Cavity Filter Assembly Air Cavity Parasitic Capacitance
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Solution Overview
Problem
Cavity filters face limitations in miniaturization and performance enhancement due to high parasitic capacitance between the transmission line and ground in coaxial type low-pass filters, which affects insertion loss and frequency cutoff characteristics, especially in ultra-high frequency bands.
Innovation Solution
The cavity filter assembly incorporates a hollow container with a second pocket portion forming an air cavity between the low-pass filter and the grounded cavity filter body, reducing parasitic capacitance by creating an open portion in the ground pattern that overlaps the transmission line, and using a dielectric substrate with impedance matching sections and open stubs to improve frequency characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a coaxial type low-pass filter with stepped-impedance is used to remove harmonics, then the harmonic removal performance is improved, but the physical length of the filter becomes very extended
Solution Approach 1:
The patent transforms the low-pass filter from a conventional coaxial type to a microstrip type, fundamentally changing the structural parameters and configuration. This parameter change enables the filter to achieve the same harmonic removal performance with a significantly reduced physical length, as microstrip structures can be more compactly designed compared to coaxial stepped-impedance structures.
Solution Approach 2:
The patent introduces a vertical air cavity dimension beneath the microstrip transmission line to reduce parasitic capacitance. By utilizing the vertical space (z-dimension) rather than only extending the horizontal length, the filter achieves improved frequency cutoff characteristics without increasing the planar footprint, effectively solving the contradiction between performance and size.
2Reliability
If the order of the low-pass filter is increased to improve harmonic removal, then the frequency cutoff characteristic is improved, but the physical length of the filter is extended
Solution Approach 1:
The patent changes the filter type from coaxial to microstrip and modifies the ground structure by introducing air cavities. These parameter changes allow the filter to achieve higher-order frequency cutoff characteristics in a more compact form factor, avoiding the need to extend the physical length proportionally with the filter order.
Solution Approach 2:
By adding the vertical air cavity dimension beneath the transmission line, the patent creates additional capacitance control capability without increasing the horizontal length. This dimensional approach allows higher-order filtering characteristics to be achieved while maintaining a compact footprint.
3Volume of moving object
If a microstrip form low-pass filter is used in ultra-high frequency band, then the miniaturization is enabled, but the parasitic capacitance between transmission line and ground increases
Solution Approach 1:
The patent introduces a vertical air cavity dimension beneath the microstrip transmission line. By creating this vertical separation space, the parasitic capacitance between the transmission line and the ground plane is significantly reduced, as capacitance is inversely proportional to the distance between conductors. This allows the filter to maintain miniaturized dimensions while eliminating the harmful parasitic capacitance effect.
Solution Approach 2:
The air cavity acts as an intermediary layer between the transmission line and the ground plane. This intermediate air gap serves as a dielectric medium with low permittivity, effectively reducing the parasitic capacitance coupling between the transmission line and ground, thereby solving the contradiction between miniaturization and parasitic capacitance reduction.
4Loss of energy
If parasitic capacitance between transmission line and ground is reduced, then insertion loss is reduced, but the filter structure becomes more complex
Solution Approach 1:
The patent utilizes the vertical dimension by introducing an air cavity beneath the transmission line. This approach reduces parasitic capacitance and insertion loss through a straightforward structural modification rather than requiring complex circuit designs. The vertical separation is achieved through simple etching or molding processes, maintaining manufacturing simplicity while achieving the desired electrical performance improvement.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly reduces insertion loss and shifts harmonics in the stopband to a higher frequency, enhancing the frequency cutoff characteristic and enabling miniaturization of the low-pass filter while maintaining performance.
Implementation Method 1
high parasitic capacitance between the transmission line and ground in coaxial type low-pass filters
Implementation Method 2
a resonance unit composed of conductive resonant rods to permit an electromagnetic field of natural frequency to exist exclusively, thereby distinctively passing only a characteristic frequency of an ultra-high frequency by resonance
Data Source
AI summary
The present disclosure provides a cavity filter assembly installed with an RF filter having an empty area formed between the RF filter and a cavity filter body serving as a ground to reduce the parasitic capacitance by forming the cavity filter body with a first pocket portion configured to install the RF filter and a second pocket portion within the first pocket portion in a position to overlap a transmission line, thereby reducing the insertion loss of the RF filter, which when serving as a low-pass filter, can position the harmonics in the stopband further away from the cutoff frequency and thus effect improved frequency characteristics of the low-pass filter through improvements of, for example, the frequency characteristics in the stopband.