Radio Frequency communication is one of the fastest-evolving technologies of the past decade. It has a significant role in our daily lives. Due to the growing number of technologies and devices, RF systems require proper filtering solutions to avoid interference and ensure coexistence in the wireless ecosystem. This article explains what low-pass filters, high-pass filters, and band-pass filters are, their specifications, and their applications.
Key Takeaways
- What RF Filters Do: RF filters selectively pass or reject specific frequency components from a signal — essential for maintaining signal quality and avoiding interference in wireless systems.
- Low Pass Filters (LPF): Pass frequencies below the cut-off frequency and reject higher frequencies. Widely used in RF transmitters, audio systems, and test laboratories.
- High Pass Filters (HPF): Pass frequencies above the cut-off frequency and reject lower frequencies. Used for harmonic measurements, satellite systems, and low-frequency noise rejection.
- Band Pass Filters (BPF): Pass a specific range of frequencies and reject everything outside that band. Critical in 5G devices, LTE smartphones, Wi-Fi routers, and IoT systems.
- Choosing the Right Filter: Cut-off frequency, insertion loss, stop-band attenuation, power handling, and form factor are the key specifications to evaluate for any filter selection.
Wireless technology helps us increase productivity and offers convenience and ease. However, all these transmitting devices contribute to radio frequency noise in their surroundings, making it very challenging to design a system without affecting others.
Each technology has to follow certain specifications to ensure wireless coexistence — such as maximum transmission power, operating frequency, and channel bandwidth.
What is RF Filtering?
RF filter solutions play an important role in improving the quality of the received signals by avoiding (filtering out) unnecessary frequency components from the received spectrum. This is significant at the receiving end since the signal strength is often too low at the receiver. Any noise could potentially impact the received signal quality and thus cause information loss.
Low Pass Filters
Low-pass filters pass low-frequency components and reject high-frequency signals above the cut-off frequency. Low-pass filters are used in many RF applications for frequency selection and to avoid harmonic signals from the transmitting antennas and power amplifiers.
Frequency response of Low Pass Filter
A low-pass filter has two regions: a pass band region and a stop band region. A typical low-pass filter will have low insertion loss across the pass-band region until the cut-off frequency, and higher attenuation after the cut-off frequency across the higher frequency range.
The electrical equivalent of a low-pass filter using an LC circuit
Passive low-pass filters can be created using different methods. Microstrip lines with varying impedance are used to design low-pass filters in the microwave frequency range. One of the simplest ways is to use the LC combination.
Low-pass filters can be designed using different topologies depending on the applications. The selection of these topologies determines the input and output impedance of filters suitable for different scenarios. Here are the most common topologies for different impedance scenarios:
Specifications – How to choose a Low Pass Filter
Frequency: operating frequency and cut-off frequency are important specifications when choosing a low-pass filter. Low-pass filters can be designed from the kilo-hertz range to the Giga-hertz range.
Power handling: Different applications have different power level requirements. It is important to make sure the LPF can handle the sufficient power required for specific applications.
Pass-band insertion loss: Pass-band insertion loss is very significant in some applications. A good filter should have an insertion loss closer to 0dB (typical).
Stop-band attenuation: Stop-band attenuation is yet another main specification for LPFs. A typical low-pass filter will have very good stop-band rejection/attenuation depending on the applications (30dB to 80dB).
Temperature range: Low-pass filters are designed to operate in different temperature ranges. Some components are sensitive to temperature fluctuations, which could affect the filter performance.
Form factor/package size: filter solutions are available in various sizes and shapes, from SMD components to inches-long solid packages. Low-pass filters used in mobile devices are of a miniaturized size; applications like base stations will have to use high-power handling filters (which increase the size).
Connector type: Standalone low-pass filters with packages will have different connector types for various applications. For example, filters used for low-frequency applications will use a BNC connector, and high-frequency applications will use an SMA or N-type connector.
