Varactor Tuners Eliminated an RF Amplifier
There is an urgent demand for low noise varactor tuners for use in TV receivers to meet the requirements of FCC rules. The RF amplifier has been frequently eliminated in mechanical tuners now in use, resulting in good cross-modulation characteristics and low cost. However, the elimination of the RF amplifier has not been attained in varactor tuners, because of high series resistance of varactor diodes. The series resistance of Si diodes at 470 MHz is around 0.7Ω, and is much higher than that of mechanical capacitor, 0.15Ω. To reduce the noise figure of varactor tuners, tuning diodes with low resistance, i. e. below 0.2Ω must be developed. However, few reports have been published on low series resistance diodes for UHF use.
GaAs Varactor Tuners for UHF TV
Recently, varactor tuners have become very popular in TV tuner, but the noise figure and selectivity of present day's varactor tuners are inferior to that of mechanical tuners. Series resistance, Rs, of Si tuning diodes manufactured is around 0.8¿in mean value at 470 MHz and at diode capacitance of 9 pF. The value of 0.8¿is so high that low resistance diodes, Rs < 0.8¿, for instance, have been selected specifically for UHF tuners. Recently, low resistance diodes with Rs = 0.68¿have been fabricated by ion implantation technique. 1) Even if these diodes are employed, the Q value of tuning circuit is still much lower than that of mechanical tuner in UHF region. Such high resistance diodes lead to a higher noise figure and lower gain in tuner performance.
Column Axial Load Effects on Performance of Panel Zone in Welded-Flange-Plate Connections
In this study the effects of different levels of column axial load on seismic performance of Welded Flange Plate (WFP) connections with different panel zone strength are presented. Finite element method was used to do this investigation and reliability of the modelling method was verified by comparison with experimental available records in documents. Fifteen sub-assemblages of WFP connections were generated by means of ABAQUS software. The aim was to enhance the knowledge about the consequence of column axial load level on cyclic behaviour and fracture potential of the connections with different panel zone strength for external joints in Steel Moment Resisting Frames (SMRFs). Results showed that cyclic behaviour including hysteresis loops and energy dissipation were not affected significantly by increasing column axial load regardless of the panel zone strength. It was also found that increase of the fracture potential due to increase in column axial load became less pronounced when the panel zone strength was decreased.