Lumped Element Filters: A Compact, Low-Frequency Filtering Option

Posted by Peter Matthews on Sep 21, 2022 8:45:00 AM
Peter Matthews

At Knowles Precision Devices, we are well known for our expertise delivering high-performance mmWave filtering options. But did you know we also excel at providing a wide variety of lower frequency filtering options using a lumped element filter construction? In this blog post, we explore the basics of lumped element filter design, general lumped element filter characteristics, and how we can push lumped element filter design limits to develop a wide variety of high-performance low-frequency filtering options.

The Basics of Lumped Element Filter Construction

In general, lumped element filters are passive filters constructed using the appropriate number of inductors (Ls), capacitors (Cs), and resistors (Rs) to meet the specific filtering needs of a particular application. At the most basic level, lumped element filters can be constructed from a collection of simple LC resonators as shown in Figure 1.


Figure 1. An example of a generic lumped element L-C structure.

The resonators used in the filter will create poles and zeros in the frequency response. A zero occurs when the function tends to zero, and a pole occurs when the roots that make the function tend towards its maximum function. By understanding how poles and zeros function (which you can learn more about in this post), we can construct resonators using Ls and Cs and place the poles and zeros where we need them to be to tightly control the frequency response.

Lumped Element Filter Characteristics

Lumped element filters offer small size at low frequencies that are not achievable with common ceramic, cavity, or waveguide implementations. Additionally, lumped element filter designs are highly customizable both in terms of electrical performance and mechanical and thermal characteristics. This is because we have a high level of control in terms of component and material choices as well as assembly techniques. For example, a lumped element filter can be constructed with withstand temperature and input power ranges that may not be possible with alternative resonator technologies.

What Filter Types Can Be Built Using a Lumped Element Construction?

All the usual filter types can be implemented in in a lumped element format including lowpass, high pass, bandpass, and band reject. As we discussed in more detail in our Designing Basic Filter Circuits blog, lowpass and high pass elements are relatively simple to develop and bypass filters can be pretty easily constructed by combining the two behaviors.

What Are the Capabilities of Knowles Precision Devices’ Lumped Element Filters?

At Knowles Precision Devices, we offer lumped element filter options that cover the frequency and bandwidth range shown in Figure 2 below.

Filter Bandwidth vs Frequency_Lumped Element

Figure 2. The frequency range covered by our lumped element filters is shown in the red area highlighted with a blue outline.

Unlike many suppliers, using our advanced modeling and design capabilities, we can achieve virtually any filter response shape and we can customize our lumped element filters to operate reliably in high-power, high-temperature, and harsh environmental conditions. More specifically, our lumped element filters can be designed with the following specifications:

  • Bandpass filters with narrow to moderate bandwidths (1 percent FBW to 70 percent FBW) and a Center Frequency (Fo) from 10 MHz to 7 GHz
  • Bandpass filters with an extra wide bandpass bandwidth (70 percent FBW to 175 percent FBW) and a Fo of 20 MHz to 11 GHz
  • Lowpass filters from 10 MHz to 22 GHz
  • Highpass filters from 10 MHz to 10 GHz
  • Bandreject filters from 20 MHz to 6 GHz that can be narrow band or wide band from 10 MHz to 6 GHz

Explore our wide-range of lumped element filter options by downloading our RF Filters catalog or learn more about some our other common filter types including cavity, ceramic coaxial, and microstrip. 

Topics: RF and Microwave, Filtering

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