Effect Of Impedance Mismatch Research Articles

Wire shaping of RLC interconnects

The optimum wire shape to produce the minimum signal propagation delay across an RLC line is shown to exhibit a general exponential form. The line inductance makes exponential tapering more attractive for RLC lines than for RC lines. For RLC lines, optimum wire tapering achieves a greater reduction in the signal propagation delay as compared to uniform wire sizing. For RLC lines, exponential tapering outperforms uniform repeater insertion. As technology advances, wire tapering becomes more effective than repeater insertion, since a greater reduction in the propagation delay is achieved. Optimum wire tapering achieves a reduction of 36 % in the propagation delay in long RLC interconnect as compared to uniform repeater insertion. Wire tapering can reduce both the propagation delay and power dissipation. Optimum tapering for minimum propagation delay reduces the propagation delay by 15 % and power dissipation by 16 % for an example circuit. The optimum tapering factor to minimize the transient power dissipation of a circuit is described in this paper. An analytic solution to determine the optimum tapering factor that exhibits an error of less than 2% is provided. Wire tapering is also shown to reduce the power dissipation of a circuit by up to 65%. Wire tapering can also improve signal integrity by reducing the inductive noise of the interconnect lines. Wire tapering reduces the effect of impedance mismatch in digital circuits. The difference between the overshoots and undershoots in the signal waveform of an example clock distribution network is decreased by 34 % as compared to a uniformly sized network producing the same signal characteristics.

RF characterization and modeling of various wire bond transitions

This paper presents radio-frequency (RF) characterization and modeling of various wire bond transitions between chips and packages. Test modules composed of Si chips and alumina packages are fabricated in conductor-backed (CB) structures, and they are characterized at frequencies up to 20 GHz. It is shown that the parallel plate resonance of the CB coplanar waveguide (CPW) persists in wire bonding transitions, and narrower-ground CPW-CPW transitions show better characteristics than wider-ground CPW-CPW transitions. The results of three-dimensional full-wave electromagnetic simulation on the test modules reproduce the measured results with a reasonable accuracy. Simple equivalent circuit modeling can reproduce the measured results of the narrower ground CPW transition with no resonance structures. Finally, the effects of bond wire length and impedance mismatch on RF performance are investigated.

On the Linearity and Efficiency of Outphasing Microwave Amplifiers

A new approach for the analysis of the efficiency and linearity of Chireix-outphasing combiners is presented. The approach takes into account, in an explicit manner, the effect of impedance mismatch between the amplifiers and the lossless combining structure. It is shown that the impedance mismatch leads to new expressions for the output voltages from both branches of the amplifier. These expressions explain the origin of the lack of linearity reported in the literature for the Chireix architecture and lead to a new expression for the instantaneous efficiency of a Chireix combiner. Various simulations using a commercial simulator are performed and their results are compared to those predicted by the derived equations. A quasi-exact agreement between the simulator and derived equations is found for all simulations conducted, including voltage expressions, linearity analysis, and instantaneous and average efficiency calculations for a code-division multiple-access signal. The impact of combiner parameters on its linearity and efficiency is also studied.

Influence of impedance mismatch effects on measurements of unloaded Q factors of transmission mode dielectric resonators

Precise measurements of the surface resistance of high temperature superconducting (HTS) thin films using transmission mode dielectric resonators requires accurate knowledge of the unloaded Q factor (Q/sub o/). So far, the most accurate method of Q/sub o/ determination for transmission mode resonators is the transmission mode Q factor (TMQF) technique based on the processing of S-parameters (S/sub 21/, S/sub 11/, S/sub 22/). The technique accounts for parasitic effects including noise, crosstalk, coupling reactance and coupling losses but does not yet take into account the impedance mismatch between the test ports of the vector network analyzer and the inputs to the resonator. Discrepancies between measured and ideal S-parameters due to the mismatch lead to inaccuracies in coupling coefficient computations. This causes undesirable error in the calculated Q/sub o/. We present measurements of Q/sub o/ obtained using the TMQF technique for mismatched and matched connections between the test ports of the vector network analyzer and the inputs to the dielectric resonator. These results are compared with values obtained using the traditional insertion loss method.

