The Application of Circular Cavity Resonators in Frequency-Tunable Microwave Components

Key Takeaways

• In microwave circuits, resonators are tuned circuits in which the stored electric field is equal to the magnetic field at the resonant frequency.

• Resonators are used in microwaves and optics to design filters, antennas, oscillators, and energy trapping devices.

• Resonators are built using lumped elements L and C, distributed elements like coaxial lines, or rectangular or circular waveguides.

Soldiers marching over a bridge creates resonance

Have you ever seen soldiers marching over a bridge? If not, it is probably because troops are usually ordered to break stride when crossing bridges to avoid the phenomenon of resonance. If soldiers didn’t break stride, resonance would arise, where the rhythm of the soldiers marching steps equals the natural frequency of oscillation of the bridge. The consequences of this situation are dire—the bridge may collapse at the resonant frequency.

While resonance can be detrimental, there are also times when utilizing resonance can be helpful.  In microwave circuits, resonators are tuned circuits in which the stored electric field is equal to the magnetic field at the resonant frequency. Resonators are used in microwaves and optics to design filters, antennas, oscillators, and energy trapping devices.

Resonators are built using lumped elements L and C, distributed elements like coaxial lines, or rectangular or circular waveguides. Rectangular cavity resonators and circular cavity resonators are the two types of cavity resonators developed from waveguides. Both of these cavity resonators are widely used in microwave circuits, as they possess natural resonant frequency and behave like an LCR circuit.

In this article, we will focus on circular cavity resonators.

Circular Cavity Resonators

When closed at two end sections by conducting sheets, a circular waveguide is called a circular cavity resonator

Circular cavity resonators are developed by placing conducting sheets on a circular waveguide at both end sections. As all the sides are closed using conducting walls, this forms a cavity inside the waveguide. When the signal flows through the cavity, electric and magnetic fields are produced. Electromagnetic energy is stored inside the cavity in the form of these electric and magnetic fields and acts as a resonant circuit.

The presence of the electric field creates a capacitance and the presence of the magnetic field creates inductance in the cavity. The circular cavity resonator can be represented as an LC circuit if it is lossless. However, the conducting wall boundaries cause losses along the walls and can be considered equivalent to resistance. Hence, the equivalent circuit of a circular cavity resonator is an LCR circuit with a natural resonant frequency associated with it.

The two modes of wave propagation possible in a circular cavity resonator are transverse electric (TE) mode and transverse magnetic (TM) mode. For each of these modes, the values of L and C change, and, therefore, the equivalent circuit is also different for each mode.

A Low Phase Noise Oscillator Employing an SIW Circular Cavity

Circular cavity resonator-based low phase noise oscillator

A substrate integrated waveguide (SIW), dual-mode bandpass filter (BPF) with a circular cavity is the most critical element in the design of X-band, low phase noise oscillators. The SIW circular cavity is used as a frequency stabilization element in the feedback loop. The transverse magnetic (TM) mode, TM110, is utilized to realize the BPF, with two poles and two zeroes in this oscillator application.

The use of an SIW circular cavity over the microstrip BPF gives a high-quality factor, which is found to be the most effective method to improve the phase noise in an oscillator. A circular cavity-based oscillator can achieve stable oscillation frequency, and the phase noise reduction is remarkable at microwave and millimeter-wave frequencies using this oscillator.

Using Power Combiners with a Circular Cavity in Microwave Applications

In microwave and millimeter frequency applications, solid-state power amplifiers are used to amplify signals. In communication and broadcasting circuits at Ku band frequencies, there is a need to combine the output power of amplifiers to achieve the required total power. To combine the amplifier output powers, resonant type and non-resonant type power combiners are used. In the resonant type, the circular cavity resonators are frequently used to construct power combiners.

The circular cavity resonator at TM mode, TM0m0 (m=1,2,..), is generally employed for power combiner applications due to advantages such as high combining efficiency, reduced path loss, compact size, and high power capability. The modified designs of circular cavity power combiners have been found to exhibit wide bandwidth and low insertion loss. One of the design modifications is the inclusion of radial slits in the circular cavity-based power combiners, as shown in the figure below, which is a method to excite the spurious mode electromagnetic fields in the cavity.

Circular cavity resonator-based power combiner: (a) Structure of combiner with circular cavity and (b) Block diagram of the power combiner

The rapid growth of frequency-agile communication systems has demanded frequency-tunable microwave and millimeter-wave components. At the circuit level, cavity resonators are widely used to realize resonant type frequency-tunable components. Among such devices, circular cavity resonator-based oscillators, power combiners, filters, and amplifiers are more efficient than their non-resonant type counterparts. Various design approaches to cavity resonator-based circuits can help achieve higher efficiency, lower insertion loss, and wider bandwidth.

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