5G Core Components: RF Filters

Ⅰ. The concept of RF filters

RF filters are some of the most important components in receivers and one of the most important components in consumer electronics. The RF filter is a filter circuit composed of capacitors, inductors and resistors.

The main function of a filter is to reject unwanted signals outside the passband of the filter and to separate signals according to their frequency. RF filters can alter the amplitude and phase of the sinusoidal waveforms passing through them. Simply put, RF filters remove unwanted frequency components from a signal while retaining desired frequency components. RF filters are designed to process signals throughout the radio spectrum, covering radio, television, wireless communications, scientific research, military and defense. Filters are also components of other RF/microwave devices such as duplexers and diplexers, and are used to combine or separate multiple frequency bands. Filter construction varies by application, size, cost and performance.

Ⅱ. Classification of RF filters

RF filters can be classified according to process materials and application scenarios.

1. Classified by process materials:

RF filters can be divided into acoustic filters, crystal filters, and ceramic filters, among which acoustic filters are currently the mainstream filters for mobile phone applications. According to different technologies, acoustic filters can be divided into two types: surface sound filters (SAW filters) and bulk acoustic wave filters (BAW filters). Among them, SAW filter products include ordinary SAW, TC-SAW filter with temperature compensation characteristics and high frequency I.H.P-SAW. BAW filter products include BAW-SMR and FBAR.

(1) Acoustic filter

① SAW filter

A basic SAW filter consists of piezoelectric material (piezoelectric substrate) and 2 Interdigital Transducers (IDT). Electrical signals are converted to sound waves by IDTs, and sound waves are converted to electrical signals by IDTs. This process mainly relies on piezoelectric materials. Piezoelectricity refers to the fact that a crystal generates a voltage when it receives external pressure. Conversely, a crystal changes shape slightly when a voltage is applied across its sides.

The frequency of the SAW is directly proportional to the velocity and inversely proportional to the spacing of the IDT electrodes. When the spacing is smaller, the current density will cause problems such as electromigration and heat generation, so SAW filters are not suitable for frequencies above 2.5GHz. In addition, SAW filters are susceptible to temperature changes. As the temperature increases, the substrate material becomes less stiff and the velocity of sound decreases, so an alternative is the temperature compensating filter (TC-SAW). Performance is improved by adding a coating to the IDT structure, which increases in stiffness as temperature increases.

A high-end smart electronic device filters the reception and transmission of 2G, 3G and 4G wireless signals in dozens of frequency bands. Signal processing in Wi-Fi, Bluetooth, and GPS receivers also requires filters. It must isolate the signals of each receiving path so that they do not affect each other, and must also monitor other unnecessary external information.

② BAW filter

The bulk acoustic wave filter uses quartz crystal as the substrate to realize the vertical transmission of sound waves. The principle is to fix a layer of piezoelectric film between two metal electrodes, and then transmit the sound wave to the piezoelectric film to generate a standing wave. The resonant frequency is determined by the thickness of the substrate and the quality of the electrodes.

BAW filters are suitable for high frequency use and become smaller as frequency increases. It is not affected by temperature, has extremely low loss and great filter suppression. Compared with traditional SAW/TC-SAW, the manufacturing process and manufacturing cost of this technology are more complicated and more expensive. Therefore, we use thin film deposition and micromachining techniques to realize the resonator structure on the carrier substrate.

(2) Crystal filter

Crystal filters have the advantages of small size and light weight. Because of the high Q value of the crystal, it is easy to implement a narrowband bandpass or bandstop filter. The relative bandwidth of a crystal filter is only one-thousandth, limiting its application in many cases.

(3) Ceramic filter

Subelectric ceramics are polarized by a DC high-voltage electric field to obtain a piezoelectric effect similar to that of a quartz crystal. Its equivalent circuit is the same as that of a crystal resonator. The quality factor of the ceramic filter is much lower than that of the crystal, but higher than that of the liquid crystal filter, and its series and parallel frequency spans are relatively large, thus obtaining a large relative bandwidth. Its operating frequency can range from several megahertz to hundreds of megahertz, and the relative frequency bandwidth can range from a few thousandths to 10%. Ceramic filters are generally composed of several ceramic resonant cavities, forming a ladder network. In the emitter loop of the amplifier, it is often used instead of the bypass capacitor because of its Q ratio.

2. Classified by application scenarios:

RF filters are most widely used in the base station market and mobile phone market of wireless communication terminals. Therefore, according to the classification of application scenarios, filters can be divided into communication base station filters and mobile phone filters. Different application scenarios have different requirements for filters, so there are obvious differences in characteristics such as volume, manufacturing process, applicable broadband, cost, and power capacity between mobile phone filters and base station filters. Base station filters pay more attention to high stability, large bandwidth, high power and other indicators, while mobile phone filters are more sensitive to price and volume.