Analysis of the impedance mismatch effect in foam-solid targets compressed by laser-driven shock waves

The impedance mismatch effect in a two-layer (low density plastic foam, and solid aluminum, respectively) plane target compressed by a laser driven shock wave is considered. In such targets the ablative pressure generated by absorption of laser light in the foam layer is amplified when crossing the foam-aluminum interface. In this paper an analytical model is developed to evaluate the shock pressure in the aluminum layer as a function of the density and thickness of the foam layer and of the laser parameters. The model is in good agreement with previously published experimental results [A. Benuzzi et al., Phys. Plasmas 5, 2827 (1998)].

Design and analysis of long absorption-length traveling-wave photodetectors

Long-absorption-length traveling-wave-photodetectors (TWPDs) are requisite for high-power and high-speed applications. In the long absorption length regime, some previously negligible bandwidth limitation factors, such as microwave loss and boundary reflection, become critical. In this paper, we calculate each limiting factor for long-absorption-length TWPDs using a photo-distributed-current model, which can be easily modified to include different effects comparing with the previous model. The simulated device structures are low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal (MSM) and n-i-n TWPDs for telecommunication wavelength applications. Our simulation results indicate that the carrier trapping time is not the dominant bandwidth limitation factor as in the short device length regime. The device bandwidth is, on the other hand, strongly affected by velocity mismatch, frequency-dependent microwave loss, and boundary reflection. By properly choosing the geometric size of the transmission line in the MSM TWPD structure, the effect of impedance mismatch can be eliminated. Also due to the enhanced microwave velocity and a lower microwave loss, a better bandwidth performance can be found in the MSM TWPD structure. With a longer carrier trapping time and a higher electrical wave velocity, high bandwidth-efficiency product can be expected for LTG-GaAs-based long-absorption length photodetectors.

Ultrahigh-speed traveling-wave electroabsorption modulator-design and analysis

Theoretical analysis and numerical calculations are presented for ultrahigh-speed (>50 GHz) traveling-wave electroabsorption modulators (TW-EAM's), including effects of velocity mismatch, impedance mismatch, and microwave attenuation. A quasi-static equivalent circuit model is used to examine the TW-EAM microwave properties, including the effect of photocurrent. Due to the optical propagation loss of the waveguide, the TW-EAM waveguide length for maximum RF link gain is currently limited to 200-300 /spl mu/m. The discussion indicates that the carrier transit time in the intrinsic layer may not severely limit the TW-EAM bandwidth. Three TW-EAM design approaches are discussed: low-impedance matching; reducing the waveguide capacitance; and distributing the modulation region.

Shock impedance matching experiments in foam-solid targets and implications for “foam buffered ICF”

We studied the influence of foams on laser produced shocks. Experiments were performed at LULI using a Nd laser converted to second harmonic, and at MPQ (Max Planck Institut für Quantenoptik) using the iodine Asterix laser converted to third harmonic. In both cases, sub-ns lasers with pulse energies of several tens of joules were focused on large focal spots (hundreds of microns) to reduce 2D effects. The laser beams were optically smoothed with phase zone plates (PZP) and directly focused on layered targets made of a foam layer on the laser side and a stepped Al layer on the other side. A visible streak camera was used to detect shock breakthrough at the base and at the step of the Al target, allowing shock velocity to be determined. Using the well known SESAME Al equation of state, we determined shock pressure. A stronger pressure increase was measured when foam was present, compared to what was obtained by focusing the laser beam directly on the Al target. This was due to the impedance mismatch effect at the Al-foam interface.

