Design, Simulation and Fabrication of a Varactor Tunable Combline Microwave Filter

Abstract

This paper devotes to design, simulation and fabrication of a varactor tunable combline microwave filter in L band. The filter is of cavity type with tapline external coupling with varactors as tuning elements. The design method used here is important because it enables the designer to use a variety of different varactors. The simulation has been done using the full wave method of finite elements in conjunction with a linear circuit analysis software, and in spite of simplicity is quite accurate. Good agreement between the results from the simulation and measurements shows this accuracy. I. IntroductionBroadband communication systems require high Q tunable filters with broad tuning bandwidth. These requirements are well met by mechanically tunable and especially YIG filters [1]. Modern communication systems, however, need a tuning speed of 1GHz/µs or even more. Neither of the mentioned filters can achieve such a speed [1]. Varactor tunable filters are suitable alternatives for these filters. Although varactor tunable filters have lower Q than mechanically tunable and YIG filters, lower size, ease of manufacturing and presenting of newer varactors with higher Q have increased their applications even in frequencies above 10 GHz. In fact, varactor tunable filters establish a trade-off between higher Q and lower tuning speed. In this paper design, simulation and fabrication of a varactor tunable combline microwave filter in L band are presented. The filter is of cavity type with tapline external coupling [2] with varactors as tuning elements. Cavity combline filter have been used because combline structures have higher second passband with lower size compared to interdigital filters. On the other hand, cavity type filters have higher Q in comparison with microstrip ones. The theory and design method of the filter are described in section II. Section III devotes to the simulation and analyzing the filter. Finally, according to the presented method a prototype filter would be designed and fabricated. Figs. 2 to 8 show the prototype and the simulated and measured results, respectively.

Capacitive model for integrated PN varactors of cells with N+ buried layer

“…As it was previously indicated, the capacitance from port 1 corresponds to the P 1 -N-well junction capacitance, C j 1 , in parallel with the parasitic capacitance, C par 1 .…”

“…The capacitances of the internal junctions, which are assumed uniformly and abruptly doped, can be estimated from Equation (1), with the corresponding built-in voltages given by (2),…”

“…Integrated voltage controlled capacitors, varactors, are used in radiofrequency integrated circuits (RFICs) to implement tunable filters [1,2], frequency multipliers [3] or VCOs [4][5][6][7]. An integrated varactor can be implemented with the MOS capacitor (MOSVAR) [8][9][10] or PN junction diode (JVAR) [11][12][13].…”