Base station RF filters are mainly divided into metal cavity filters and dielectric filters. Their corresponding manufacturing processes are metal precision machining and dielectric sintering. Mobile phone RF filters are mainly acoustic wave filters, including SAW, TC-SAW, BAW, FBAR, etc., and the corresponding manufacturing process is semiconductor manufacturing process. The main research direction of this report is the RF filter (acoustic filter) applied to the mobile phone.

Ⅲ. The function of RF filters

In RF front-end modules, RF filters play a vital role. In many cases, bad signals (known as interference) can cause system degradation or even damage. In wireless communication systems, various types of RF filters are used at the receiver input to attenuate signals outside the desired frequency band. RF filters are also used to reduce harmonics, spurious content and out-of-band leakage from the transmitter circuitry. In many applications of modern electronics such as smartphones, these devices are equipped with several wireless communication technologies. Without proper isolation using RF filters, these technologies can interfere with each other—a so-called coexistence design challenge.

As communication protocols become more complex, the requirements for both in- and out-of-frequency ranges are increasing, creating greater challenges for filter design. In addition, as the number of frequency bands that a mobile phone needs to support increases, the number of filters that need to be used in a mobile phone is also increasing. Currently, 4G mobile phones require more than 30 filters.

Ⅳ. How to choose the right RF filters

In today's wireless world, intense competition to expand bandwidth has forced people to pay more attention to filter performance. Failure to accurately parameterize the filters can create frequency conflicts that can expose design teams to issues such as crosstalk, dropped calls, lost data, and disrupted network connections.

Part of the problem with incomplete filter definitions is the current enthusiasm for digital electronics in the electronics market. According to some statistics, 80% to 90% of new electronic design engineers are software and digital. Therein lies the knowledge gap. Because regardless of whether the transmitted information is in digital form, when information is transmitted by radio or microwave, the carrier signal always obeys the physical laws of electromagnetism.

Fortunately, a quick recap of some important basics of filter performance parameters can help engineers correctly identify the right filter for a particular application. Making the right choice at the start can save time and money, and you can be assured of the best value for money when ordering these must-have components. RF filter selection can refer to the following five elements.

1. Understand the basic response curve

The basic response curves of filters include: bandpass, lowpass, highpass, bandstop, duplexer. Each specific shape determines which frequencies are allowed to pass and which are not.

Undoubtedly, the most common of this group is the bandpass filter. As all engineers know, bandpass filters allow signals between two specific frequencies to pass and reject signals at other frequencies. Examples include surface acoustic wave filters (SAW), crystal filters, ceramic and cavity filters. As a reference, the frequency coverage of the cavity bandpass filter manufactured by Anatech Electronics is 15 MHz to 20 GHz, and the bandwidth is in the range of 1% to 100%. All manufacturers have adopted the method of defining the passband with 0.5 dB, 1 dB, or 3 dB attenuation points on either side of the filter center frequency.

2. Find a manufacturer that can balance requirements

Although the filter vendor has nothing to do with the inherent characteristics of the filter performance, we still have to pay attention to the selection of the filter vendor. A good filter manufacturer can often produce specific parts to compensate for product design flaws.

3. Consider Power Handling Capabilities

Power capacity is the average rated power in watts. If this value is exceeded, the performance of the filter will degrade, or malfunction. In addition, the size of the filter depends to some extent on the power capacity of the filter. Generally speaking, the higher the power supply, the larger the board area occupied by the filter. Vendors like Anatech are working hard to adopt new algorithms to solve these challenging problems.

4. Strive for a reasonable VSWR

Usually, the voltage standing wave ratio (VSWR) is used to represent the efficiency of the filter, and the size is between 1 and infinity, which is used to represent the magnitude of the reflected energy. 1 means all energy is transferred without loss. If the value is greater than 1, it means that part of the energy is reflected, that is, wasted. But in a real circuit, it is difficult to achieve a VSWR of 1:1. Generally speaking, 1 to 5 is more practical. If the desired figure is lower than this, then the benefit-to-expense ratio falls.

5. Include all necessary technical parameters

It often happens that the engineer gives a short request for "a 100 MHz bandpass filter," which is clearly too little information. It is really difficult for filter suppliers to sign orders based on such little information. We will start by specifying all the frequency parameters, giving all the necessary information, such as:

(1) Cut-off frequency (Fc): It is the conversion point where the pass band of the low-pass filter or high-pass filter starts, and the conversion point is generally the 3 dB point.

(2) Suppression frequency: A specific frequency or group of frequencies at which the signal attenuates some specific value or set of values. Sometimes the frequency region outside the ideal passband is defined as the suppression frequency or frequency group, and the attenuation passed is called suppression.

(3) Center frequency (Fo): The midpoint between the two 3dB points of the bandpass filter (or bandstop filter), generally represented by the arithmetic mean of the two 3dB points.

