Varactor Diode Selection Based on Spice Simulation for Application in Discrete Nonlinear Transmission Lines

Abstract

Varactor diode is a semiconductor component widely used in a telecommunication system, such as amplitude modulation, due to the capacitance variation with the voltage. When reversely polarized, the PN junction capacitance decreases with voltage, which is a nonlinear characteristic used for pulse sharpening 1 . This nonlinear capacitance behavior is also employed in nonlinear transmission lines (NLTL) to produce radiofrequency waves in the same way used with ceramic capacitors for high power applications 2 . Low power discrete nonlinear transmission lines use a net of inductors and varactors, where the latter is a nonlinear element 1 . In this work using SPICE simulation, we investigated five different commercial varactor diodes for applications in NLTLs. The first parameters observed were the nominal capacitance and the breakdown voltage. The highest capacitance variation observed was with the 1SV149 model of about 96.67% near the breakdown voltage of 9 V and the nominal capacitance of the order of 500 pF. But the better result, looking for the highest frequency oscillation generated by the NLTL, was obtained for the BB405 model, with a frequency generated along of the line of the order 100 MHz with 30 line sections and a load of 50 Ω. For BB405, the nominal capacitance was 19 pF with a variation of 90.00% near the breakdown voltage of 28 V. However, the oscillation obtained in the NLTL using the 1SV149 diode was only 12.5 MHz, using the same number of sections and load. In conclusion, simulation results show that the highest nonlinear behavior of the varactor does not mean the highest frequency oscillation generated in an NLTL. Also, it is necessary to take both the nonlinearity and initial or nominal capacitance for the project.