Low Pass, High Pass and Band Pass Filters – Simple Explanation

Radio Frequency communication is one of the most evolving technologies in the past decades. It has a significant role in our daily life. Due to several technologies and a large number of devices, RF technology requires proper filtering solutions to avoid interference and ensure coexistence in the wireless ecosystem. This article explains what is low pass filters, High Pass Filters, and Band Pass Filters, their specifications, and their applications.

Wireless technology is inevitable in our daily life. It helps us to increase productivity and offers convenience and a life of ease. However, all these transmitting devices contribute radio frequency noise to their surroundings. It is very challenging to design a device or system without affecting the existing systems.

Each technology has to follow certain specifications to ensure wireless coexistence. For example maximum power of transmission, frequency, bandwidth, etc…

What is RF Filtering?

RF filter solutions play an important role to improve the quality of the received signals by avoiding (filtering out) unnecessary frequency components from the received spectrum. This is significant at receiving end since the signal strength is often too low at the receivers. Any noise could potentially impact the received signal quality and thus information loss.

Low Pass Filters

As the name implies, Low Pass Filers removes (rejects) any low-frequency components from the signals. Low pass filters are used in many RF applications for frequency selection and to avoid harmonics 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 low pass filter using 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 using 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 the 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 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 size and shape from SMD components to inches-long solid package. Low pass filters used in mobile devices are miniaturized size; applications like base stations will have to use high power handling filters (which increases 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 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 at 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 filter, high pass filters also has two regions. A pass-band region that 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 specification of high pass filters.

Other specifications are similar to Low Pass Filters

Applications of High Pass Filters

• HPFs are used to reject any low-frequency components for 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 which require low-frequency isolation.
• High Pass Filters are used in harmonics measurements to avoid fundamental signals from the source and only allow 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 selects 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: 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.

Applications of Band Pass Filters

• Bandpass filters are used in a wide range of applications such as mobile devices.
• 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.
• 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 are using bandpass filters to improve signal selectivity and avoid other noise from 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 is using bandpass filters in their transmission modules.
• Many IoT devices are using 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 uses high pass filters to avoid interference with other operating frequency bands.

Find More about What is Notch Filters and applications

Source: Basic Knowledge of LC filters