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Using Re-Entrant Cavity Resonators in High-Frequency Applications

Key Takeaways

  • At frequencies above 3MHz, transistor-based oscillators and amplifiers become obsolete due to the skin effect and stray reactances. 

  • To efficiently generate oscillations and amplification at higher frequencies, cavity resonators are used instead. 

  • Increased bandwidth is the main advantage of re-entrant cavity resonators. 

View of a transistor circuit

Transistor circuits exhibit stray reactances and the skin effect at frequencies above 3MHz

At frequencies above 3MHz, transistor-based oscillators and amplifiers become obsolete due to the skin effect and stray reactances. Instead, to efficiently generate oscillations and amplification at higher frequencies, cavity resonators are used. 

What are Cavity Resonators?

Cavity resonators are hollow, closed compartments made of conducting material. RF signals are given as input and output within the compartment through input and output ports. The compartment is analogous to an inductor, and its mouth acts as the capacitor for radio frequencies. 

There are several types of cavity resonators, characterized based on their structure and function:

  • Regulated cavity resonators

  • Unregulated cavity resonators 

  • Co-axial cavity resonators 

  • Inductive cavity resonators 

  • Capacitive cavity resonators 

  • Waveguide cavity resonators 

  • Re-entrant cavity resonators

Re-entrant cavity resonators are used for oscillation filtering and amplification in the 3MHz-300MHz frequency range. Let’s take a closer look at this type of cavity resonator.  

The Structure of a Re-Entrant Cavity Resonator

A re-entrant resonant cavity is made from two cavity resonators connected perpendicularly by another rectangular waveguide at both ends. Increased bandwidth is the main advantage of the re-entrant cavity resonator, which makes this type of resonator applicable as a wide-band amplifier and oscillator in the frequency range of 3MHz to 300MHz. 

Efficient energy transfer occurs from the electron beam to the high-quality factor cavity resonator when electrons cross the cavity field region in minimum time. To keep the time this takes to a minimum, cavity grids need to be placed close to each other in the re-entrant cavity structure. 

The electric field is concentrated across gap ‘g’ on the capacitance region, allowing the electrons to flow through it. Electric energy stored in the cavity can be increased by increasing the capacitance, C. This type of re-entrant cavity resonator is tuned by varying the short plunger. The resonant length ‘d’ can be varied by using the short-plunger as well. 

If the re-entrant cavity's length is greater than the gap thickness, then such a structure would be considered a coaxial line with the radii of the inner and outer conductor. At resonance frequency 𝜔r, the gap capacitance, C, and the coaxial line below the gap provide reactances, which are equal and opposite. This conduction obeys the following equations:

equation for conduction

Z0 is the characteristic impedance of the coaxial line and ƛg is the guided wavelength corresponding to the resonance frequency 𝜔r.

Modes for Re-Entrant Cavity Resonators

There are two modes for re-entrant cavity resonators:

  1. Transverse electromagnetic (TEM) type, where d> (b-a);b<g / 4

  2. Transverse magnetic (TM) type, where d<(b-a); b=g / 4 and the electric field is along the cavity axis.

Applying Re-Entrant Cavity Resonators to Solid-State Microwave Oscillators 

A microwave solid-state oscillator (patented as US 3639856) is an application of re-entrant cavity resonators. The main advantage of the re-entrant cavity resonator-based microwave oscillator is that oscillating output changes slightly with the frequency change. 

In this oscillator, the solid-state oscillating element is positioned with the re-entrant cavity resonator, making it compact, light, and easy to manufacture. 

Superconducting Re-Entrant Cavity Filters

In communication circuits, frequency selective filters can be attained using re-entrant cavity resonators. These filters exhibit higher-quality factors compared to planar transmission line filters. They show the property of reducing interference from adjacent channels. These types of filters can be applied in circuits where space is not a limitation and filtering quality is of primary importance. The construction of re-entrant cavity structures from superconducting materials is a new approach, and it reduces losses caused by ordinary conductors. 

In re-entrant cavity oscillators, fluctuating frequency can be changed by varying the gap capacitance. Distinct characteristics of re-entrant cavity filters make them the best fit for signal processing circuits. When working in frequency ranges above 3MHz, using cavity resonator-based oscillators, filters, and amplifiers is highly recommended. 

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