The Compact Microwave High Pass Filter

Microwave highpass filter is a device that allows frequencies higher than cut-off frequencyωc to pass, rejects frequencies below cut-off frequencyωc. Generally, highpass filter can be constructed from quasilumped elements or distributed elements perioadically loaded, since any commensurate network exhibits periodic frequency response, in paractical application, there is no ideal highpass response. So we also employ bandpass filter with broad passband to use as highpass filter, so called quasi highpass filer.In this article, several methods and theories to implement highpass filer are researched, aslo prototypes are constructed,tuned and measured. Main job is shown as follows:1, Fundamentals to design highpass filer are presented. The design is initiated from normalized low-pass prototype, then applied frequency and element transformations to transform frequency and impedance to the desired level, finally highpass and bandpass filter prototype is obtained. Richards'transformation establishes approximate equivalence between lumped and distributed elements.2, Optimum distributed highpass filer model is introduced, which consists of a cascade of shunt shorted-circuited stubs, because connecting lines between short-circuit stubs are non-redundant, so it is called optimum model. In the article, highpass filters are constructed using microstrip line and stripline structure sperately, both of which have a fractional bandwidth of 120% and 50 dB rejection from 0 to 2.4 GHz. The measured results are well agreed with simulation results. The relation between impedance of short-circuit stub and fractional bandwidth is discussed, also the relation between impedance of short-circuit stubs and return loss is discussed, method to improve return loss within passband is presented.3, Theory of direct-coupled resonator filter is introduced, which is applied to construct quasi highpass filter. To improve selectivity of the direct-coupled filter, rejection resonator is employed to extract pole (the position of transmission zero extracted in this way is arbitrary, which is relevant with resonant frequency of rejection resonator). Method of extracting coupling coefficient is introduced in detail. A quasi highpass filter prototype is constructed using comb line structure adding two rejection resonators, which has a fractional bandwidth of 40% and achieve 30 dB rejection 400 MHz away from lower passband edge. The mearsured result is basically agreed with the simulated result.