Current status and challenges of SAW filters and introduction of new IHP SAW filters
I’ve been swearing before, I’m sorry, I’m sorry.
IHP SAW is also called incredible high performance SAW, which is called super SAW. When I saw this name for the first time, I couldn’t help but be shocked and sighed with a rare straightforwardness. Let's take a look at Murata's promotional documents here;
The radio communication terminal includes a radio frequency filter having a frequency ranging from 800 to 2500 MHz. At present, the most widely used RF filter is a sound table filter (commonly referred to as SAW), which is a piezoelectric crystal such as quartz, lithium niobate or lead titanate, which is evaporated on the surface after polishing. Metal film, through the photolithography process to form two sets of interdigitated metal electrodes with energy conversion function, respectively called input interdigital transducer and output interdigital transducer. When the input interdigital transducer is connected to the alternating current When the voltage signal is applied, the surface of the piezoelectric crystal substrate vibrates and excites sound waves of the same frequency as the applied signal. The sound wave mainly propagates along the surface of the substrate and the direction in which the interdigital electrodes rise, so it is called sound. Surface filtering, in which the sound wave in one direction is absorbed by the divisor sound absorbing material, and the sound wave in the other direction is transmitted to the output interdigital transducer, which is converted into an electrical signal output).
In recent years, we have also seen more and more applications of BAW filters (unlike SAW filters, vertical propagation of sound waves in BAW filters. For BAW resonators using quartz crystals as substrates, they are embedded in The metal on the top and bottom of the quartz substrate excites the sound waves to cause the sound waves to bounce from the top surface to the bottom to form a standing acoustic wave. The thickness of the slab and the mass of the electrodes determine the resonant frequency. The BAW filter is highly visible. The frequency of the piezoelectric layer must be on the order of a few micrometers. Therefore, the resonator structure is realized by thin film deposition and micromachining techniques on the carrier substrate. In some frequency bands with higher difficulty, BAW technology has the Q value and bandwidth characteristics that SAW can't match.
The incredible high performance of the new SAW filter newly developed by Murata, Murata calls it the IHP SAW filter, which overcomes the shortcomings of the traditional SAW filter and achieves better performance than the BAW filter. In this article, Murata introduces the new IHP SAW filter.
1. Current status and challenges of SAW filters
RF filters are widely used in the transmit and receive links of RF terminals, allowing signals of a specific frequency or frequency band to pass while filtering out unwanted interference or spurious signals. It consists of several resonant units connected in different circuit forms (as shown in Figure 1 is a typical ladder circuit), providing a filter circuit with a certain bandwidth.
Fig. 1.Schematic diagram of an RF filter (ladder circuit)
An important characteristic in RF filters is the steepness of the transition band, which is the curve between the passband and the stopband in the filter bandpass characteristics. If the transition bandwidth between the passband and the stopband (as indicated by the red arrow in Figure 1) is too narrow, the design difficulty of the filter is greatly increased. In order to increase the steepness of the transition zone, the design of the connection circuit between the resonators constituting the filter is important, but the characteristics of the resonance unit itself (quality factor or Q value) are also key factors. In order to meet the filter design requirements of these difficult frequency bands, manufacturers have been working hard to increase the Q value of the resonant unit. SAW devices based on single crystal piezoelectric LiTaO3 substrates have been widely used in RF filters. The filter has an interdigital transducer mounted on the surface of the piezoelectric substrate. The resonant characteristics of the surface acoustic waves excited by the IDT determine the characteristic band of the RF filter (Figure 2).
Fig. 2. Surfaceacoustic wave device
Another common new RF filter on the market is the bulk acoustic wave filter BAW. Unlike the SAW filter, the acoustic waves in the BAW filter propagate vertically. For a BAW resonator using a quartz crystal as a substrate, the metal attached to both sides of the top and bottom of the quartz substrate excites the sound wave so that the sound wave bounces from the top surface to the bottom to form a standing acoustic wave. The slab thickness and the mass of the electrode determine the resonant frequency. Although BAW is more complex to produce, it can provide higher Q values ​​and provide a narrower transition band.
