A Complete Guide to the Filters

By assypcbPublished On: April 18th, 2025Categories: Blog, Electronic Components

Table of Conent

Filters are circuits composed of capacitors, inductors, resistors, and other components that purify signals by selectively transmitting or suppressing specific frequency signals.

1. Filters Overview

A filter is an electronic circuit or device used for signal processing. Its core function is to improve signal quality by selectively allowing specific frequency components to pass through while suppressing or attenuating other frequency components. Its application scenarios include communication systems, power management, audio processing, and RF front-end.

2. What are the Types of Filters?

1) Classification by Frequency Characteristics

Low-pass filter (LPF): allows signals below the cutoff frequency to pass through, suppresses high-frequency noise, and is often used for power supply ripple smoothing.

High-pass filter (HPF): allows signals above the cutoff frequency to pass through, and removes low-frequency interference (such as DC offset).

Band-pass filter (BPF): only allows signals in a specific frequency band to pass through, and is used for signal frequency division in communication systems.

Band-stop filter (BSF): suppresses signals in a specific frequency band (such as ground reflection interference in radar systems).

2) Classification by Implementation Method

Passive filter: composed of passive components such as resistors, capacitors, and inductors, with low cost, but performance limited by component parameters.

Active filter: combined with active components such as operational amplifiers, with high gain and strong stability.

Digital filter: processes discrete signals through algorithms (such as FIR and IIR filters) with high flexibility.

3) Special Types

RF filter: used in wireless communication equipment (such as mobile phones and base stations) to solve the problem of interference between frequency bands. Typical types include surface acoustic wave filters (SAW) and bulk acoustic wave filters (BAW).

3. What are the Key Performance Parameters of Filters?

Center frequency: the reference frequency of the filter passband (such as the midpoint frequency of the bandpass filter).

Bandwidth: the frequency range allowed to pass.

Q value (quality factor): the core indicator for measuring frequency selectivity. The higher the Q value, the stronger the frequency selectivity of the filter.

Insertion loss: the power loss when the signal passes through the filter, which needs to be reduced as much as possible.

4. Where are Filters Used For?

1) Communication System

Transmitter: located behind the power amplifier (PA) to filter out harmonic interference.

Receiver: located in front of the low noise amplifier (LNA) to suppress out-of-band noise.

2) Power Management

Filter out ripple and noise in the power supply voltage and provide stable DC output.

3) Biomedicine and Image Processing

A high-pass filter enhances image edge details, and a band-stop filter removes interference in specific frequency bands.

5. What are the Advantages and Disadvantages of Digital Filters?

1) Advantages of Digital Filters

High Precision and Stability

Digital filters are based on digital operations and are not affected by analog component errors. They have high precision and environmental interference resistance. Their filtering characteristics are stable and will not drift due to temperature, device aging, and other factors during long-term use.

Flexible and Adjustable

Filter parameters (such as cutoff frequency, order, etc.) can be quickly adjusted through software programming without modifying the hardware circuit. Supports a variety of filtering algorithms (such as FIR, IIR) and complex functions (such as adaptive filtering).

Strong Anti-Interference Ability

Digital signal processing can suppress noise and interference through algorithms and is particularly suitable for separating target components from aliased signals.

Diverse Applicable Scenarios

Low-frequency and narrow-band signals can be processed, and special functions that are difficult to accomplish with analog filters (such as linear phase characteristics and steep cutoff characteristics) can be realized.

Higher Design Freedom

Supports frequency domain and time domain processing, which can not only restore signal waveforms (such as denoising and smoothing), but also separate frequency domain conflicting signals (such as multi-band separation in speech processing).

2) ‌Disadvantages of Digital Filters‌

‌Processing Delay‌

Digital filters need to complete sampling, calculation, and other steps, resulting in processing delays and making it difficult to meet scenarios with extremely high real-time requirements (such as high-speed communication systems).

‌High Computing Resource Requirements‌

High-order filters (such as high-order FIR) require a lot of computing resources, which may exceed the carrying capacity of low-performance processors.‌

‌High Design Complexity‌

Some advanced algorithms (such as IIR filters) need to solve stability problems, and the design and debugging complexity is relatively high.

