Adjustable Analog Filter Circuit for Low-Noise Frequency Tuning
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Solution Overview
Problem
Existing analog filters with adjustable frequency, such as bandpass and band blocking filters, face challenges in minimizing noise and effectively adjusting frequency without introducing significant noise amplification, particularly in applications like ultrasound fill-level measuring devices where disturbance signals are prevalent.
Innovation Solution
The design incorporates an oscillatory circuit with parallel or series branches containing capacitance and inductance, along with an amplifier with adjustable amplification, which allows for the adjustment of resonance frequency while minimizing noise by controlling the electrical current through these components, thereby reducing noise amplification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If passive filters are used to reduce noise, then noise level is reduced, but frequency adjustment becomes mechanically complex requiring switching inductances and capacitances
Solution Approach 1:
The patent replaces mechanical switching of inductances and capacitances with an electrical control method. An amplifier with controllable gain adjusts the effective impedance of the oscillatory circuit, enabling frequency adjustment without mechanical switches or variable capacitors. This substitution eliminates the mechanical complexity while maintaining passive filter noise characteristics.
Solution Approach 2:
The patent changes the impedance parameter of the oscillatory circuit dynamically through amplifier control. By adjusting the amplifier gain, the effective impedance seen by the oscillatory circuit changes, which shifts the resonance frequency. This parameter change approach enables continuous frequency adjustment without physically switching components.
2Adaptability or versatility
If active bandpass filters with multiple cross coupling are used for frequency adjustment, then filter frequency is adjustable, but noise increases due to resistances
Solution Approach 1:
The patent replaces the active amplification stage with multiple cross-coupling resistances with a controlled impedance transformation approach. Instead of using resistive networks that generate thermal noise, the invention uses a single amplifier to dynamically adjust the effective impedance, thereby reducing noise while maintaining frequency adjustability.
Solution Approach 2:
The patent extracts and removes the noisy resistance elements from the frequency adjustment mechanism. By separating the frequency control function from resistive elements and implementing it through amplifier-controlled impedance adjustment, the design eliminates the noise source while preserving the adaptability of frequency tuning.
3Adaptability or versatility
If bandpass filters with very large bandwidth are used to cover total resonance frequency range, then frequency coverage is extended, but disturbance signal suppression deteriorates
Solution Approach 1:
The patent implements dynamic bandwidth control through the amplifier. Instead of using a fixed large bandwidth to cover all possible frequencies, the system dynamically adjusts the bandwidth to match the actual resonance frequency and required suppression level. This dynamic adaptation maintains narrow bandwidth for optimal suppression while covering the full frequency range through tuning.
Solution Approach 2:
The patent changes both the center frequency and bandwidth parameters of the filter dynamically. By coordinating adjustments of these parameters through the amplifier control, the system maintains optimal disturbance suppression at each frequency point while still covering the entire resonance frequency range of the ultrasonic sensor.
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 results in low-noise analog filters with adjustable frequency, capable of effectively filtering measurement signals with reduced noise levels compared to conventional active filters, and allows for precise tuning of filter frequencies without amplifying noise associated with resistances.
Implementation Method 1
an oscillatory circuit, whose resonance frequency equals the filter frequency of the filter
Implementation Method 2
an amplifier with adjustable amplification, whose output is connected with its inverting input via the frequency determining element arranged in such circuit branch, and which, in filter operation, amplifies, according to the adjusted amplification, a voltage applied across the frequency determining element arranged in such circuit branch and thereby effects a corresponding change of an electrical current flowing through such frequency determining element
Data Source
AI summary
A low noise analog filter with adjustable filter frequency includes an oscillatory circuit, whose resonance frequency equals the filter frequency of the filter. The oscillatory circuit has a first circuit branch. One of the frequency determining elements is a capacitance and the other an inductance. The low noise analog filter further includes an amplifier with adjustable amplification installed in one of the two circuit branches. The output of the amplifier is connected with its inverting input via the frequency determining element arranged in such circuit branch. In filter operation, the amplifier amplifies, according to the adjusted amplification, a voltage applied across the frequency determining element arranged in such circuit branch and thereby effects a corresponding change of an electrical current flowing through such frequency determining element.