Tunable Lowpass Filter with Varactor and Damping Resistor
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Solution Overview
Problem
Existing lowpass filters, such as Chebyshev filters, are not tunable in cutoff frequency and lack adequate out-of-band rejection, especially when subjected to input spectra with multiple tones, which limits their linearity and spurious signal management.
Innovation Solution
A lowpass filter design featuring a series of inductors with electrically tunable capacitors, including varactors, and a resistance-capacitance combination across the inductor winding to enhance out-of-band rejection, allowing for tunable cutoff frequency and high linearity, with a control circuit for frequency adjustment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If fixed values for inductors and capacitors are used in a Chebyshev filter, then the filter structure is simple and manufacturing is easier, but the cutoff frequency cannot be varied without changing components
Solution Approach 1:
The patent applies the dynamics principle by replacing fixed capacitor values with electrically tunable capacitors (varactors) that can dynamically adjust their capacitance values based on control voltages. This allows the cutoff frequency to be varied without changing the physical structure or component connections, resolving the contradiction between adaptability and device complexity
Solution Approach 2:
The patent implements parameter changes by using varactor diodes whose capacitance can be electrically tuned through voltage control. The capacitance values of C1, C2, and C3 can be dynamically adjusted by applying different control voltages to the varactors, enabling continuous cutoff frequency tuning while maintaining the same filter topology
2Object-affected harmful factors
If a tunable high-order Bessel low pass filter is used, then the cutoff frequency can be varied, but adequate out of band rejection is not provided
Solution Approach 1:
The patent introduces an intermediary resistance element R1 connected in parallel with the series combination of inductor L1 and capacitor C1. This resistance acts as a mediator that provides additional attenuation paths for out-of-band signals, enhancing out-of-band rejection by approximately 20 dB without interfering with the normal tuning operation of the varactors
Solution Approach 2:
The patent creates a composite filter structure by combining multiple filter sections with different characteristics. The first filter section (L1-C1 with R1) provides enhanced out-of-band rejection, while the second filter section (L2-C2-C3) provides the tunable cutoff frequency response, creating a composite structure that achieves both objectives simultaneously
3Reliability
If the filter is subjected to an input spectrum with multiple tones, then linearity becomes critical, but additional spurious signals are generated
Solution Approach 1:
The patent converts the potentially harmful effect of varactor nonlinearity into a beneficial outcome by using the resistance R1 to suppress spurious signals. The resistance provides a damping effect that reduces the generation of intermodulation products and spurious signals, transforming the nonlinear behavior into a controlled response that maintains signal integrity
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
The design achieves tunable cutoff frequency and high linearity with minimal spurious signal generation, maintaining high out-of-band rejection several octaves beyond the initial stop band, while maintaining low and flat insertion loss, effectively managing spurious signals and improving filter performance.
Implementation Method 1
The electrically tunable capacitors may each include a varactor. Each of the varactors may include two diodes coupled together anode to anode or cathode to cathode.
Implementation Method 2
Each of the varactors may include one diode. Each of the varactors may include a p-n junction.
Implementation Method 3
at least one electrically tunable capacitor coupled to a node of one of the inductors; at least one of the inductors including a winding that includes a first end, a second end and a pair of nodes defining a portion of the winding between the first end and the second end
Data Source
Figure 1
Figure 2
Figure 3a~3b
AI summary
A low pass filter includes an RF input terminal, an RF output terminal, a plurality of inductors coupled in series between the RF input and output terminals, at least one electrically tunable capacitor coupled between ground and a node of one of the inductors. At least one of the inductors includes a winding, and a resistance and a capacitance coupled in series across a portion of the winding to enhance the out of band rejection of the low pass filter.