Why are RF filters necessary?

Wireless communication systems require the selective transmission and reception of only the necessary signals, while eliminating unwanted signals and noise. Especially in recent years, with the increasing frequency and multi-channel nature of wireless systems, signal selection performance to efficiently utilize limited frequency resources has become more crucial than ever. RF filters are important passive components in such wireless communication systems, playing a vital role in allowing signals within a specific frequency band to pass through while suppressing other unwanted signals. Proper use of filters reduces interference from noise and unwanted waves, leading to improved overall communication quality and measurement accuracy.

Furthermore, RF filters are used in a wide range of RF and microwave systems, including cell phone base stations, wireless LANs, satellite communications, radar, and various measuring instruments, playing a crucial role in supporting diverse applications from communication satellites to mobile networks.

This article explains where and how RF filters are used, using transceivers as an example, and introduces typical filter types and key terms to understand when selecting a filter.

Where are RF filters used in wireless communication systems?

RF filters are used in various parts of the transmitting and receiving sections of wireless communication systems. By allowing signals within a specific frequency band to pass through and removing unwanted signals and noise, they contribute to maintaining the overall communication and signal quality of the system. In transceivers, one of the most common wireless communication systems, multiple RF filters are placed in the signal transmission and reception paths. Filters play an important role in suppressing unwanted harmonics and spurious signals on the transmitting side, and in selecting the target signal and removing unwanted signals on the receiving side.

The following is a typical transceiver configuration. The diagram shows multiple filters (BPF, LPF), as well as high-frequency circuit blocks such as amplifiers and mixers. These circuit blocks work together to generate the transmitted signal and process the received signal.

Example transceiver configuration

Key basic characteristics in RF filter design

There are various types of RF filters, but to understand their basic characteristics, it's important to know about three types: low-pass filters (LPF), high-pass filters (HPF), and band-pass filters (BPF). These filters differ not only in the frequency band they pass through, but also in their design philosophy, depending on the required cutoff characteristics, phase characteristics, and impedance characteristics.

This section explains the characteristics and design points of each filter.

Low-pass filter (LPF)

A low-pass filter (LPF) is a filter that allows signals below a certain frequency to pass through while attenuating signals above that frequency. In wireless communication systems, it is widely used to remove harmonics and unwanted high-frequency components.

■ LPF frequency characteristics
An LPF (low-pass filter) allows frequencies below the cutoff frequency to pass through while attenuating higher frequencies.

Note: The vertical axis represents attenuation. Attenuation is small in the passband and large in the stopband.

■ Key characteristics in low-pass filter design
The performance of a low-pass filter is not simply about "passing low frequencies." Multiple characteristics are important depending on the application.

• Steep cutoff characteristics
In filters, the key factor is how rapidly the signal can be attenuated near the boundary between the passband and the stopband.
In particular, when unwanted signals are present near the target signal, filters with steeper cutoff characteristics can remove the unwanted signals more efficiently.
Therefore, it is an important design requirement in wireless communication systems where it is necessary to separate signals of closely spaced frequencies.

- Linear phase characteristics
When a signal passes through a filter, it undergoes not only a change in amplitude but also a change in phase.
Filters with linear phase characteristics can easily maintain the shape of the input waveform and suppress waveform distortion in pulse signals and digitally modulated signals.
Therefore, in communication systems where waveform quality is critical, not only amplitude characteristics but also phase characteristics become important evaluation criteria.

High-pass filter (HPF)

A high-pass filter (HPF) is a filter that allows signals above a certain frequency to pass through while attenuating signals below that frequency. In wireless communication systems, it is used to remove unwanted low-frequency components and noise.

■ Frequency characteristics of HPF
A high-pass filter (HPF) attenuates frequencies below the cutoff frequency while allowing higher frequencies to pass through.

Note: The vertical axis represents attenuation. Attenuation is small in the passband and large in the stopband.

■ Key characteristics in high-pass filter design
• Steep cutoff characteristics
In high-pass filters, the key is how efficiently unwanted low-frequency components adjacent to the passband can be removed.
The steeper the cutoff characteristic of a filter, the better it can remove unwanted signals while minimizing the impact on necessary high-frequency signals.
Therefore, in applications where it is necessary to suppress unwanted signals that are close to the passband, a steep cutoff characteristic is important.

Bandpass filter (BPF)

A bandpass filter (BPF) is a filter that allows signals within a specific frequency band to pass through while attenuating other frequency components. In wireless communication systems, it is widely used to select only the desired signal and remove unwanted signals and noise.

■ Frequency characteristics of BPF
A band-pass filter (BPF) allows only the set frequency band to pass through, while attenuating frequencies below and above that band.

Note: The vertical axis represents attenuation. Attenuation is small in the passband and large in the stopband.

■ Key characteristics in bandpass filter design
• Passband selectivity
In wireless communication systems, it is necessary to extract only the frequency band of the desired signal.
Filters that attenuate signals sharply near the boundary of the passband are more effective at removing unwanted signals in their vicinity.
Therefore, filter selectivity is important in applications where you want to suppress adjacent channel interference.

