Microwave Reflection Coefficient Research Articles

Optimum carrier lifetime of semiconductor substrate in optoelectronic microwave grating structures

An analytical approach to the optimisation of substrate carrier lifetimes in optoelectronic microwave grating structures is reported. The method is based on the single-section approximation of the diffusion-controlled multisection grating profile, and utilises the specific slope behaviour to maximise the differential microwave reflection coefficient per unit length. The resultant optimum carrier lifetime is presented as a function of optical excitation wavelength, surface recombination velocity and grating centre frequency. In silicon, optimum values of roughly 10-6-10-5 s have been obtained.

Spectral dependence of light-induced microwave reflection coefficient from optoelectronic waveguide gratings

After a short description of the method of operation of optoelectronic microwave filters, a detailed analysis of the spectral dependence of light-induced microwave reflection coefficient from an optoelectronically generated distributed Bragg reflection waveguide grating is reported. The theory is based on an improved stepped-impedance model utilizing a diffusion-controlled abrupt-profile approach of photoconductivity along with a conformal mapping technique for the quasi-static evaluation of the spectral performance of photoinduced wave attenuation. The validity of theory is clarified and the calculated results are compared with experimental results. As a useful result for future applications, an optimum excitation wavelength of about 825 nm for a fiber-optically controlled lab-tested 50 /spl Omega/ full-substrate silicon coplanar waveguide has been obtained. >

A microwave measurement technique for characterizing the I-V relationship for negative differential conductance devices

The practical procedures of a recently proposed technique to experimentally determine the I-V characteristics of negative differential conductance (NDC) devices are presented in this paper. The technique employs microwave reflection coefficients together with the measurable portions of the device's I-V characteristics to infer the I-V curve in the negative conductance region where oscillations tend to occur under normal DC bias conditions. The effects of higher harmonics at large signal levels are also taken into account in the calculation. The advantages of the method for high NDC devices have been pointed out in the stability analysis contained in a previously published work. The technique was demonstrated in the case of a low NDC tunnel diode where the agreement between the deduced I-V curve using the described method and the DC measured I-V curve is within 5%. The I-V curve in the negative conductance region for another tunnel diode with much higher negative conductance is also obtained and a self-consistent accuracy of 5% is found. >

Calibration techniques for free space reflection coefficient measurements

Owing to the limitations of presently available calibration standards and techniques, free space microwave reflection coefficient measurements, commonly used for characterising RAM reflectivity and material parameters, are prone to significantly more errors than those made in coaxial lines or waveguide. In this paper, the most commonly used free space calibration procedure using an empty room and a metal plate is described, and the errors arising critically examined. Alternative calibration procedures using both two and three term error correction are investigated both theoretically and experimentally and shown to be efficacious in reducing measurement errors still further. The paper also discusses the need for a free space reflectivity standard, which would enable the verification of free space reflection coefficient measurement systems to be achieved.

Flanged coaxial microwave probes for measuring thin moisture layers.

We obtain an analytical, closed-form expression for the input admittance of a flanged, open-ended coaxial line radiating into a layered lossy material. The effect of higher modes as well as the dominant mode have been taken into consideration. The results obtained in the experimental measurement of the microwave reflection coefficient of the probe radiating into very thin water layers are described and compared with the results obtained from numerical calculations based on a theoretical model. The measurements were made in the 5.0-7.0 GHz range. The results demonstrate, for example, the usefulness of this type of device in measuring thin moisture layers in the outer layer of the human skin.

A new method of determination of the I-V characteristics of negative differential conductance devices by microwave reflection coefficient measurements

A technique to map out all of the I-V characteristics of negative differential conductance (NDC) devices is described. This method uses the DC measurable positive conductance portions of the I-V curve together with the measured microwave reflection coefficients at different RF signal levels and fixed DC bias voltage. The advantages of the method for high NDC devices are pointed out in a stability analysis. The complete I-V curve of a tunnel diode has been obtained with an accuracy within 5% in a proof-of-principal test of this method. >

Use of the linear electrooptical effect for diagnostics of microwave radiation

An optical method for diagnostics of microwave radiation, enabling absolute measurement of the power of an electromagnetic wave, is described. Theoretical dependences are presented for the magnitude of the linear electrooptical effect, the microwave reflection coefficient, and the mean effective modulation field on the frequency of the electromagnetic wave entering the receiving transducer, are presented. Working formulas are given. The sensitivity, the SWR, and the measurement error are evaluated. The reliability of the method is demonstrated experimentally.

An approach to diagnostics along HF high pressure plasma columns by microwave reflectometer

Diagnostic results relating to the full length of high frequency discharge plasma columns produced by a VHF surface wave generator (50 W max) are presented and discussed. Such plasmas, highly collisional, are generated in narrow quartz tubes having inner diameters of 2 and 4 mm, in xenon at pressures of 10 and 40 Torr respectively. The radius of the plasma column, the radial profile of the electron density, and the global collision frequency are estimated by measuring the microwave reflection coefficient in the X-band. The experimental results are fitted to the results obtained by a numerical calculation derived from an exact theory of electromagnetic scattering in a rectangular waveguide. Different plasma regimes along the columns may be diagnosed.

