What is BPF Filter(Band-pass Filter)?
What is BPF (Bandpass Filter)?
Band-Pass Filter is one of the most fundamental and important components in electronic systems. It is designed to allow signals within a specific frequency range to pass through while attenuating all other frequencies outside this range. From the perspective of a waveform diagram, it is like a semicircular protective shield that covers and safeguards the desired signal area, blocking external noise and ensuring that the protected signal is transmitted without interference.
The two ends of the waveform curve gradually descend from the center point. The steeper the slope of this descent, the better the performance of the band-pass filter. However, achieving a steeper slope also becomes more challenging. Theoretically, the ideal shape of a band-pass filter curve would immediately and significantly suppress signals beyond the cutoff frequencies of both the high and low-frequency ends, forming a rectangular shape. However, in practice, it will be affected by various physical factors and present different waveform curve results.
In standard communication systems, band-pass filters play a crucial role. They are responsible for filtering out unnecessary data from the signal aggregate received by the antenna, leaving only the required information. By controlling the range of frequencies allowed to pass, they help organize and refine the signal. Conversely, when transmitting signals, a band-pass filter is used to organize and enhance the quality of the outgoing signal before it is transmitted through the antenna.
Performance parameters of BPF Filter
The main performance parameters of the bandpass filter (BPF) are as follows:
1. Center Frequency (Center Frequency)
The center frequency of a band-pass filter refers to the midpoint of its passband, typically defined as the frequency at which maximum gain occurs or the middle point of a symmetrical frequency band. In RF design, the center frequency determines the primary operating frequency of the filter. For example, Wi-Fi operates at 2.4 GHz or 5 GHz, while 5G NR operates at 3.5 GHz or 28 GHz. Selecting the appropriate center frequency ensures the smooth transmission of the desired frequencies while suppressing unwanted bands, thereby enhancing system performance and reducing interference.
2. Passband Frequency Range (Bandwidth, BW)
The passband range is the frequency range that the filter allows the signal to pass, usually defined by the -3 dB point. In RF communications, different wireless technologies have different passband ranges, such as LTE Band 3, which has a passband range of 1805–1880 MHz. Choosing the appropriate passband range ensures that specific signals pass through completely without excessive attenuation. Additionally, wide passband filters are suitable for high-data-rate applications, while narrow passband filters help select specific frequencies and reduce interference.
3. Insertion Loss
Insertion loss refers to the power attenuation a signal experiences when passing through a filter within the passband, measured in decibels (dB). It indicates the extent to which the filter affects the target signal—the smaller the value, the lower the energy loss, resulting in better signal quality and higher transmission efficiency. For example, an insertion loss of 1 dB corresponds to approximately a 20% power loss, while 3 dB represents about a 50% power loss. In RF design, low insertion loss reduces transmission power loss and improves receiver sensitivity, which is crucial for the performance of base stations, antennas, and RF front-end systems. Therefore, selecting low-loss materials and optimizing matching circuits is essential during design to enhance overall efficiency.
4. Stopband Rejection
The rejection range refers the ability of the filter to attenuate unwanted signals in the stopband, and the unit is dB. The larger the value, the stronger the filter's ability to suppress noise or interference signals. For example, a suppression range of 60 dB means that the signal power at that frequency is attenuated to one millionth of its original value. In RF communications, good rejection range helps reduce interference between different frequency bands, such as the coexistence between LTE, Wi-Fi, and 5G bands, ensuring signal quality and system stability.
The features of BPF(Band-pass Filter)?
1. Consider more factors than other filters (LPF/HPF/BSF)
The design of a bandpass filter (BPF) is more complicated than that of a low-pass filter (LPF), a high-pass filter (HPF), and a band-stop filter (BSF) because it requires controlling two critical frequencies at the same time: the low-frequency cutoff frequency and the high-frequency cutoff frequency, so that the correct frequency signal can pass through.
