Single and Dual Mode Resonator

What is a resonator? In simple words, it is any circuit that produces a resonance at a particular frequency. A resonance frequency is when a frequency is exhibiting greater amplitude when compared to other frequencies. In some applications, resonance frequencies are undesirable as it could mean an anomaly in the response graph. For example in sound applications, a resonance frequencies usually means a buzzing or noise that stands out.

However, in the field of RF and Microwave, resonance frequencies found its usage. This is because, at resonance frequencies, it translates to maximum transference of signal or data from input to output. Logically, one would desire for resonance at the center frequency because this would mean the design will have optimal performance at the frequency its designed to operate.

Because of this, resonators form the basic building blocks of resonators.

Resonators in Filters

Filters are designed based on the number of resonators and inter-resonator couplings to achieve the required filtering response such as the passband bandwidth and out-of-rejection level for undesired signal attenuation. Each resonator can be constructed using a single mode resonator which produces single resonance at the passband. For instance a 10th order filter will require 10 resonators to construct the whole filter. This is most evident in the construction of LC ladder filter where the resonators are depicted by the repeating LC pair.

A 6th order LC filter with the red box depicting one LC resonator pair.

Dual-Mode Resonators

The addition of each resonator constitute a volume resulting in different filter size depending on the order. In order to reduce the size of the filter, one solution is to incorporate dual mode resonator which produces two resonances at the passband at the cost of a single resonator structure and volume.

This means that to achieve the same 10th order filter, one can construct 5 dual mode resonators which will result in the equivalent of 10 resonances for 10th order response. Depending on the type of dual mode resonator, a good design may reduce the overall filter volume and size by approximately closed to 50%. Reduction of cost is achieved as it is proportional to size and material consumption.

How To Construct Dual Mode Resonators

For aspiring RF designers, the knowledge in designing dual-mode resonators is important for producing commercially viable designs. In this two part videos, we give a simple overview of how to do that using FILPAL EDS HF. In the first video, we explore on how to construct a single-mode resonator and proceed to convert it into a dual-mode resonator.

In the second part of the video, we will explore on how to convert a lumped dual-mode resonator into a microstrip line filter.

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