One article on understanding filters in 5G RF front-end modules

With the development of RF technology, many modules in RF circuits have also been discovered to have more functions. To tap into the potential of these modules, it is necessary to have a deeper understanding of them, and filters are such a module that can be discovered. Filters are one of the important modules in the RF front-end. As the name suggests, the main function of a filter is "filtering", which means blocking interference information through useful signals.

     Image: Filter in RF Front End

In RF communication systems, "spectrum" is a very valuable and crowded resource. In addition to 5G, 4G, Wi Fi, GPS, and Bluetooth signals that are closely related to our daily lives, there are also signals from communication satellites, military satellites, and meteorological monitoring. In real life, wireless signals are everywhere, so RF "filters" are needed to clean up useless signals.

      Image: Crowded Spectrum Resources

Filters in 5G RF front-end modules

At present, the main frequency bands for large-scale commercial use of 5G are below 6GHz. In this frequency band, it is divided into Sub-3GHz and Sub-6GHz according to different frequencies. The Sub-3GHz frequency is located below the 3GHz frequency and is an upgraded and reused frequency band from the original 4G LTE, hence it is also known as the Re farming frequency band. The characteristics of this frequency band are numerous spectra, narrow bandwidth, and a large number of FDD frequency bands, which require precise filtering of signals to meet normal communication needs. 5G Sub-6GHz generally refers to the part of the new frequency band below 6GHz and above 3GHz. Currently, the most important frequency bands are n77 (including n78) and n79. This part of the frequency band has a wide bandwidth, almost no interference in the surrounding frequency band, and is a TDD frequency band. There is no need to consider interference between transmission and reception, which can reduce the need for filter out of band suppression.

Figure: Frequency coverage of 5G RF front-end and requirements for filter characteristics

The filters used in 5G RF front-end modules are mainly classified as follows:

     Image: Filter classification of 5G RF front-end module

Piezoelectric filter

A piezoelectric filter is a filter designed using the piezoelectric effect of materials. The characteristic of piezoelectric materials is that they can convert electrical signals into mechanical signals. The piezoelectric effect commonly used in radio frequency is to convert electrical signals into elastic wave signals in mechanical signals, process them in mechanical signals, and then convert them into electrical signals for output. Due to the fact that the elastic waves used are within the frequency range of sound waves, this device is also known as an acoustic wave device.

     Figure: Working principle of acoustic wave device

In acoustic wave devices, the most common are SAW and BAW devices, which are respectively called surface acoustic wave devices and bulk acoustic wave devices. From the names SAW and BAW, it can be seen that both are devices designed using acoustic properties, but one is to transmit sound waves on the surface, and the other is to transmit sound waves inside the body. The following are schematic diagrams of the working principles of SAW and BAW devices, respectively.

   Figure: Schematic diagram of the working principle of BAW filter

The advantage of piezoelectric filters is that they can utilize the extremely high Q value of acoustic devices to design narrowband, high suppression, and low insertion loss filters. The disadvantage is that piezoelectric materials must be used, which is incompatible with semiconductor processes in integrated circuits. And the process is sensitive, which puts high demands on design and manufacturing processes. The comparison of different filters in the 5G RF front-end module is shown in the following figure:

   Figure: Comparison of Different Filters in 5G Modules

Filter Integration in 5G RF Modules

Whether it is LC filters designed for IPD and LTCC, or piezoelectric filters designed for SAW and BAW technologies, they are incompatible with semiconductor processes such as PA, LNA in RF front-end and GaAs, SOI, CMOS in switch design. In module design, System in Package (SiP) is used to integrate and achieve module integration.

      Image: Process implementation of 5G RF front-end module

In the 5G integrated module, in addition to the main measurement indicators for the above-mentioned filters, higher requirements are also put forward for filter size, secondary packaging capability, etc. Due to the inability to perform secondary debugging and replacement after packaging into the module, filters in 5G integrated modules also require high consistency and reliability.

Summary

Filters are important components in the RF front-end, responsible for completing the "filtering" function of signals. In filter evaluation, frequency and bandwidth, insertion loss, suppression, temperature characteristics, and power characteristics are important evaluation indicators. The commonly used filters in 5G mobile terminals include LC filters and piezoelectric filters, which are respectively used in broadband scenarios with low suppression requirements, as well as narrowband scenarios with high suppression requirements. In the evolution of 5G high integration modularization, filters are also continuously integrated into RF front-end modules. In the integration process, in addition to considering conventional filter specifications, it is also necessary to consider indicators such as filter size, secondary packaging capability, and reliability.