・ 50Ω impedance matching
Mini-Circuits' constant impedance bandpass filters achieve 50Ω impedance matching across the entire frequency band, including not only the passband but also the stopband.
Insufficient impedance matching can lead to signal reflection and loss, potentially negatively impacting system performance.
In particular, impedance mismatches can cause intermodulation distortion (IMD) in nonlinear devices such as mixers and oscillators.
Therefore, impedance matching is an important design element for maintaining the overall signal quality of the system.

Key terms in RF filter selection

When selecting an RF filter, it's necessary to check not only the passband but also the filter's frequency characteristics and specifications related to signal quality. Mini-Circuits uses insertion loss, passband, stopband, cutoff frequency, VSWR, and phase characteristics as indicators to evaluate filter performance. Here, we introduce important terms for understanding filter specifications.

■ Insertion Loss
Insertion loss is an indicator that shows the power difference measured at the filter input and output. Generally expressed in dB (decibels), it represents the signal loss caused by inserting a filter into a circuit. Insertion loss consists of the following three elements:
1. Losses due to impedance mismatch at the input port
2. Losses due to impedance mismatch at the output port
3. Dissipation loss (internal loss) due to reactance elements (L, C) inside the filter.
Mini-Circuits filters are primarily designed for 50Ω systems and are evaluated in a 50Ω environment during measurement.

■ Passband
The passband refers to the frequency range where the insertion loss of a filter is less than a specified value. For example, many Mini-Circuits low-pass filters are specified to have a maximum insertion loss of 1 dB or less within the passband. When selecting a filter, you need to check whether the signal frequency you want to use falls within this passband.

■ Stopband
The stopband is the frequency range in which the insertion loss of a filter exceeds a specified value. Many Mini-Circuits low-pass filters define the stopband as the region where the insertion loss exceeds 20dB or 40dB. These values are representative indicators set to allow engineers to quickly compare and evaluate the selectivity of filters. Note that some models actually have attenuation performance exceeding 60dB in the stopband.

■ Cut-off frequency (fco)
The cutoff frequency (fco) is the frequency at which the insertion loss of a filter is 3dB. The cutoff frequency is often used to define the boundary between the passband and the stopband. It is also used as a reference frequency to normalize the frequency response of a filter. For example, in a standard low-pass filter from Mini-Circuits,
　Upper limit of passband ≈ 0.9 × fco
It is defined as such. Therefore,
　Required passband: DC to 225 MHz
in the case of,
　fco = 250 MHz
You need to select a model of a certain degree.

Relationship between cutoff frequency (fco) and currency band/stopband

■ VSWR (Voltage Standing Wave Ratio)
VSWR (Voltage Standing Wave Ratio) is an indicator of the impedance matching state of a filter. It quantifies the matching state as seen from the other port when one port is terminated with 50Ω, and can also be evaluated by converting it to return loss (reflection loss). In many filters,
Passband: Good matching (1.2:1 or less near the center frequency)
Stopband: High reflectivity (approximately 18:1)
It is designed to be such. However, the constant impedance bandpass filter mentioned above is an exception, as it maintains good impedance matching in both the passband and stopband.

■ Center Frequency (f₀)
The center frequency is the frequency located in the middle of the passband of the bandpass filter. The 3dB cutoff frequency is...
Low frequency range: f₁
High frequency side: f₂
In that case, the center frequency can be calculated using the following formula.

This is the definition using the geometric mean. Note that for narrowband filters with a narrow bandwidth, the center frequency calculated using the geometric mean and the center frequency calculated using the arithmetic mean will be approximately the same value.

■ Linear Phase and Flat Time Delay
Linear phase characteristics refer to the property where phase change occurs at a constant rate in response to frequency changes. Mini-Circuits' PBLP series employs the Bessel-Thomson design to achieve linear phase characteristics. This characteristic ensures that the various frequency components constituting the pulse signal pass through with approximately the same delay. As a result, it offers advantages such as "suppression of signal waveform distortion" and "ease of maintaining pulse shape."

■ Group Delay
Group delay is an indicator of the delay time when a signal with a finite time width, such as a pulse, passes through a filter. Ideally, all frequency components should be delayed by the same amount of time. However, in general filters, the group delay varies with frequency. On the other hand, linear phase filters have a nearly constant group delay, which makes them good at suppressing waveform distortion. Therefore, group delay is an important evaluation item in "pulse signals," "high-speed digital communications," and "systems where waveform fidelity is important."

Related Information

This article explained the role of RF filters in wireless communication system design, the basic characteristics of LPF, HPF, and BPF filters, and the terminology to check when selecting them. Mini-Circuits offers a wide range of RF filter products, including bandpass filters, low-pass filters, high-pass filters, bandstop filters, deplexers, and triplexers. We also support various design technologies and mounting configurations, such as LTCC, thin film, cavity, and ceramic resonators. For more information on our RF filter product lineup, specifications, and product selection based on your application, please visit the product page below.

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