• page_banner1


What is an RF filter and why is it so important?

What is an RF filter and why is it so important? 

Filters are necessary to filter unwanted signals entering the radio spectrum. They are used in combination with various electronic devices. However, its most important use is in the RF domain.


What is an RF filter

Radio frequency filter is an important part of wireless technology. It is used together with radio receiver to filter other unnecessary frequency bands and only receive the correct frequency. RF filters are designed to operate easily in the frequency range from intermediate frequency to very high frequencies (i.e. megahertz and gigahertz). Because of its working characteristics, it is most commonly used in radio stations, wireless communications, television and other equipment. 

Generally, most RF filters are composed of coupled resonators, and their quality factors can determine the filtering level in RF. According to the application and size of wireless equipment, there are many filter types, namely cavity filter, plane filter, electroacoustic filter, dielectric filter, coaxial filter (independent of coaxial cable), etc. 

Basic Types of Radio Frequency Filter

The RF filter is a special circuit that allows correct signals to pass through while eliminating unwanted signals. In terms of filter topology, there are four basic RF filter types, namely, high pass filter, low pass filter, band pass filter and band stop filter 

Low Pass Filter:

As the name implies, a low-pass filter is a filter that only allows low frequencies to pass through and attenuate other signal frequencies at the same time. When a signal passes through a bandpass, its frequency reduction is determined by many factors, such as filter topology, layout and component quality. In addition, the filter topology also determines the transition speed of the filter from the passband to achieve its final suppression. 

Low pass filters come in various forms. The main application of the filter is to suppress the harmonic of RF amplifier. This feature is important because it helps prevent unnecessary interference from different transmission bands. Primarily, low pass filters are used for audio applications and filter out noise from any external circuit. After the high-frequency signal is filtered, the obtained signal frequency has a clear quality. 

High Pass Filter:

In contrast to the low pass filter, the high pass filter allows only high frequency signals to pass through. In fact, the high pass filter and the low pass filter are very complementary, because both filters can be used together to produce a band-pass filter. The design of the high pass filter is direct and attenuates the frequency below the threshold point. 

Generally, high pass filters are used in audio systems, through which all low frequencies are filtered. In addition, it is also used to remove small speakers and bass in many cases; These filters are specifically built into the speakers. However, if any DIY project is involved, the high pass filter can be easily connected to the system.

Band Pass Filters:

A band-pass filter is a circuit that allows signals from two different frequencies to pass through and attenuate signals that are not within its acceptable range. Most band-pass filters rely on any external power source and utilize active components, namely integrated circuits and transistors. This type of filter is called active band-pass filter. On the other hand, some band-pass filters do not use external power supply and rely heavily on passive components, namely inductors and capacitors. These filters are called passive band-pass filters. 

Bandpass filters are commonly used in wireless receivers and transmitters. Its main function in the transmitter is to limit the bandwidth of the output signal to the minimum, so that the required data can be transmitted at the required speed and form. When the receiver is involved, the band-pass filter only allows decoding or hearing the required number of frequencies, while cutting off other signals from unwanted frequencies. 

In a word, when a band-pass filter is designed, it can easily maximize the signal quality and minimize the competition or interference between signals.

Band Reject:

Sometimes called band stop filter, band stop filter is a filter that allows most frequencies to pass through without changing. However, it attenuates frequencies below a very specific range. Its function is completely opposite to that of the band-pass filter. Basically, its function is to pass the frequency from zero to the first cut-off point of the frequency. In between, it passes all frequencies above the second cut-off point of the frequency. However, it rejects or blocks all other frequencies between these two points. 

In a word, a filter is something that allows signals to pass through with the help of the passband. In other words, the stopband in the filter is the point where some frequencies are rejected by any filter. Whether it is high pass, low pass or band pass, the ideal filter is a filter without loss in the pass band. However, in reality, there is no ideal filter because bandpass will experience some frequency loss and it is impossible to achieve infinite suppression when the stopband is reached. 

Why are Radio Frequency Filters so Important?

RF filters are used to classify signal frequencies, but what makes them so important? In short, RF filters can filter out noises that may affect the quality or performance of any communication system or reduce the interference of external signals. Lack of appropriate RF filter may damage the transmission of signal frequency, and ultimately may damage the communication process. 

Therefore, RF filters play an important role in wireless communication systems (i.e. satellite, radar, mobile wireless systems, etc.). When it comes to the operation of unmanned aerial vehicles (UAS), the importance of RF filters is obvious. Lack of proper filtration system will affect UAS in many ways, such as: 

The communication range can be reduced to interference caused by external environmental factors. In addition, the availability of a large number of RF signals in the atmosphere may cause serious damage to the UAV communication system. Malicious signals from other platforms include but are not limited to:; Intensive Wi Fi signal activity and other communication systems operating within the UAS. 

Interruptions from other communication systems will interrupt the UAS communication channel, thereby reducing or limiting the communication range of such systems. 

Interference will also affect the GPS signal reception of UAS; This increases the possibility of errors in GPS tracking. In the worst case, this may result in a complete loss of GPS signal reception. 

With proper RF filter, external interference and signal interference generated by adjacent communication systems can be easily eliminated. This maintains the quality of the desired signal frequency while easily filtering out all unwanted signal frequencies. 

In addition, RF filters also play a crucial role in the mobile phone environment. When it comes to mobile phones, they need a certain number of frequency bands to work properly. Due to the lack of appropriate RF filters, various frequency bands will not coexist at the same time, which means that some frequency bands will be rejected, namely, Global Navigation Satellite System (GNSS), public safety, Wi Fi, etc. Here, RF filters play an important role by allowing all bands to coexist at the same time. 

In general, filters are light in weight and help improve the performance of the signal frequency. If the RF filter does not provide the desired performance, then you can explore a variety of other options, one of which is to add amplifiers to your design. From grid amplifier to any other RF power amplifier, you can convert lower signal frequency to higher signal frequency; So as to improve the overall performance of RF design. 

Si Chuan Keenlion Microwave a large selection in narrowband and broadband configurations, covering frequencies from 0.5 to 50 GHz. They are designed to handle from 10 to 30 watts input power in a 50-ohm transmission system. Microstrip or stripline designs are utilized, and optimized for best performance. 

We can also customize the RF Filter according to your requirements. You can enter the customization page to provide the specifications you need.


Sichuan Keenlion Microwave Technology Co., Ltd.




Post time: Dec-22-2022