‌Hardware Dependency‌

Depending on dedicated digital processors (such as DSP) or microcontrollers, the hardware cost may be higher than that of simple analog filters.

‌Parameter Sensitivity Issues‌

If the parameters are set improperly (such as insufficient sampling frequency or too high order), it may cause signal distortion or reduced computing efficiency. ‌

6. How to Choose Filters?

Application Scenario Requirements: Base station filters require high power capacity and stability, and mobile phone filters require miniaturization and low cost.

Environmental Interference Type: Select low-pass, high-pass, or band-stop type according to the noise frequency band.

Integration Process: SMT (surface mount technology) is suitable for miniaturized RF filter design.

7. Typical Brands for Filters

SCHURTER

MOLEX

TDK

Murata

Xilinx

And so on…

8. Filters FAQs

1) How do filters work? ‌

Filters achieve their functions through a frequency selection mechanism: they allow signals to pass with minimal attenuation in the passband, while greatly attenuating interference signals in the stopband. For example, a low-pass filter allows low-frequency signals to pass while suppressing high-frequency noise.

2) What is the core difference between digital filters and analog filters? ‌

‌Analog Filters‌: They are composed of passive components such as resistors, capacitors, and inductors, and process continuous-time signals. They have a simple structure but low adjustment flexibility.

‌Digital Filters‌: They process discrete signals based on algorithms, have strong programmability, and are suitable for high-precision scenarios (such as IIR/FIR filters in communication systems).

‌3) What should be noted when installing filters? ‌

‌Wiring Requirements: Reserve a “clean ground” at the cable port to avoid direct coupling between the signal ground and the filter ground.

‌Electromagnetic Shielding‌: The filter and the chassis must be reliably overlapped, and metal plates or sealing gaskets must be used to reduce RF impedance when necessary.

‌Installation Location: Keep as close to the interference source or sensitive equipment as possible to shorten the length of the wire after filtering.

‌4) What are the special requirements for RF filters? ‌

‌High-frequency Performance‌: Need to support GHz frequency bands, such as SAW/BAW filters commonly used in 5G communications.

‌Manufacturing Process‌: Use surface acoustic wave (SAW) or bulk acoustic wave (BAW) technology to improve quality factor and temperature stability‌.

5) What are the common causes of filter failure? ‌

‌Environmental Factors: High temperature causes capacitor capacitance drift, and high humidity causes leakage current or component corrosion.

‌Overload Damage‌: Exceeding the rated voltage/current causes inductor saturation or capacitor breakdown‌.

‌Design Defects‌: Failure to match system impedance causes signal reflection or abnormal insertion loss‌.

6) What are the components of an active filter?

Active filters are a type of electronic filter that uses active components, including op-amps and transistors. They actually work with passive components as well, like resistors and capacitors, but not inductors.

7) What are the differences between active and passive filters?

Passive filters do not require a power source, whereas active filters need a power supply to operate. Active filters can provide gain and have greater control over filter characteristics, while passive filters are limited in their ability to shape the frequency response.

8) How do I choose between a passive and an active filter for my application?

Consider factors such as the desired frequency response, signal levels, power handling requirements, and available space and budget.

Passive filters are often preferred for high-power applications, low-frequency designs, and situations where simplicity and reliability are crucial.

Active filters are suitable for applications requiring high Q factors, precise control over filter characteristics, and the ability to introduce gain.

9) What are some common applications of active and passive filters in engineering?

Passive filters are used in power supply filtering, audio crossover networks, and RF and microwave signal conditioning.

Active filters are employed in applications such as audio equalizers, biomedical signal processing, and anti-aliasing filters for data acquisition systems.

Both active and passive filters are used in telecommunications, control systems, and instrumentation to remove unwanted frequencies and improve signal quality.

9. Summary

Filters can effectively optimize signal integrity and improve the anti-interference ability and performance of electronic systems. With the popularization of high-frequency communications (such as 5G/6G) and Internet of Things technologies, filters tend to be high-frequency, integrated, and low-loss. The application of new materials (such as gallium nitride and high-temperature superconductors) and processes (such as MEMS technology) will further promote performance improvements.