TABLE OF CONTENTS This chapter describes a hybrid lumped-distributed highpass filter structure. Highpass filters require series capacitors which are difficult to realize in distributed form. The hybrid highpass uses distributed stubs and series lumped capacitors.
The L-C highpass filter structure can be transformed directly from the lowpass prototype values, making this structure as easy to synthesize as the L-C lowpass filter. However, the absence of a convenient series capacitor equivalence presents a realization hurdle for distributed highpass filters. Transmission lines placed end-to-end must have an impractically narrow gap to develop adequate capacitance for highpass filters. Edge-coupled lines increase the adjacent area and therefore the capacitance, but as they become sufficiently long to develop adequate capacitance they no longer behave as pure capacitance. This hurdle is often overcome by using lumped or semi-lumped capacitors. High dielectric constant materials make chip capacitors more practical and inexpensive at microwave frequencies than inductors.
Consider a 7th order Chebyshev highpass filter with a cutoff frequency of 2000 MHz. The initial =M/FILTER= screen is shown in Figure 9-1. The substrate is 31 mils thick with a nominal dielectric constant of 2.55 and a loss tangent of 0.001. The impedance of the shorted stubs is 80 ohms. The shorted stubs are the equivalent of shunt inductors in the L-C highpass circuit. In a direct transformation, the series capacitors would be connected directly to the stubs. However, the stubs would then be close enough to couple to each other, degrading filter performance. To solve this...
