Tunable Filter With Distributed Switched Capacitors For Constant Bandwidth
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Solution Overview
Problem
Tunable filters with constant bandwidth and rejection across varying frequencies are difficult to achieve due to the inherent proportionality of filter quality factor with tuning frequency, leading to challenges in maintaining constant impedance and bandwidth in radio transmitters/receivers, especially in the VHF and UHF domains.
Innovation Solution
The filter design maintains constant internal impedance by distributing switched capacitors of variable values along the inductance, with heavier capacitors for low frequencies positioned farther from the ground and lighter capacitors for high frequencies closer to the ground, allowing for a constant impedance ratio throughout the tuning band, thus ensuring a constant bandwidth and rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If fixed inductors or transmission lines are used with switched capacitors for tuning, then the filter can be tuned to different frequencies, but the quality factor Q becomes proportional to the tuning frequency, causing the filter bandwidth to widen proportionally with frequency
Solution Approach 1:
The inductor is divided into multiple sections with access points distributed along its length, allowing selective connection of different capacitor values at different positions. This segmentation enables independent control of capacitance values to compensate for the frequency-dependent Q factor variation, maintaining constant bandwidth across the tuning range.
Solution Approach 2:
Different sections of the inductor are associated with different capacitor values based on their position. The capacitor values are specifically designed to vary locally along the inductor length, with heavier capacitors for low frequencies positioned farther from ground and lighter capacitors for high frequencies closer to ground, creating non-uniform local characteristics that compensate for global Q factor variations.
2Device complexity
If the internal impedance of the filter is proportional to the tuning frequency (as in classic filter topology), then the filter structure is simple, but it becomes technically and physically difficult to achieve constant bandwidth over a wide tuning range
Solution Approach 1:
The filter structure transitions from static fixed impedance to dynamic adjustable impedance. By selectively switching different capacitor values at different positions along the inductor, the internal impedance can be dynamically adjusted to compensate for frequency variations, maintaining constant bandwidth while keeping the overall structure relatively simple.
3Manufacturing precision
If tunable impedances are used for coupling to achieve constant bandwidth, then the bandwidth can be maintained, but filter losses increase
Solution Approach 1:
The filter structure serves itself by using the same inductor-capacitor network to achieve both tuning and impedance matching functions. The distributed capacitors along the inductor simultaneously provide frequency tuning and maintain constant internal impedance, eliminating the need for separate tunable impedance coupling elements and reducing overall filter losses.
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 approach enables a natural, constant band response and filter optimization over wide tuning bands without compromising performance, maintaining a quasi-constant resonator impedance and bandwidth across the frequency range, even exceeding an octave for power filters.
Implementation Method 1
several switched capacitors comprising several capacitors of variable values depending on the position
Implementation Method 2
at least one inductor or transmission line
Implementation Method 3
at least two tunable resonators, a first resonator comprising at least one inductor or transmission line
Implementation Method 4
the impedance of the resonator is kept constant throughout the tuning band of the tunable filter
Data Source
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AI summary
A tunable filter comprising at least two tunable resonators (30, 40), one resonator comprising at least one inductance or transmission line (31), said inductance or transmission line having a first end (31A) connected to earth M and a second end (31B) coupled to another resonator via a coupling impedance (32), and a plurality of switched capacitors (33i), characterised in that: · said inductance or transmission line (31) comprises a plurality of access points (31i) or connection points distributed along said inductance (31) making it possible to connect or not a switched capacitor (33i) the value of which is defined according to its position on the inductance, · the low-weight capacitors are located close to the end of the inductance connected to earth M. Use of the filter in radio transmitters/receivers.