2. The using purpose is different from other filters (LPF/HPF/BSF)
The main purpose of the bandpass filter (BPF) is to only allow signals within a specific range to pass through, which is different from the low-pass filter (LPF), high-pass filter (HPF) and band-stop filter (BSF). LPF is mainly used to filter out high-frequency noise and allow low-frequency signals to pass normally, such as preventing noise in audio processing. HPF is used to remove low-frequency interference, such as power supply noise or certain unwanted signals. BSF completely blocks a certain frequency range, such as 50/60Hz noise from the power supply. BPF is mainly used to extract signals in a specific frequency range, such as selecting a specific channel in wireless communications or analyzing certain bands of an electrocardiogram (ECG) in medicine. So when designing a BPF, we must ensure that it can effectively filter out the required frequencies and block other unnecessary signals.
3. Design method is different from other filters (LPF/HPF/BSF)
The design of bandpass filter (BPF) is different from low-pass filter (LPF), high-pass filter (HPF) and band-stop filter (BSF) because it usually combines high-pass filter and low-pass filter together or uses resonant circuit to select a specific frequency band. Common BPF design methods include LC resonant circuit (for passive BPF), Sallen-Key amplifier architecture (for active BPF), and quartz filter or SAW filter (for high-frequency circuits). In contrast, LPF and HPF can be designed using simple RC or RL circuits, which is relatively easy to do. BSF usually blocks a specific frequency band by connecting in parallel or in series. Since BPF requires precise frequency selection, special attention should be paid to the quality of inductors and capacitors, signal loss, gain adjustment, and impedance matching during its design to ensure that the filter works stably within the required frequency band and maintains good performance.
What kinds of BPF are there?
1. Cavity Bandpass Filter (Cavity BPF)
Cavity Bandpass Filter (Cavity BPF) is composed of a metal cavity with a resonant cavity formed inside, which selects a specific frequency by electromagnetic resonance. The passband range and frequency response are designed by adjusting the cavity size and coupling structure. The frequency characteristics can be fine-tuned by adjusting the screws inside the cavity.
Cavity BPF has the highest Q value and low insertion loss, but is large and expensive. Compared with DR BPF, Cavity BPF has higher power handling capability and is suitable for base stations and radars. Compared with Helical BPF, it is suitable for higher frequency bands (above GHz). Compared to SAW BPF, Cavity BPF has better frequency stability, but is much larger and cannot be used in small devices such as mobile phones.
2. Dielectric Resonator Bandpass Filter(DR BPF)
Dielectric Resonator Bandpass Filter(DR BPF ) is composed of a dielectric resonator made of a high dielectric constant material, and uses the resonance of electromagnetic waves in the dielectric to select the frequency. Its small size makes it easy to match with other RF components.
The Q value of DR BPF is lower than that of Cavity BPF, but higher than that of SAW BPF and Helical BPF, thus providing better selectivity and lower insertion loss. Compared to Cavity BPF, it is smaller in size and suitable for high-frequency communication equipment in the 1–20 GHz frequency band. Compared with Helical BPF, DR BPF can be applied to higher frequencies and has better frequency stability. Compared to SAW BPF, DR BPF is larger in size but has a higher Q value in high-frequency applications, making it more suitable for base stations and microwave communication equipment.
3. Helical Bandpass Filter (Helical BPF)
Helical Bandpass Filter (Helical BPF) uses a spiral conductor as an inductor and forms an LC resonant circuit with distributed capacitors to select the passband frequency. The helical structure provides a high Q value in a relatively small volume and is usually packaged in a metal shielded box to reduce external interference. The frequency can be adjusted via the top screw, allowing for immediate fine-tuning of the frequency.
The Q value of Helical BPF is lower than that of Cavity BPF and DR BPF, but higher than that of SAW BPF. It is smaller than the Cavity BPF but still larger than the SAW BPF and is suitable for use in the VHF/UHF bands (30 MHz–3 GHz), such as radio communications and TV broadcasting. Compared with DR BPF, Helical BPF is suitable for lower frequencies, has a simpler structure and lower cost. Compared to SAW BPF, Helical BPF has better power handling capability, but is bulky and unsuitable for small devices such as mobile phones.
4. Surface Acoustic Wave Bandpass Filter (SAW BPF)
Surface Acoustic Wave Bandpass Filter (SAW BPF) is composed of a piezoelectric substrate and an acoustic wave transmission line. When an RF signal is applied to the piezoelectric material, it is converted into a mechanical acoustic wave and propagates on the substrate. The passband frequency is controlled by different electrode patterns. SAW BPF has the smallest size and is suitable for high-integration applications such as mobile phones and Wi-Fi devices. Compared with Cavity BPF and DR BPF, SAW BPF has the lowest Q value and is suitable for medium frequency applications within 30 MHz–3 GHz. Compared to Helical BPF, SAW BPF has smaller size and can be mass-produced, but has lower power handling capability and higher loss in high-frequency applications (>3 GHz), so it is not suitable for mmWave 5G or satellite communications.