Applications of Low Pass Filters
- Cut off high-frequency components in any system above its operating frequency range.
- Low-pass filters are used in radio receivers to avoid high-frequency interference. Example: FM radio broadcasting operates in the 88MHz to 108 MHz range, a low-pass filter with a cut-off frequency just above 108MHz is used in FM radio receivers.
- In audio devices, low-pass filters are used to filter treble sound from 2.5 kHz to 20 kHz (high-frequency components of the audio spectrum) to subwoofers. This frequency selection significantly improves the audio quality. Each speaker (subwoofer, mid-range speakers, and tweeter) is designed to operate within a specific frequency range.
- In RF test laboratories, low-pass filters are used to construct complex test setups. The main purpose of low-pass filters is to avoid high-frequency interference and improve test setup accuracy.
- In RF transceivers, LPFs are used to significantly improve the low-frequency selectivity and signal quality.
High Pass Filter
High-pass filters are filters used in RF and wireless applications to reject low-frequency components from a signal and pass the high-frequency range. These filters are used in many applications for frequency selectivity and provide higher isolation from low-frequency components.
For example, in a multi-band transmission system, antennae will receive multiple signals from various sources. This could interfere with the main signals in the system and cause distortion known as intermodulation distortion. HPFs will block any low-frequency components that could mix with the fundamental signal.
Frequency response of a High Pass Filter
Similar to low-pass filters, high-pass filters also have two regions. A pass-band region allows the high frequency to pass with minimum insertion loss, and a stop-band region where it has maximum attenuation or rejection.
Electrical Equivalent of High Pass Filter
Specification of High Pass Filter
Operating Frequency: Cut-off frequency and operating range are the most important specifications of high-pass filters.
High-pass filters share the same core performance metrics as low-pass designs, such as power handling, insertion loss, and temperature stability. The key difference is the cutoff frequency, which establishes the lower edge of the passband rather than the upper limit
Applications of High Pass Filters
- HPFs are used to reject any low-frequency components from the system
- In audio devices, HPFs are used to filter low-frequency components below 2.5 kHz.
- RF laboratories use HPFs to build various test setups that require low-frequency isolation.
- High-pass filters are used in harmonics measurements to avoid fundamental signals from the source and only allow the high-frequency harmonics range.
- HPFs are used in radio receivers and satellite technology to attenuate low-frequency noise.
Band Pass Filters
Bandpass filters are designed to filter a particular range of frequencies and reject other frequency components from the input signal. These filters are used in applications where high selectivity is required and in noisy environments.
A multi-band RF transceiver system will have multiple antennas to send and receive information using radio waves. If the frequencies are close to each other, it could interfere with other neighboring bands and distort the data. Band pass filters with a specific frequency range ensure it only select the desired frequency spectrum within that operating band.
Frequency Response of Band Pass Filter
Band Pass Filters have one pass-band region with insertion loss close to zero and two stop band regions with a maximum attenuation of more than -60 dB (it may vary in different cases and applications)
Electrical Equivalent of Band Pass Filter
Specification of Band Pass Filter
Pass-band: The operating passband is the most important specification of a bandpass filter.
Lower cut-off frequency: this is the frequency point at which the filter starts to reject lower-frequency components. Any frequency below this point will have higher attenuation than the pass band.
Upper cut-off frequency: this is the upper passband frequency point of the filter. After this point, the filter will have more attenuation moving to a higher frequency range.
Bandpass filters follow the same performance metrics as low-pass and high-pass filters — power handling, pass-band insertion loss, stop-band attenuation, temperature range, and form factor all apply. What sets a BPF apart is that it has two cut-off frequencies instead of one.
