Filter Topologies
Have you ever asked yourself, “why did Jim quote me a cavity filter when I was sure a discrete component filter was what I wanted”? In this post, I’ll detail several different filter topologies and reasons for using them. In advance, however, please understand that this is a general discussion – each situation is unique, and a given set of requirements may cause a deviation from the guidelines to follow.
Discrete Component: A discrete component (sometimes called an LC) filter is flexible in size and power handling capability. Selecting the right components can yield extremely small, surface-mount-sized units suitable for densely packed circuit cards or extremely large packages ideal for handling 10 kW or more. In addition, the flexibility of mixing and matching circuit designs make this an excellent choice for asymmetrical responses, though they are frequency limited to about 5 GHz.
Cavity / Combline / Interdigital: I’ll group these together due to their similarity. The main difference between these three topologies lies in a given filter’s percent bandwidth (%BW), necessitating a slightly different mechanical construction. Cavity / Combline / Interdigital filters advantage is having high unloaded Q. This offers the ability to have low insertion loss in a filter with a small %BW while maintaining good out-of-band performance. These units can be large at low frequencies or when designed to handle high power. These units are suitable for frequencies between 1 and 40 GHz.
Waveguide: A terrific choice for extremely small %BW designs where having low loss is a requirement. Waveguides are often a choice for high-power applications, and most designs are suitable for pressurization. Despite being a waveguide filter, you are not limited to waveguide flanges. These can also be outfitted with an RF connector of your choosing. Waveguides will be large at low frequencies; we typically suggest a 5 to 67 GHz frequency range to keep size in check.
Suspended Substrate: Rare among filter manufacturers, suspended substrate filters are best suited for two applications: extremely wideband requirements and high-order multiplexing. If you are looking for a bandpass filter to cover 1 to 18 GHz or 18 to 40 GHz, suspended substrate is the design we would use. This topology is uniquely suited to handle wide passbands because it is a printed rather than machined filter. When seeking a pentaplexer, octoplexer or any high-order multiplexer, suspended substrate is our topology of choice. Even at lower frequencies, the printed nature allows us to place the channels near the common port for ease of tuning. Generally, suspended substrates work well in a frequency range of 1 to 50 GHz.
Ceramic: Generally reserved for low-cost, high-volume requirements, ceramic filters offer the ability for automated assembly by our customers. Monobloc or resonator ceramic filters are small and designed for surface mount applications, though they can be placed into a housing with RF connectors. In other applications, we use ceramic resonators in some notch filter designs to increase performance. Ceramic designs are suitable for frequencies from 400 MHz to 5 GHz.
Tubular: The old-school workhorse of the filter industry. Being practically limited to a Chebychev response has not slowed down tubular filters’ continuance as a viable option. While more labor-intensive during manufacturing than several other options, tubular filters can handle high power and are uniquely suited to insertion into a cable fun for last-minute spurious response issues. Tubular filters are an option at frequencies ranging from 10 MHz to 20 GHz.
While I only briefly touched on topology options, there is undoubtedly a larger story to tell. If you are interested in a more substantial discussion of filter topologies, please contact me directly by email at jassurian@reactel.com.
At Reactel, we manufacture RF & Microwave Filters, Multiplexers and Multifunction Assemblies with satisfied customers across the globe. If you want to explore a filter for your requirement, please visit www.reactel.com or email me directly at jassurian@reactel.com.
