MICROWAVE BANDPASS FILTER :COMBLINE FILTER

COMBLINE FILTER:

The resonators in this type of filter consist of quasi transverse electromagnetic(TEM)-mode transmission lines that are short-circuited at one end and have a lumped capacitance C○ between the other end of each resonator line element and ground.

Coupling between resonators is achieved in this type of filter by way of fringing fields between the resonator lines. If the resonator sections are long at the primary passband(f○), then thesecond passband will be centered on 4f○. If the resonator line elements are made to be less at the primary passband, the second passband will be even further removed from the primary passband.

Microwave network parameters :

Visulization of microwave network seems to be an interconnection of many microwave devices like attenuators, resonators etc coupled by transmission lines or waveguide. For high frequency wave length is small so parameters like terminal voltage or current can not remain constant and so microwave ckts are analyzed through scattering matrix i.e. s- parameters.

Scattering matrix : We know that

Vr exp(+γz)

Г = -------------------------

Vi exp(-γz)

Where Vr and Vi are –ve and +ve voltage traveling waves i.e. reflected and incident wave respectively and γ

is propagation constant and z is the point on transmission line and Г is voltage reflection coefficient.

For scattering matrix incident and reflected microwave amplitudes at any port are used to characteristic the

Ckts.

v1i v2i

a1 = ------------ a2= ------------

(z01)½ ( z01)½

v1r v2r

b1= ----------- b2= ---------

(z01)½ (z01)½

b1 = s11a1 +s12a2

b2 = s21a1 +s22a2

where s11 s12

is scattering matrix

s21 s22