Comparison – LPF, HPF, and BPF
| Specification | Low Pass Filter | High Pass Filter | Band Pass Filter |
| Passes | Below cut-off | Above cut-off | Within a defined band |
| Rejects | Above cut-off | Below cut-off | Outside the defined band |
| Stop bands | One (upper) | One (lower) | Two (upper and lower) |
| Typical attenuation | 30–80 dB | 30–80 dB | 60 dB+ |
| Common applications | Harmonics suppression, audio | Harmonic measurement, satellite | 5G, LTE, Wi-Fi, IoT |
| LC topology | Series inductor, shunt capacitor | Series capacitor, shunt inductor | Combined LC resonator |
Applications of Band Pass Filters
- Bandpass filters are among the most widely used filter types in modern RF systems, appearing in everything from smartphones to satellite ground stations.
- Modern smartphones support multiple LTE bands used by telecom operators across the globe. Each handset is equipped with several band-pass filters within the communication module to make sure it can function properly in all frequency bands from 700MHz up to 2700 MHz.
- The introduction of 5G technology requires higher frequency bands in the sub-6GHz spectrum. High-performance Bandpass filters are used in 5G-supported devices to improve signal quality.
- Wi-Fi routers use bandpass filters to improve signal selectivity and avoid other noise from the surroundings.
- In high-definition audio systems, bandpass filters are used to select the mid-range frequency in the audio spectrum.
- Satellite technology uses bandpass filters to choose the desired spectrum.
- Automated vehicle technology uses bandpass filters in its transmission modules.
- Many IoT devices use bandpass filters to improve the selectivity of the received signal spectrum.
- Other common applications of bandpass filters are RF test laboratories to simulate test conditions for various applications.
- High-power transmitting and receiving stations use band-pass filters to avoid interference with other operating frequency bands.
Conclusion
RF filters are fundamental building blocks in every wireless system — from the FM radio in your car to the 5G module in your smartphone. Choosing the right filter type comes down to understanding your frequency requirements: whether you need to pass low frequencies, pass high frequencies, or select a specific band. The specifications — cut-off frequency, insertion loss, stop-band attenuation, and power handling — translate those requirements into a component you can design with.
As wireless systems continue to evolve toward 5G mmWave and Wi-Fi 7, filter performance requirements are becoming more demanding. Higher frequencies, tighter band spacing, and stricter coexistence requirements mean filter design is more critical than ever in modern RF systems.
FAQ
A low-pass filter passes frequencies below the cut-off frequency and attenuates everything above it. A high-pass filter does the opposite — it passes frequencies above the cut-off and rejects lower frequencies. The cut-off frequency is the -3dB point where the filter transitions between the pass band and stop band. Both filter types are defined by this boundary, just from opposite directions.
The cut-off frequency is the frequency point at which the filter begins to attenuate the signal, typically defined as the -3dB point, where signal power drops to half its pass-band value. For a low-pass filter, this is the upper boundary of the pass band. For a high-pass filter, it is the lower boundary. Selecting the correct cut-off frequency is one of the most critical decisions when choosing or designing an RF filter.
Insertion loss is the reduction in signal power caused by inserting the filter into the signal path, measured in dB. A perfect filter would have 0dB insertion loss in the pass band — meaning it passes the signal without any reduction. In practice, all filters introduce some loss. For receiver applications where signal strength is already low, keeping pass-band insertion loss as close to 0dB as possible is critical for maintaining system performance.
5G devices rely heavily on Band Pass Filters for frequency selectivity across multiple bands. In sub-6GHz 5G, high-performance BPFs select the required operating band while rejecting adjacent channel interference. For mmWave 5G above 24GHz, the filtering requirements become more demanding — SAW filters are no longer effective at these frequencies, which is why BAW (Bulk Acoustic Wave) filters have become the standard solution in 5G smartphones and modules.
A band-pass filter passes a specific range of frequencies and rejects everything outside that range. A notch filter — also called a band reject filter — does the opposite: it rejects a narrow band of frequencies and passes everything else. Notch filters are used to eliminate a specific interference source, such as a known interfering carrier or harmonic, while leaving the rest of the signal spectrum unaffected.
Find More about What is Notch Filters and applications
Source: Basic Knowledge of LC filters