What product can BPF be used for?
Bandpass filter (BPF) is a common filter that is widely used in many fields such as wireless communication, radar, satellite, and broadcasting. The following are some specific applications of BPF in some products:
1. Mobile communication devices
Smartphones: BPF is used in RF front-end modules to help select specific frequency bands and filter out unwanted signals, thereby improving call quality, data transmission rate, and anti-interference capabilities. Wireless base stations (such as 4G/5G base stations): In wireless base stations, BPF is used to filter out unnecessary frequency band signals to ensure stable signal transmission and maximize spectrum efficiency.
2. Satellite communication
Satellite receiving system: BPF is often used in satellite receiving equipment to filter out signals in the required frequency band and isolate other unnecessary frequencies, which is crucial to ensure clear signal reception. Satellite ground stations: Ground stations require highly selective filters (such as Cavity BPF and DR BPF) to avoid interference between different satellite frequency bands.
3. Radar system
Military radar: Bandpass filters are used in radar systems to screen specific frequency ranges, effectively filtering out background noise or other irrelevant frequency bands, thereby improving target recognition capabilities. Weather radar: Weather radar uses BPF filters to filter radar echo signals and accurately detect meteorological phenomena.
4. Radio Communication Systems
Amateur Radio: Bandpass filters are used in amateur radio to select signals in specific frequency bands. These filters can improve signal clarity and reduce interference. Public safety communications: Public safety communication systems such as police, fire, and emergency use BPF to filter unnecessary frequencies to ensure the stability and reliability of communications.
5. Wireless Sensors and Internet of Things (IoT)
IoT devices: IoT devices usually use low-power wireless technologies (such as LoRa, NB-IoT), and bandpass filters help select the correct frequency and reduce electromagnetic interference to ensure accurate data transmission. Smart Home: For example, in wireless communication technologies such as Wi-Fi, Zigbee, and Bluetooth, BPF is used to filter frequency ranges to ensure efficient data exchange and system stability.
6. Broadcasting System
AM/FM Broadcasting: BPF is used in broadcast receiving equipment to filter out unnecessary frequency band signals to ensure that the received audio signals are clear and accurate. Digital broadcasting (such as DAB and DAB+): BPF is also used in digital broadcasting systems to filter out digital signals in specific frequency bands to avoid interference and ensure clear playback of digital audio.
7. GPS System
Global Positioning System (GPS): BPF can filter the received satellite signals in the GPS receiver and selectively transmit the signals of the required frequency band to improve positioning accuracy and signal reception stability.
8. In-vehicle communication system
Vehicle-to-everything (V2X): The in-vehicle system uses BPF to filter out unnecessary communication frequency bands to ensure the stability of wireless communication between vehicles and between vehicles and base stations, thereby improving driving safety.
9. Measurement and test equipment
RF test equipment: RF signal analyzers, spectrum analyzers and other equipment use BPF to filter input signals to help measure and analyze the signal strength and quality of a specific frequency band.
Products and Services Provided by Temwell
Temwell is a leading company specializing in the design and manufacture of radio frequency (RF) filters with over 27 years of professional experience. We are dedicated to providing high-performance, high-precision filter solutions for global customers, widely applied in wireless communication, radar, satellite, broadcasting, medical, and other fields.
As a professional leader in the industry, Temwell provides customized BPF (bandpass filter) and other RF filter services, designing and producing tailor-made products according to different customer needs and project requirements. Our technical team has a strong background in RF design and can provide comprehensive support to ensure that customers can achieve their technical and commercial goals during the development process.
Whether you need to enhance communication quality, optimize radar signals, reduce interference, or solve any other RF filtering challenge, our professional team can provide you with a customized solution. Temwell has strong technical support and and free consulting services to ensure the successful implementation of each project.
If your team has any needs for RF BPF, please feel free to contact us and get free consultation services so that we can provide you with the best solution.
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