RS Microwave Technical Corner

February 2004

EVANESCENT MODE BANDSTOP FILTERS

Size reduction using components built in dispersive media (waveguides) operating below cutoff is well-known.  As an example, when an evanescent mode coupling, represented by a trio of Pi or Tee equivalent inductive elements, is applied to bandpass filter design, the physical dimension of the filter can be reduced while simultaneously obtaining a wide spurious free rejection bandwidth [1]. A similar approach can be applied to bandstop filter structures to reduce the overall size of the filter, by the use of high-dielectric constant dispersive transmission line operated below cutoff.

Fig.1 shows a configuration for the in-line implementation of a six-pole notch 1030 MHz filter using the new technique.  In some applications, it is required to combine the notch filters with DPDT switches to allow for selecting or bypassing the filter [2].  For that purpose, filters built in-line or with this new approach must be implemented in a physically “folded” configuration, shown in Fig.2.


 
 

Fig.1 In-line 6-pole bandstop filter
 

 Fig.2  Folded 6-pole bandstop filter.



Temperature stability is excellent, based on the dielectric material inherent properties.  The filter operates at RF power levels of at least 300 W peak, 120 W average power.  Actual response is shown in Fig.4 which has the notch frequency over 1023-1037 MHz.

The notch filter using short-circuited coaxial resonators centered at 1030 MHz (Left: 63531A-2A); a size reduced evanescent mode notch filter  (Right: 21901B-3).

Fig. 3.  Photograph of  folded-type notch filters

Fig.4  Actual response of the evanescent mode notch filter (21901B-3).

References

[1] Ralph Levy, Richard V. Snyder, George Mattahaei, “Design of Microwave Filters”, IEEE Transactions on Microwave Theory and Tech., Vol.50, No.3, pp783-793, March 2002.

[2] Richard V. Snyder, “Quasi-Elliptic Compact high-Power Notch Filters Using a Mixed Lumped and Distributed Circuit”, IEEEE Transaction on Microwave Theory and Tech., Vol.47, No.4, pp. 518-522, April 1999.