Filters 1 Comprehensive Introduction Article

Filters: 1 Comprehensive Introduction Article

This article originally appeared on Network World and was written by IDG staff writer Brent Dietz.

Filter This Few people will think about the filter before servicing their car for 50,000 kilometers. Look at the items listed on the maintenance invoice, including air filter, oil filter, fuel filter, cabin air filter, transmission filter…

Oh my gosh, how many filters are there? !

We may also think of filter during family dinners. You probably have at least one relative who uses a filter. Yes, that’s you, Aunt Sondra.

But most of the time, we don’t see the filter or think about it. Most people would be surprised to find a dozen or more filters in their pocket, but they’re not meant for you to snap photos and post on pocket lint, Snapchat, or Instagram.

RF filters basics

Like antennas, filters are becoming an increasingly important part of networked mixers.The design process of L-type filter entails selecting appropriate components such as inductors and capacitors to meet specific filtering needs.

The rapid growth of mobile wireless data and 4G LTE networks has also led to growing demand for new frequency bands and the combination of frequency bands through carrier aggregation to accommodate wireless traffic. 3G networks only used about five frequency bands, LTE networks now use more than 40 frequency bands, and with the arrival of 5G, the number of frequency bands used will increase further.

Connected devices must send cellular, Wi-Fi, Bluetooth and GPS signals across multiple frequency bands while avoiding interference. We may immediately think of smartphones, but so are shark fins mounted on car roofs, cellular base stations, radar and communications systems, and industrial, scientific or medical applications connected to the Internet of Things (IoT). This is when the filter comes into play.

Devices receive a variety of frequencies, and filter allow the desired frequencies to pass while rejecting unwanted frequencies. In other words, the filter is like Gandalf from J.R. Tolkien’s Lord of the Rings:

“Don’t even think about passing through here!”

Today’s equipment typically has 30 to 40 filters to avoid interference. This situation will become more complicated as the next generation of high-end smartphones requires more filters.

A smartphone without a filter is a brick. Optimization techniques are employed to enhance the performance during the design process of LCL-type filter.

Power filters
Power filters

Filtering Challenge

Filters are an essential tool for RF design engineers, but they also come with many challenges. For starters, filter performance changes with temperature. Filters in today’s equipment are exposed to average temperatures of 60 degrees Celsius (140 degrees Fahrenheit) or more, while the average temperature for indoor filters is 25 degrees Celsius (77 degrees Fahrenheit). Filter embedded in shark fins or car roofs are subject to even higher temperatures.

The hotter the filter, the harder it is to filter out specific frequencies, and the more likely the signal is to “drift” into adjacent frequency bands.

“I particularly like static white noise, phone lines, and dropped calls,” no one ever says.

Because many of the newly allocated frequency bands are very close to existing frequency bands, managing temperature drift is particularly important. Meanwhile, carrier aggregation (CA) is also evolving rapidly, allowing cellular service providers to combine up to five carrier channels to improve network performance, where precise filtering is a must.

To address temperature issues, the RF industry is developing low-drift and drift-free filter technologies. Surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters are highly stable over temperature changes to meet the stringent performance requirements of emerging equipment.

As mentioned above, the next generation of high-end smartphones will also need to be equipped with more filters. Like all other components of RF, there is very little room left for filter. Engineers must be able to integrate multiple filter into a smaller space to achieve higher performance.

Here are some RF components that I really like.

Diplexers, triplexers, quadplexers, and even hexaplexers are collectively called multiplexers. Multiplexers combine multiple filters into a single device, helping designers save space, simplify designs, and meet performance requirements while avoiding interference.

The final challenge is Wi-Fi. While some people may still use their smartphones to text and make phone calls, most people use their smartphones to browse the web, watch videos, and access social media. Because LTE and Wi-Fi operate in very close frequency bands, unfiltered Wi-Fi signals can cause a device to be less sensitive to LTE signals. And reduced sensitivity will bring us a lot of trouble, such as dropped calls or interruptions.

For this we need a special filter. Collaboration among experts accelerates the evolution of the design process of LLCL-type filter.

Coexistence filters allow Wi-Fi and LTE signals to coexist harmoniously. Absolutely worthy of the name. This filter suppresses closely adjacent frequencies so we don’t miss our mom’s call while scrolling through our friends’ Instagram stories. They also play an important role in connected vehicles, as LTE, Wi-Fi, Bluetooth, GPS, and vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications must coexist without interference.

Without filters, wireless traffic jams would occur.

In today’s mobile environment, the number of frequency bands required for a device is staggering, and with the advent of 5G, this trend will only intensify. Although supporting all frequency bands will cause interference problems, the problem can be solved by using filters. Without filters, the network simply cannot function.

High pass filters
High pass filters