High-Selectivity Microstrip Filters Based on Structures With a Limited Number of Hairpin Resonators [Application Notes]

High‐performance two‐pole filter using self‐coupled resonators with frequency‐variant internal and external couplings

“…The phenomenon is also termed as TZ splitting. 18 For illustration, simulated   S 21 parameters of the weakly excited filter, designed on a 0.8-mm thick substrate with a dielectric constant of 2.55, with and without self-couplings are shown in Figure 1B. TZs are obtained around the passband at f zm and f zs1 according to Equations ( 1) and ( 2).…”

“…The couplings through the primary and the additional paths cancel out at a certain frequency producing another TZ. The phenomenon is also termed as TZ splitting 18 …”

“…These include source-load coupling, [10][11][12] zero-degree tapped feed structure, [13][14][15] terminating half sections, 16 and frequencyvariant couplings (FVCs) realized as mixed electric and magnetic couplings with tapped feed lines. [17][18][19] To achieve very high selectivity, stronger coupling between the source and load is required, 10 which requires placing the feeds in close proximity 10 or extended feed line structures. 11,12 For the same objective, both electric and magnetic components in a mixed coupling have to be strong, 20 which is difficult to achieve in planar structures with complete ground.…”

“…High‐selectivity two‐pole filters are designed by inserting two transmission zeros (TZs) on opposite sides of the passband using various methods. These include source–load coupling, 10–12 zero‐degree tapped feed structure, 13–15 terminating half sections, 16 and frequency‐variant couplings (FVCs) realized as mixed electric and magnetic couplings with tapped feed lines 17–19 . To achieve very high selectivity, stronger coupling between the source and load is required, 10 which requires placing the feeds in close proximity 10 or extended feed line structures 11,12 .…”