A New Analytical Method for Robust Extraction of the Small-Signal Equivalent Circuit for SiGe HBTs Operating at Cryogenic Temperatures

We present a new analytical direct parameter-extraction methodology for obtaining the small-signal equivalent circuit of HBTs. It is applied to cryogenically operated SiGe HBTs as a means to allow circuit design of SiGe HBT low-noise amplifiers for cooled radio astronomy applications. We split the transistor into an intrinsic transistor (IT) piece modeled as a Pi-topology, and the quasi-intrinsic transistor (QIT), obtained from the IT after that the base resistance (R_b) has been removed. The relations between Z-Y-parameters of the IT and QIT are then established, allowing us to propose a new methodology for determining R_b. The present extraction method differs from previous studies in that each of the model elements are obtained from exact equations that do not require any approximations, numerical optimization, or post-processing. The validity of this new extraction methodology is demonstrated by applying it to third-generation SiGe HBTs operating at liquid-nitrogen temperature (77 K) across the frequency range of 2-22 GHz.     

Quantile curves and dependence structure for bivariate distributions

Within the context of a general bivariate distribution an intuitive method is presented in order to study the dependence structure of the two distributions. A set of points—level curve—which accumulate the same probability for a fixed quadrant is considered. This procedure provides four level curves which can be considered as the boundary of a generalization of the real interquantile interval. It is shown that the accumulated probability among the level curves depends on the dependence structure of the distribution function where the dependence structure is given by the notion of copula. Furthermore, the case when the marginal distributions are independent is investigated. This result is used to find out positive or negative dependence properties for the variables. Finally, a nonparametric test for independence with a local dependence meaning is performed and applied to different data sets.     

Dielectric properties of guava,mamey sapote,prickly pears,and Nopal in the microwave range

Dielectric properties (DPs) of selected fruits: guava (Psidium guajava), mamey sapote (Pouteria sapota), red prickly pear (fruit from Opuntia streptacantha), and white prickly pear (fruit from Opuntia ficus-indica), as well as nopal (young cactus pads or cladodes from Opuntia ficus-indica) were studied. DPs were determined using the open-ended coaxial probe method at microwave frequencies (500 MHz–2 GHz) and temperatures of 20, 40, and 60ºC. At 20ºC and 915 MHz for unripe fruits, dielectric constant values ranged from 65.7 to 70.9, while the loss factor had values between 8.4 and 20.7. Both dielectric constant and dielectric loss factor were affected by temperature and frequency (p < 0.05). For example, loss factor for red prickly pear at 2450 MHz decreased from 13.88 at 20ºC to 12.8 at 40ºC and 11.7 at 60ºC. DPs of fruits were also affected by their ripening (quantified through the maturity index). In addition, penetration depth decreased with increasing frequency, ranging from 1.98 to 4.80 cm at 915 MHz and from 0.25 to 0.41 cm at 5800 MHz. Results are valuable to develop further applications with microwave technology for these foods, such as microwave-assisted disinfestation treatments or microwave drying.     

Dielectric Properties of Beans at Different Temperatures and Moisture Content in the Microwave Range

Dielectric properties of common Mexican beans (Phaseolus vulgaris L.) were determined and analyzed at microwave frequencies (800–2500 MHz). The free-space transmission technique was employed for the measurements of three varieties (“Flor de mayo,” “Bayo,” and “Negro”) with different moisture content (8.8–12.3%, w.b.) at 20, 30, 40, 50, and 60°C. The dielectric constant and loss factor of beans decreased with increasing frequency for a fixed temperature, and increased with increasing temperature at a fixed frequency. The dielectric constant increased with increasing moisture content, while the loss factor remained nearly constant. With these results, disinfestation or quality control measurements can be proposed for beans using microwaves.