At the lower mode of frequencies the oscillation and amplification is carried out by conventional wires and transistors networks. We can design the oscillator with the use of coil and capacitors. We can also make an amplifier using transistors and FET’s.
At the higher mode of frequencies above 3 MHz all the methods mentioned above are not applicable due to the skin effect and stray inductance/capacitance. The cavity resonator is the device which is used for oscillation and amplification above 3 MHz efficiently.
It is a close compartment made of conductor and hollow from its interior. The input and output ports are made in this compartment to carry the R.F signal. The conducting area of the cavity works as inductors in parallel and its mouth works as the capacitors for the microwave length of frequencies. Hence, a perfect oscillator is formed when the input is provided to the cavity resonator at high frequency the output of the cavity resonator is higher than it input, so it also works as an amplifier.
Types of Cavity Resonators
The cavity resonator is designed in different types according to its structure and working capabilities. Some of the types are mentioned below.
- Regulated Cavity Resonator
- Un Regulated Cavity Resonator
- Co-axial Cavity Resonator
- Capacitive Cavity Resonator
- Inductive Cavity Resonator
- Waveguide Cavity Resonator
- Reentrant Cavity Resonator
Regulated Cavity Resonator
As shown in the given diagram. The regulated cavity resonator is designed in such a way that we take a piece of circular waveguide. This piece of circular waveguide is covered with the conducting plates from open ends. The input and output ports are made in this piece in order to provide the external input to the cavity and obtain the output for further use.
When the input is provided to the cavity, the oscillation takes place in the cavity at higher mode of frequencies. The E-field and the H-field is developed and the output is taken from the output port for further use.
Un-Regulated Cavity Resonator
As shown in the given diagram, the un-regulated cavity resonator is designed in such a way that two pieces of regulated cavity resonator is joint together with the help of circular waveguide. In one piece of the regulated cavity resonator input port is made and in the second piece output port is designed.
When the input is provided to the un-regulated cavity resonator the oscillation take place in this portion of the resonator. After a movement this oscillation is transferred to the output port gradually through the circular waveguide in the second piece of regulated cavity resonator. The advantage of un-regulated cavity resonator is its greater band width as compare to regulated cavity resonator. But the disadvantage is its less gain.
Co-axial Cavity Resonator
As shown in the given diagram, the co-axial cavity resonator is designed in the rectangular shape. In which internal portion is hollow. The surrounding is covered with the conduction mesh which is of the co-axial cable and the broad dimension is taken λ/2 of the operating frequency. The input and output ports are designed in the broad dimension of the resonator.
The band width for this type of cavity resonator is high so that the wide range of input frequencies can be amplified.
Capacitive Cavity Resonator
As shown in the given diagram of capacitive cavity resonator, we come to know that it is designed like the waveguide cavity resonator with some changes. A piece of rectangular waveguide is taken. The conducting plates of broad dimension are flexible. Pressure is applied over the flexible plates and can be varied.
As the E-field is perpendicular to the broad dimension therefore, increasing in the pressure results in disturbance of the electric field which varies the frequency of the cavity. It is due to the reason that with the increase of pressure the broad dimension also increases. It means that λ/2 of the broad dimension. Hence the operating frequency varies by applying pressure. In this type of cavity resonator the different input frequency of microwave length can be handled where the gain will remain the same.
Inductive Cavity Resonator
The inductive cavity resonator is a piece of rectangular waveguide. The narrow dimension is made flexible. The mechanical pressure is applied over its narrow dimension. As the H.field is perpendicular to the narrow dimension, therefore increase in the pressure will increase the length of this dimension.
If the length is increased the wave length of this dimension will also increase. As a result the operating frequency of the inductive cavity resonator will decrease.
The inductive cavity resonator is used for the TM mode of operation for different microwave length of frequency with the constant gain output.
Waveguide Cavity Resonator
The waveguide is designed such a way that a piece of rectangular waveguide is taken and its open ends are covered by the conducting plates the input and output ports are made for the R.F energy in microwave length.
When the input is provided to the waveguide cavity resonator the oscillation takes place. This oscillation is in such a way that E.Field is perpendicular to the broad dimension and H-field is perpendicular to the narrow dimension. The frequency of operation is equal to half wave length of the broad dimension.
Reentrant Cavity Resonator
The reentrant cavity resonator is designed in such a way that two pieces of cavity resonator are taken and they joined perpendicularly (or 90 Degree) with another piece of rectangular waveguide at its either side. The input and output ports are designed as shown in the diagram.
The advantage of reentrant cavity resonator is increase band width of microwave length of frequency. It is used as wide band amplifier and oscillator from 3 MHz to 300 MHz.