In this context, Murata has successfully developed a new IHP SAW technology that overcomes some of the weaknesses of traditional SAW. The new IHP SAW is able to achieve near or better performance than BAW, while having better temperature characteristics.
2. Characteristics of IHP SAW
IHP SAW is a SAW filter with the following characteristics: (1) high Q, (2) low frequency temperature coefficient (TCF) and (3) good heat dissipation
(1) High Q value
IHP SAW has a high Q value. This filter uses a new structure that focuses the energy of the surface acoustic wave on the surface of the substrate, allowing the wave to propagate without loss on the substrate. In the 1.9 GHz band, the peak Qmax of the Q value of the resonant unit reaches 3000 or more, and the resonant unit Qmax of the conventional SAW filter is about 1000.
Figure 3 is an example of an IHP SAW filter. The design band is Band25, which is traditionally considered to be the most difficult to handle. The measured data is more satisfactory. The typical insertion loss of the Tx band is 1.5dB, and the insertion loss of the Rx band is about 2.1dB. The typical value of the isolation is 57dB in the Tx band and 59dB in the Rx band. This has been fully satisfied. The extent of need.
Fig. 3. TheI.HP SAW Band25DPX (left: transmission characteristics, right: isolation characteristics)
(2) Low frequency temperature coefficient (TCF)
The IHP SAW filter can improve the frequency temperature coefficient TCF, which achieves satisfactory temperature characteristics by simultaneously controlling the linear expansion coefficient of the substrate and the speed of the sound. Figure 4 shows the characteristics of the filter when the temperature is from 35 ° C to + 85 ° C. The traditional SAW filter has a very large transition in TCF, about 40 ppm / ° C, while the IHP SAW filter can show An improved TCF characteristic that achieves about ±8 ppm / ° C or less. The TCF has been improved by approximately 30 ppm / °C and can be further increased to 0 ppm / °C with a properly designed substrate structure.
Fig. 4. Filterwaveform under the variation of temperature (blue: -35°C, black: +25°C, red: +85°C)
The TCF characteristics of IHP SAW are also superior to BAW filters. The range of common BAW filter TCF is 20-30 ppm / °C. Therefore, the IHP SAW filter can achieve better TCF characteristics than the BAW filter.
(3) Good heat dissipation
The IHPSAW filter also exhibits good thermal characteristics. When there is a high power electrical signal input, the IDT generates heat. Entering a stronger signal produces more heat, which can cause equipment failure, including electrode failure. The IHP SAW filter can efficiently dissipate the heat generated by the electrode to the substrate, thereby reducing the temperature rise, which is nearly half lower than the conventional SAW filter. The low TCF and better heat dissipation characteristics ensure the stability of the SAW filter at high temperatures.
3. Future can be expected
The IHPSAW filter can perform excellently in a wide frequency range from 800M to 2.5GHz. In recent research, we have also verified the performance of 3.5GHz, which is also considered to be difficult for traditional SAW filters. Frequency band (Q value comparison curve is shown in Figure 5). The evolution of mobile terminals to high-speed communication tools is an inevitable path for future development. IHP SAW filter products are also potential options for meeting the challenges posed by next-generation communication terminals.
Fig. 5. Qcharacteristics comparison of the IHP SAW and traditional SAW filters
Another feature of the IHP SAW filter is the freedom of bandwidth adjustment. The IHP SAW filter allows designers to choose whatever bandwidth they want. In addition, the IHP SAW filter can be used for the miniaturization of the filter, and the above three characteristics will contribute to the miniaturization of the device compared with the conventional SAW filter. As the available size on mobile terminals has shrunk, there has been an increasing demand for the size of radio frequency devices. Under this premise, the IHP SAW filter is an ideal product that offers a size advantage.
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