Thickness dependence of the reflection coefficient from thin semiconductor films and measurements of the conductivity

Two approaches to the measurement of the resistivity of thin films of semiconductors deposited onto metal plates are compared. The results of these approaches demonstrate that the microwave reflection coefficient varies with the cube of the film thickness. This is experimentally supported by measurements previously reported in the literature.

Fluctuation Effects in the ac Conductivity of Thin Superconducting Lead Films at Microwave Frequencies

The microwave transmission and reflection coefficients have been measured for thin (150 \AA{}) lead films and for a lead film with thin manganese overlays (total thickness of overlay 5 \AA{}) in the temperature range 5.0-7.3 K at frequencies of 23.89, 37.2, and 69.5 GHz. Well below ${T}_{c} [({T}_{c}\ensuremath{-}T)\ensuremath{\le}0.2 \mathrm{to} 0.3 \mathrm{K}]$, the temperature and frequency dependence of these ratios are in good agreement with theoretical values calculated using the complex conductivities derived by Mattis and Bardeen. Best fits with the experimental results were obtained using complex conductivities corresponding to a superconducting energy gap of $(4.5\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{k}_{B}{T}_{c}$. Above ${T}_{c}$ both the microwave and dc data implied an excess conductivity for each film studied. This excess conductivity has been attributed to thermodynamic fluctuations in the Ginzburg-Landau order parameter for temperatures near the normal-to-superconducting phase transition. In agreement with the predictions of Aslamazov and Larkin for the two-dimensional limit, the dc measurements indicate a value of 0.152 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ for $\frac{{\ensuremath{\tau}}_{0}}{{R}_{n}}$. Both the temperature and frequency dependence of the implied excess ac conductivity above ${T}_{c}$ are accounted for by fluctuation-effect calculations of Schmidt. In the temperature region just below ${T}_{c} [({T}_{c}\ensuremath{-}T)\ensuremath{\sim}0.1 \mathrm{K}]$, the experimental results indicate an excess ac conductivity compared to the predictions of the Mattis and Bardeen (MB) theory. Recently, Schmidt has extended his theory of fluctuation effects in the ac conductivity to temperatures below ${T}_{c}$; combining these fluctuation-induced conductivities with those of MB (due to quasiparticles) permits the calculation of transmission coefficients in good agreement with experiment. We believe this to be direct evidence for fluctuation effects in a two-dimensional superconductor in the presence of quasi-long-range order. The deposition of a manganese overlay of approximately 2 \AA{} upon a 150-\AA{} lead film resulted in a 46% reduction in the microwave-measured energy gap. Essentially no charge was observed in the dc transition temperature.

A Homodyne System for the Measurement of Microwave Reflection Coefficients (Correspondence)

The homodyne detection principle is applied to the measurement of microwave reflection coefficients. The system described is considerably more sensitive than video detection, is linear over a wide dynamic range, and allows simultaneous phase and amplitude measurements, with phase measurements amplitude insensitive. Two channels are used, one being amplitude modulated.

Electromagnetics of colloidal plasmas

This paper presents an analytical study of the conductivities and the wave propagation phenomenon in a colloidal plasma. A rigorous kinetic approach is used to obtain the expression for the current density. Variations of dc and microwave conductivities, refractive index, attenuation, and reflection coefficients are studied as a function of relevant colloidal plasma parameters.

Fast Sensitive Smith Chart Plotter or Microwave Reflectometer

Real and imaginary components of microwave reflection coefficient are observed simultaneously with a response time of 1 μsec for use in paramagnetic resonance experiments. At a signal strength of one μW and an i.f. bandwidth of 3 MHz a change in reflection coefficient of 5×10−4 is detectable. When the experiment permits a larger signal strength or a narrower system bandwidth, increased sensitivity is available. Adjustment procedures that use a sliding short and a matched load are described.

Precursor Effect in Electro-Nagnetioally Driven Shock Tube

For the origin of the precursor electron in the E-M shock tube, various hypotheses were proposed by many authors. But up to now there is no definite conclusion of its origin nor the informations of its density. Now we report the axial distribution of the precursor electron in a T-shaped shock tube which has been measured by using the automatic measuring apparatus of the microwave reflection coefficient.

Microwave Reflection Coefficient of Plasma as a Function of Electron Density, Collision Frequency and Cyclotron Frequency

We had already reported some properties of shock waves in argon measured by microwave techniques.Now we have calculated the characteristics of microwave reflection coefficient from the sharp boundary of plasma as a function of electron density, collision frequency and cyclotron frequency by a electronic digital computer, MADIC II-A. We have also constructed an automatic apparatus to measure the microwave reflection coefficient. Applying the experimental results to the calculated charts of reflection coefficient, we can easily find out the final data concerning the electron density and collision frequency of plasma under the various conditions of magnetic fields.In this report, the charts of the reflection coefficient and the system of the measuring apparatus are presented. The detail results of experiments on the plasma shock waves will come out in the next paper.

Magnified and Squared VSWR Responses for Microwave Reflection Coefficient Measurements

In conventional microwave impedance measuring the measured ratio of maximum to minimum detector signal level is ideally equal to the voltage standing-wave ratio (VSWR) of the termination. In this paper, it is shown how radically types of response are obtainable in which the observed ratio may approximately equal the square of the VSWR or may be magnified any desired amount. Theory k given enabling accurate ments by interesting techniques. Accuracies of 0.1 per cent in VSWR to 2.0 have been achieved using magnitied response techniques.