As radio architectures evolve, the need for filters is also evolving. At the same time, the industry is working to miniaturizes mmWave devices while continually minimizing costs. This means RF designers need filter solutions that offer a smaller footprint while keeping prices manageable.
This is not an easy task. Filtering at mmWave frequencies can be much more difficult as there are a variety of unique challenges to consider beyond size and costs. Through extensive experience working on mmWave filtering applications, we have determined that there are five common RF filtering challenges present. Below is an overview of these five challenges and our approach to tackling each one.
1. Finding a Filter Technology that Works Across All mmWave Bands
Since there is not a single filter that can operate from 24.25 GHz to 52.6 GHz, it is ideal to identify one filtering technology that can be used across this entire spectrum. We’ve found that Planar thin-film implementations are most desirable from the standpoint of size, cost, and performance.
2. Determining How “Good” Your Filter Really Needs to Be
If the majority of your driving is commuting a short distance to and from the office, it likely doesn’t make sense to spend the money for a high-performance vehicle like a Ferrari when a Ford Focus would get the job done at a fraction of the cost. The same is true when it comes to selecting an RF filter – it’s not practical to use the expensive high-performance filter in every application.
3. Working with Strict Size Constraints
In mmWave applications, space generally comes at a premium. After comparing the size of a variety of common bandpass filters, we determined surface mount technology (SMT), and a microstrip approach in particular, is a great choice.
4. Monitoring the Impact of Tolerance on Overall Cost and Performance
Poor tolerance encroaches on potential board space or layers that could be used for adding other devices or functionality. Using a fully integrated design featuring thin-film technology and a high-permittivity dielectric lets RF designers shrink the overall size and integrate the resistor, reducing variation from resistor tolerances and improving overall RF performance at the lowest cost.
5. Managing Temperature Stability Issues
In these densely packed systems, there is no way to control temperature, which means frequent variations may occur and systems will run hot. Therefore, filters must have the ability to perform within specification over a wide range of temperatures with a good temperature stability of approximately 3 ppm/°C. By designing with the right dielectric material and filter topology, temperature-stable SMT filters with high rejection and low loss can be produced.
Graph A shows the response of microstrip bandpass filters built on alumina and graph B shows the response of microstrip bandpass filters built on CF dielectric.
The Right Filter for Any mmWave Application
At Knowles Precision Devices, we aren’t your typical commodity components supplier. Our goal is to help RF designers overcome the unique challenges of mmWave by developing filters that are smaller, more temperature stable, and allow for more design flexibility. Get started today by talking to us about the challenges you are facing with your mmWave applications so we can determine the best filter options for your unique application.