EC-22.5 Passive Microwave Structures: Filters, Couplers and Circulators
You can specify a filter from a required response, explain what a directional coupler and a power divider do in terms of their scattering matrices, and say why a circulator is the one common passive component that is not reciprocal.
At these frequencies passive components are shapes rather than parts. A filter is a pattern of coupled lines, a divider is a junction with a resistor across it, and a coupler is two lines running close enough to share energy. This topic connects the response you want to the geometry that produces it, and it is the point at which the waveguide theory of the earlier module becomes something you can order.
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Specifying a filter: passband, stopband, ripple and order
A filter is specified by how flat it is where you want signal, how steeply it falls, and how much loss you will accept. Those requirements determine the number of resonators before any geometry is chosen.
NPTEL: Design Principles of RF and Microwave Filters and Amplifiers (IIT Kharagpur) · CourseThe standard responses, and what each one trades away
Flat passband, steep skirt and constant delay cannot all be maximised, and each classical response picks a different one. Knowing which property the application actually needs prevents a great deal of over-specification.
NPTEL: Design Principles of RF and Microwave Filters and Amplifiers (IIT Kharagpur) · CourseCoupled lines, resonators, and turning a response into a layout
A distributed filter is realised as coupled sections whose lengths and spacings come from the prototype values. Seeing that translation once explains why microwave boards look the way they do.
NPTEL: Microwave Engineering (IIT Guwahati) · CoursePower dividers and directional couplers, read from their matrices
A divider splits power between two ports while keeping them isolated from each other, and a coupler samples a known fraction of a travelling wave in one direction only. Both are defined by the entries of their scattering matrices.
NPTEL: Microwave Engineering (IIT Guwahati) · CourseCirculators and isolators, and where non-reciprocity comes from
A magnetised ferrite lets energy pass one way around a junction and not the other, which is how one antenna serves a transmitter and a receiver at once. It is the one common passive part that breaks reciprocity.
NPTEL: Passive Microwave Circuits, Devices and Measurements (IIT Bhubaneswar) · CourseAttenuators, terminations and the loaded quality factor
The unglamorous parts set the accuracy of everything else, and a termination that is not quite the right impedance corrupts every measurement made through it. Quality factor is what connects a resonator's loss to its bandwidth.
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- FreeNPTEL / IIT Kharagpur · CourseNPTEL: Design Principles of RF and Microwave Filters and Amplifiers (IIT Kharagpur)
- FreeNPTEL / IIT Guwahati · CourseNPTEL: Microwave Engineering (IIT Guwahati)
- FreeNPTEL / IIT Bhubaneswar · CourseNPTEL: Passive Microwave Circuits, Devices and Measurements (IIT Bhubaneswar)
Links last checked 4 Sept 2026.
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