How Does RF Filters Look Like? Part II

In the last article we discuss and show the practical implementation of RF filter in mobile phone application. It demonstrated how filter technologies emerged from SAW to BAW following the evolution of mobile generation and the ever increasing demand for device miniaturization.

Similar to the network infrastructure which works as the brain of the mobile network, transmitting and receiving signals between deployed base stations as well as terminal devices such as mobile phones, the demand for miniaturization and greater performance is also facing a greater height.

As we move into the 5G era and beyond, it is expected that terminal devices will not be limited to personal mobile phones but also any portable devices that can have an uplink to the network, for instances vehicles, instruments, machines and etc. In this article, it is intended to extend the discussion into some other practical implementation of RF filter technologies that are rarely understood and alien to engineering graduates to certain extent.

“Antenna” Towers?

It is straightforward understanding that every wireless device would need RF filter for noise filtering with no exception. The figure below is picture of a typical mobile base stations that can be realized and found in any community. Most of the time, these white blocks are regarded as “Antenna” which transmit and receive signals. However this is not entirely accurate. The white blocks also consist of RF filters. Compared to the antennas, they are almost the same size!

Now you may be puzzled by the size of the filter as it is millions times bigger than what is being discussed in the previous article which was demonstrated in chip level form factor. Why do we need such a big RF filter in this white box? How do these filters look like? Do they follow the L-C circuit implementation that we learnt in the textbook? And how are these filters different from the SAW or BAW filter found in our smartphone?

Macro Base Station for Mobile Network Communication

The Cavity Filters

Uncovering the white box, the RF filter is what you can see as shown in figure below. Don’t be surprised, this entire block is a RF filter! Apparently the picture has addressed some of the doubts such as the size and appearance of the RF filter in a base station. Same question to be asked here: Can you locate the ‘LC filter’ in the picture? Or could you even spot the L or C circuit?

And yes, again they are not as simple as what we learnt from the textbook! Known as the cavity filter, the principle of this filter is to construct multiple “cavity-like” resonators cascaded together in order to exhibit its filtering operation. The resonator structure for this implementation is a form of coaxial resonator which are demonstrated by the rods in the center of each hollowed square.

Why Use Cavity Filters?

The fundamental reason for having this type of resonator implementation is due to the stringent requirement of power handling, RF performance as well as the environmental factor such as temperature and pressure taking into consideration the base station may possibly be deployed and operate nominally at high altitude with extremely harsh environment and unpredictable weather.

RF chip filter will not be an option as it deals with low power RF signal which interfaces very close to the human subject. A metric called SAR is a measure of the rate of RF (radiofrequency) energy absorption by the body from the source being measured — in this case, a cell phone. SAR provides a straightforward means for measuring the RF exposure characteristics of cell phones to ensure that they are within the safety guidelines set by the FCC. This is another topic on radiation and will not be the focus of this article.

The typical power level of mobile phone is limited to mW range whereas the power handling of a base station can range from 100W — few kW. A simple analogy is the size of a filter or resonator is proportional to the power handling. The bigger the size, the better the power handling. Hence, it is now clear why the size of a base station RF filter is many times of a mobile phone RF filter.

Size vs Cost vs Performance

Of course there are some other parameters which determine the size of the filter such as the loss of the filter device and how selective the filtering can be in order to get rid of all possible noises transmitting in the air. Loss is a very important consideration in designing the RF filter as this would impact the total power consumption required in order to drive the whole system. A poor design will lead to higher power consumption to drive the system and hence high cost for utility. This in turn translates to consumers paying for higher cost bill in order for the mobile phone operator to maintain their profit!

Having realized the big size RF filter implementation for base station, could you imagine the weight, cost and the challenges for the installation if this has to be placed on a snowy mountain for better coverage? As a result of this great challenge, its understandable how important is the impact of miniaturizing the filter without sacrificing the technical performance and perhaps even giving better performance.

The Market Trend

Scientists, engineers, mathematicians are constantly putting great effort just to solve this problem and this filter alone has become an industry of its own. Not only miniaturization, demand is further pushing the physical, performance and also the functional limits. It has been reported that the worldwide Market for 5G Base Station is valued at 73980 million USD in 2020 and is expected to reach 1303860 million USD by the end of 2026, growing at a CAGR of 50.1% during 2021–2026.


FILPAL Cavity Filters

For us in FILPAL, cavity filters are one of the exciting design work we do. Not only is there a demand for them with the advent of 5G technology, but we also appreciate the challenge and excitement when designing industrial standards cavity filters. So let me take this opportunity to show some of our past designs, to give you an idea how cavity filters can look like.

So far we have only discussed about the implementation of RF filter in mobile application. What about our satellite communication and defense application? Nowadays, with the various emergent applications, it has further push demand for advanced functionalities to be incorporated into RF filter such as the tunability, self adaptive and etc. Perhaps I can talk more of this in future articles.

Originally published at on September 5, 2021.

FILPAL designs, and builds RF and Microwave software and hardware for Cellular, Military, Academia and Test & Measurement applications.