The filter type determines the specific frequency. For band-pass and band-stop filters, the center frequency is the specified frequency. For low-pass and high-pass filters, the specific frequency is a cut-off frequency. For completeness, the engineer should also define the following properties, such as:

(4) Isolation: In the duplexer, Rx/Tx isolation is to suppress the transmission frequency (Tx) when considering the Rx channel, and to suppress the reception frequency (Rx) when considering the transmission frequency (Tx). The higher the isolation degree, the greater the ability of the filter to separate the RX signal from the TX signal, and vice versa. The result of this is that both sending and receiving become clearer.

(5) Insertion loss (IL): Indicates a value of power loss in the device, IL=10Log(Pl/Pin). This has nothing to do with frequency. Among them, Pl is the load power, and Pin is the input power from the generator.

(6) Working temperature: The working temperature range of the filter design.

(7) Group delay (GD): Indicates the size of the device phase linearity.

(8)Impedance: Filter source impedance (input) and termination impedance (output) in ohms. In general, the input impedance and output impedance are the same.

(9) Stopband: The frequency band between specific frequency values that the filter does not transmit.

(10) Return loss (RL): Indicates the degree to which the input and output impedance of the filter is close to the ideal impedance value. Return loss is defined as: RL = 10Log(Pr/Pin). It is frequency independent, where Pr is the power reflected back to the generator.

(11) Shape factor (SF): The shape factor of the filter is usually the ratio of the stopband bandwidth (BW) to the 3 dB bandwidth. It is a measure of the steepness of the filter edges. For example, if the 40 dB bandwidth is 40 MHz and the 3 dB bandwidth is 10 MHz, then the form factor is 40/10=4.

(12) Relative attenuation: the difference between the attenuation at the measured minimum attenuation point and the attenuation at the ideal suppression point. Typically, relative attenuation is expressed in units of dBc.

Ⅴ. Piezoelectric materials for RF filters

1. Aluminum nitride

Aluminum nitride (AlN) is a covalent bond compound with a hexagonal wurtzite structure, which has the advantages of high thermal conductivity, high temperature insulation, good dielectric properties, high material strength at high temperatures, and low thermal expansion coefficient. AlN has two applications in radio frequency filters, one of which is as a piezoelectric film for the manufacture of film bulk acoustic wave filters (FBAR).

Since the piezoelectric effect of AlN along the c-axis orientation is obvious, it is possible to obtain a high-performance thin-film bulk acoustic wave device by preparing an AlN film with a preferred orientation of the c-axis on the electrode material. The substrate made of aluminum nitride ceramics as the main raw material has the characteristics of high thermal conductivity, low expansion coefficient, high strength, and corrosion resistance. It is an ideal heat dissipation substrate and packaging material.

2. Lithium tantalate crystal

Lithium tantalate(LT) is a trigonal chemical crystal. LT is widely used in high-frequency broadband filters, second harmonic generators, electro-optic Q-switching components, and laser frequency doublers due to its excellent nonlinear optical effects, piezoelectric effects, and photorefractive effects. It has a wide range of uses in military and civilian fields. In the field of radio frequency filters, it is widely used as the substrate material of SAW filters because of its excellent piezoelectric properties. In particular, it has an absolute advantage in the production of SAW device substrates with a frequency below 3GHz, and no other material can replace its position.

3. Lithium niobate crystal

Similar to lithium tantalate, lithium niobate(LN) is also a chemical crystal of the trigonal system, which belongs to the ilmenite structure. The distorted lithium niobate crystal has many properties such as piezoelectricity, ferroelectricity, photoelectricity, nonlinear optics, and pyroelectricity. It has a wide range of uses in military and civilian fields, and can be used to manufacture Q-switches, photoelectric modulation, etc.

LN crystals doped with a certain amount of iron and other metal impurities can be used as holographic recording medium materials, second harmonic generators, phase grating modulators, large-scale integrated optical systems, high-frequency broadband filters, etc. LN can also be widely used as a piezoelectric substrate in the manufacture of SAW filters, and its usage is second only to LT.

4. Other piezoelectric materials

There are also a variety of piezoelectric materials that can also be used in the manufacture of radio frequency filters, including quartz, potassium niobate, lithium tetraborate, gallium strontium germanate and gallium lanthanum series, etc. With the continuous exploration of people, a variety of piezoelectric crystals with excellent performance have been continuously excavated and developed. However, lithium tantalate and lithium niobate are still the most used in the piezoelectric materials used to manufacture SAW filters at this stage. Aluminum nitride is the most used piezoelectric material in the manufacture of FBAR. Among them, quartz is the earliest piezoelectric crystal used to manufacture SAW filters, but due to the limitation of its own electromechanical coupling coefficient, it is difficult to apply to high-frequency and broadband RF filters, and has been gradually eliminated.