Input impedance conditions for minimizing the noise figure of an analog optical link

We use an equivalent-circuit model to derive the minimum noise figure (NF) for an amplifierless optical analog link with a perfect lossless input impedance-matching circuit. This minimum, which is 3 dB, is called the lossless passive match limit. We contrast the link model with an analogous equivalent-circuit model for a transistor amplifier, which does not have the same limiting NF under this impedance-matching condition. It turns out that the link's 3-dB limit arises from the ohmic nature of the impedance of the device that modulates the light, and not from the electro-optic conversion processes in the link. In a prior experimental link, with near-perfect impedance matching, dissipative loss in our input circuit precluded achieving a measured NF of less than 4 dB. Investigation of the effects of input impedance mismatch indicates that mismatch can actually lower the NP to below 3 dB even when dissipative loss is present in the input circuit. We have used this mismatch effect to reduce the measured NF of our link to 2.5 dB at 130 MHz. We believe this is the first demonstration of amplifierless-link NF of less than 3 dB.

Bat sonar: an alternative interpretation of the 10-ns jitter result.

In 1990 Simmons et al. reported evidence of a time resolution hitherto unknown in any animal, namely a 10-ns jitter detection threshold in echolocating bats. This result is discussed. The calibration data from the original papers are examined. Observations indicating other cues than delay being presented to the bats are given. We offer an alternative explanation for the psychometric jitter function, based on the assumption of a subtle distortion due to impedance mismatch in the delay-producing apparatus. We also report that effects of impedance mismatch are detectable by a human subject in a model experiment.

Effects of impedance mismatch and imperfect isolation on distortion cancellation in feedforward amplifiers

Feedforward linearisation offers a viable way to reduce intermodulation distortion generated in power amplification, the actual reduction depending on the degree of cancellation in the feedforward cancellation loops. Commonly, cancellation is calculated from design formulae developed by assuming perfect impedance matches between subcircuits and perfect isolation in the power splitting/combining networks, but these are difficult to achieve in practice. The effects that mismatches and imperfect isolation have on cancellation are studied and investigated and it is shown that the transmission characteristics of the cancellation loops can deviate significantly from those calculated using the above assumptions, resulting in poorer cancellation. The use of tuning circuits to compensate for these effects is also examined and simulation shows that mismatch and imperfect isolation can effectively be compensated over a very narrow frequency band by this technique and good cancellation achieved, though it becomes less effective as bandwidth increases. Experimental results are presented to verify the simulation results.

EHV Transformer Bank Unbalance: Practical Issues and Solutions

An unconventional tap configuration has been implemented in AEP's South Canton 765/345/34.5 kV 3*500 MVA transformer bank to counter the combined effects of impedance mismatch within the bank and the natural unbalance of the EHV transmission system. These effects manifest themselves as unequal phase voltages and loadings and as elevated neutral and tertiary circulating currents in the bank, capable of reaching high magnitudes during heavy loading conditions. This paper describes the South Canton bank unbalance, the concerns it raised, and the actions taken by AEP to control its possible adverse effects on the integrity and operation of the South Canton Station (USA) facilities.

Wideband dispersion in baseband systems

To avoid dispersion caused by an explicit /spl lambda//sup 2/ in either the one-way, or two-way range equations, the antenna gain product must be proportional to frequency squared. For the one-way case this is equivalent to one antenna with gain independent of frequency, and the other antenna with effective area independent of frequency. Both gain and effective area must include impedance mismatch effects. Antennas in general are examined to determine gain and effective area variation over a wide frequency band. The effects on representative short pulse waveform of systems with a gain product constant with frequency, and of systems with an effective area product constant with frequency are explored. >

Postpolarizer mismatch effects in waveguide radiating systems

Postpolarizer impedance mismatch effects are known to have a degrading influence on the performance of a circularly polarized waveguide radiating system. These effects are analyzed in this paper.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pure mode versus mixed mode interfacing between analog circuits

The concepts of transmission error∈ and mismatch factor∈−1 are introduced to evaluate the effect of impedance mismatch on the accuracy of broadband signal transmission between two feedback amplifiers. It is shown that, in comparison with the error introduced by the feedback amplifiers,∈ becomes negligible for pure V.M. (voltage mode) or C.M. (current mode) signal transmission; in the mixed mode, in which a V.M. output circuit feeds a C.M. input circuit, or vice versa,∈ may become significant. Computer simulations show that the pure mode also yields reduced T.H.D., improved bandwidth and improved transient response. It is also shown that a particular combination between the kind of feedback and the active circuit to which it is applied, termed “enhancing combination,” further increases the accuracy of signal transmission.

