenna theory and a load port in view of the transmission line theory. The proposed feed model is constructed by combining the infinitesimal-gap source condition of the antenna and the equivalent load condition of the feed line. It leads to perform no additional FDTD cell modelling of the line. The transient reflected voltage and the input impedance of cylindrical monopole antennas fed by coaxial lines are calculated numerically and then compared with the accurate measurement and a full fine-grid. The FDTD results of the proposed model have a good agreement with the measured data and the fine-grid results.     

High-frequency input impedance characterization of dielectric films for power-ground planes

Broadband impedance characterization of high dielectric constant (high-k) films was performed using a coaxial test fixture configuration. The presented coaxial test fixture and broadband measurement methodology of impedance for high-k films minimizes systematic uncertainties by reducing the interconnection inductance and improving the calibration procedure. In the APC-7 configuration, the technique enables accurate evaluation of impedance at frequencies of 100 MHz to 10 GHz with resolution of 0.01 /spl Omega/. The electrical characteristic of high-k films was found to be consistent with a capacitive load without significant contribution from the circuit inductance that typically dominates the high-frequency response. The experimental data and numerical simulations showed that high-k organic-ceramic composite materials could considerably suppress resonant behavior of the power-ground planes. It was found that high-k organic resins filled with ferroelectric ceramic powders exhibit a high-frequency dielectric loss that increases with increasing volume fraction of the ceramic component. The dielectric dispersion and the corresponding dielectric loss of organic-ceramic hybrid materials can serve as an effective mechanism for suppressing the resonant standing waves in power-ground planes.     

Characteristic impedance of unbalanced TDR probes

Time domain reflectometry (TDR) may be used to make simultaneous measurements of both dielectric constant and conductivity by means of a probe inserted into the medium. The air-spaced characteristic impedance of the probe is required in order to estimate the conductivity from the final value of the TDR waveform. Unbalanced probes derived from a coaxial line by replacing the outer shield with regularly spaced wires are favored for many applications because they eliminate the need for a balun. Until now, it has been necessary either to measure the probe characteristic impedance or to calibrate it in solutions of known conductivity. This paper shows how to deduce an expression for the probe characteristic impedance by means of a conformal transformation. For thin wires, a simpler formula has been derived which is suitable for hand calculation. The accuracy of these analytic expressions has been assessed by a method of moments numerical solution which exhibits very rapid convergence as extra basis functions are added. The characteristic impedance formula obtained by conformal transformation and its thin wire approximation are found to be accurate to 0.1% and 1%     

On strategies for automatic bridge balancing

Automatic balancing of coaxial AC impedance bridges asks for simple, robust and efficient strategies suitable to be easily implemented on PC controllers. The paper considers the problem of balancing a bridge with n detectors to be equivalent to the problem of finding the root of a complex-valued vector function F. To avoid electrical modeling of the bridge, F is considered generally unknown except at specific points evaluated by experiment. In this framework, efficient root-finding strategies can be employed; in particular, secant method and two generalizations of it to the case n>1 are proposed. Experimental results on a test circuit are given.     

Performance analysis of piezoelectric composite plates with consideration of the internal losses

The performance of a 2.3 MHz composite transducer with 1-3 connectivity is analyzed using the transmission line model. The basic transducer parameters, including the attenuation coefficient, are determined from the electrical impedance in the vicinity of the resonance. Using these parameters, the effects of internal losses on round trip insertion loss (RTIL) are investigated. Also, electrical impedance, pulse-echo, and RTIL response are calculated and found to be in good agreement with the experimental data.     

Precision Coaxial VSWR Measurements by Coupled Sliding-Load Technique

The method described employs a slotted line for VSWR measurement and a precision coaxial line as an impedance standard. A sliding load terminates the standard line during the process of transforming the impedance of the slotted line to that of the standard line. This load is coupled mechanically to the slotted-line probe so that the two move together. The VSWR seen by the probe is then, to first approximation, only that due to the reflection from the junction of the two lines. It is simple, therefore, to tune out the residual reflection without requiring a perfect load. The slotted line is then effectively transformed to the impedance of the standard line, and subsequent measurements made with it at the same frequency indicate VSWR with respect to the standard line. In order to minimize the effect of amplitude fluctuations of the RF source and to expand the scale, the detected probe output is compared with the detected output of a directional coupler monitoring the incident power. The difference is recorded with a strip recorder; the paper transport of the recorder is synchronized with the probe travel. In describing the method, considerable background material is presented on other precision impedance-measuring methods and techniques and on the general state-of-the-art. Theory, accuracy, system details, and results are given. The system appears capable of making absolute impedance measurements on a low VSWR termination to a maximum possible error in reflection coefficient of 0.0012.     

Two Methods for Measuring High Permittivity at Microwave Frequencies

Permittivity is measured by impedance measurement on thin cylindrical samples placed in an ordinary rectangular waveguide and in a cavity formed by an ordinary coaxial transmission line. The first method does not depend on resonance and is characterized by a rather high accuracy. The second, more conventional method, is used in conjunction with the first for study of dc bias dependence of the permittivity.     

Assigning RF/DC Transfer Difference to High Frequency Voltage Primary Standard up to 1 GHz at NPLI

National Physical Laboratory (NPLI) India is the premier research and development center and the National Metrology Institute, which provides traceability in measurements by calibration throughout the country. High frequency (HF) voltage is one of the important parameter in electrical metrology. At NPLI the primary standard of HF voltage at frequencies from 1 to 1,000 MHz is a twin resistance coaxial power mount. The calibration technique and establishing the traceability by assigning the RF/DC transfer difference to HF voltage primary standard are described in this paper. The HF voltage primary standard has been characterized by assigning RF/DC transfer differences to it in terms of effective efficiency, RF impedance and DC resistance. The calibration results of the primary standard have been discussed in this paper. The assigned RF/DC transfer difference (δ) and the expanded uncertainty of HF voltage primary standard at 1.0 GHz is (?5.2 ± 5.4) mV/V.     

SPICE model for lossy piezoelectric polymers

This work presents the transmission line equivalent model for lossy piezoelectric polymers and its SPICE implementation. The model includes the mechanical/viscoelastic, dielectric/electrical, and piezoelectric/electromechanical losses in a novel way by using complex elastic, dielectric, and piezoelectric constants obtained from the measured impedances of PVDF and PVDF-TrFE samples by nonlinear regression technique. The equivalent circuit parameters are derived from analogies between a lossy electrical transmission line and acoustic wave propagation. The simulated impedance and phase plots of various samples, working in thickness mode, have been shown to agree well with the measured data.     

Exact one-dimensional computation of ultrasonic transducers withseveral piezoelectric elements and passive layers using the transmissionline analogy

We present a computation technique for one-dimensional piezoelectric transducers, resonators, or interferometers having an arbitrary number of layers and piezoelectric elements electrically in parallel. Explicit and exact expressions are derived for the electric impedance and acoustic velocities. This method is based upon a systematic use of the electrical transmission line analogy, each passive or active medium being represented by a line. It has successfully been applied to various structures, particularly a symmetrical resonator with an intermediate liquid medium and a total of 29 media