Analysis of ceramic resonator triplet filter with transmission zeroes

 
In [1] a ceramic 3-pole band pass filter is designed, manufactured and characterized. The resonators are arranged in a triplet which allows non-adjacent couplings and hence inclusion of transmission zeroes.
 
From the data given in the article the corresponding filter has also been synthesized with the CMS software.
A comparison is made below and it is seen that the CMS tool is in excellent agreement with the measurements.
 

Measured result presented in [1]*

CMS synthesized filter
In [1] a wider measured sweep is also presented which shows two extra notches, which were not intentionally designed in. The article concludes that these extra notches originate from coupling between the input and output probes.

This situation is also analyzed with the CMS tool and displayed below
   

Measured result presented in [1]*

CMS synthesized filter
In the CMS tool it is also possible - through the topology matrix - to investigate which coupling arrangements can give a characteristic as shown above.
The coupling matrix shown below corresponds to the 3 notch characteristic pictured above.
 
 

The non-adjacent coupling between resonator 1 & 3 (red circle) gives the wanted notch closest to the pass band.

The other two notches are explained by the weak stray-couplings marked by the yellow circle and involves: 
1)  coupling directly between the input/output probes (leftmost notch)
2)  coupling between resonator 1 and the output probe (rightmost notch).
 

Other stray-coupling paths can give the same characteristic (e.g. couplings: S/2, S/3 and S/L) but they involve much stronger couplings and are therefore not relevant.

 

[1]   A. Abramowicz, J. Krupka, and K. Derzakowski,
Triplet dielectric resonator filters with direct coupling
Proc. Int. Conference on Electromagnetics in Advanced Applications, ICEAA'03, Torino, Italy, 2003, pp.143-146

* Figure reprinted with permission from Dr.Abramowicz