Numerical study of the impedance mismatch effect in laser-irradiated layered targets

The effect of impedance mismatch on shock-pressure enhancement in laser-irradiated layered targets has been investigated by numerical simulation of the entire time history of shock generation. The model is a simple, one-dimensional, single-temperature Lagrangian hydrodynamic code incorporating a simplified laser-absorption model and an equation of state suitable to describe the thermodynamic state of matter in a wide range of density and temperature. Effects of change in layer materials and other experimental parameters on the attainable shock pressure and compression have been studied. From an exhaustive study of such targets, it is found that it is possible to enhance the attainable values of compression (ρ/ρ0) and shock pressure with a given laser pulse by suitable optimization of the target. A pressure of more than 60 Mbar can be generated with a 1014 W/cm2, 300-ps rise-time laser pulse in such targets. In order to generalize the applicability of the results, within existing experimental uncertainties of measurements, and facilitate a suitable target design for any laser system, simple scaling laws are obtained in terms of parameters like intensity and rise time of the laser pulse.

On the Measurement of Shock Waves

The classic theoretical formulations of finite-amplitude distortion invariably depict continuous sinusoidal waves which develop into sawtooth or N-shaped waves. In practice, where ultrasonic wave- forms at high intensities are measured by a PVDF hydrophone con- nected to an oscilloscope, waveforms are observed with gross asym- metries between positive and negative pressure amplitudes, high- amplitude spikes, notches, ringing, and other peculiarities. In princi- ple, additional nonlinearities in the driving stages or the receiving sys- tem may influence the observed waveform. However, it is not necessary to invoke nonlinearities in the transmit or receive systems to explain the time-domain appearance of many waveforms. Instead, linear mechanisms exist which affect the waveshape. The first is a relative phase shift betwen the fundamental and second (and higher) harmonics which occurs in a diffraction-limited focused field. This alters the time- domain appearance but not the spectral content of the observed signal. The second class of linear mechanisms involves hydrophone cable and impedance mismatch effects which alter both time- and frequency-do- main characteristics of the actual pressure waveform incident on the hydrophone. These effects and others are illustrated. The results form the basis for some recommendations on experimental procedures in nonlinear wave propagation.

Effect of half-wavelength membranes on the axial resolution of real-time ultrasonic scanners.

The effect of interposing a half-wavelength membrane between the transducer and the load in mechanically sectored ultrasonic scanners was investigated. In particular, the effects of impedance mismatch and detuning on broadband spectra were theoretically and experimentally studied. These studies indicate that for certain common cases, a bandwidth reduction of 20%-30% can be anticipated, resulting in a proportional decrease in the axial resolution.

Effects of impedance mismatch on the strength of waves in layered solids

This research program was conducted to study the effects of acoustic-impedance mismatch between materials in a layered elastic solid on the amplitudes of the head waves generated at the interface as a stress wave develops and propagates in one of the layers. Dynamic photoelasticity methods were employed. The isochromatic-fringe patterns used for analysis were recorded with a Cranz-Schardin multiple-spark camera operating at a framing rate of approximately 188,000 exposures per second. Acoustic-impedance ratios from a low of 1.7∶1 to a high of 17.4∶1 were studied. Small charges of lead azide were used to generate the original dilatational (P1) wave.

Connectors in Very-High-Speed Digital Circuits

The properties of standard card edge, printed circuit, and flat cable connectors have been investigated from the point of view of transmitting pulses with very short rise times. A simple theory based on linear approximation of the leading and trailing edges of the actual pulse waveform is derived to predict the effect of impedance mismatch caused by the connector. Experimental confirmation of the theory, measurements of popular types of connectors, and extrapolation of experience with a 350-Mb/s logic network confirm that, under certain conditions, the currently available connectors can be used with pulse rise times longer than 250 ps and with interconnection impedances from 50 to 100 ohms. Transmission lines matched at both ends and/or balanced pairs are preferred for interconnections between different parts of a very fast (greater than 250-Mb/s) pulse system.