How Do Printed RF Filters Work?
If you pick up an RF module off-the-shelf, it may not be obvious how it works or what is going on inside the module. If you open up the packaging, you may see a printed circuit that provides the filtering action in that module. Printed RF filters provide these functionalities directly on a PCB as passive elements, and they are useful for designing signal chains when certain off-the-shelf components are unavailable.
If you’re building an RF system, which types of printed RF components should you use, and how can they be analyzed at the system level? Printed RF filters operate based on wave propagation on transmission line sections, which makes them very easy to place on a PCB alongside other components. We’ll look at some of the core concepts in RF filter design with printed elements and how some of the most common components work.
RF Printed Filters
RF printed filters all take advantage of wave propagation to transmit or block signals on a PCB within specified bandwidths. Outside of the passbands in these circuits, the filter functions as either a reflector or absorber, which will prevent unwanted power from traveling into the filter element. There are many types of printed components that provide filtering action that is analogous to discrete filter circuits. Some of the most common are listed below.
Transmission Line Impedance Transformers and Stubs
The transmission line section used in an RF interconnect can also act like a type of filter. The way these filters work is quite simple: they provide a passband at specific frequencies (and their harmonics) through impedance matching, but they reject at all other frequencies. By matching impedance at the target passband, therefore unwanted frequencies experience strong return loss and are rejected.
These filters can be very high-Q at the target frequency with bandwidths of less than about 5%. Standard impedance transformer transmission line sections include stubs and series impedance transformers.
Transmission line stub and series impedance transformation as a filter.
Cascaded Filters
A variation on the above filter style is a cascaded transmission line filter, which can provide much broader bandwidth impedance matching. The output impedance (normally the system impedance at 50 Ohms) is taken as the load and cascaded back to the input port across a series of transmission lines. By matching back to the input port in this way, the passband can be rather large but the rejected frequencies can be strongly absorbed (experiencing high insertion loss) throughout the stopband. An example with a low-pass filter built from cascaded transmission line sections is shown below.
Cascaded transmission lines functioning as a low-pass filter.
A variation on this filter uses cascaded stubs instead of cascaded line sections, giving high-pass filter functionality. The same analysis technique with matching back to the input port applies in this filter design as well.
Wilkinson Dividers
A Wilkinson power divider is a variation on the typical series transmission line impedance transformer that will split power from one input port to two or more output ports. The Wilkinson divider operates at one frequency and certain harmonics and therefore acts like a bandpass filter. With precise impedance matching across the length power divider and when fabricated on a low-loss laminate, power can be divided down very close to the theoretical limit of approximately 3 dB in each leg.
Couplers
Couplers operate through intentional crosstalk; they provide power transfer between elements by exploiting parasitic capacitance and inductance between transmission line sections. These filters can couple strongly as moderate-Q bandpass filters. The spacing between these line sections will determine the coupling strength and the total power that can be ideally transferred through these devices.
Coupler functioning as a bandpass filter.
A Process For Incorporating Printed RF Filters
Although passive RF systems can technically operate entirely with printed components, the typical approach is to mix printed elements with off-the-shelf parts, including with active components. Starting at the system level helps develop specific component requirements, then the components can be selected off-the-shelf and printed directly on a PCB. Some of the design requirements listed above that must also be determined in your system include:
- Carrier frequency
- Bandwidth
- Loss budget and power budget
- System size target
With these factors in mind, designers can determine which components can be taken off-the-shelf and which can be selected as printed elements. In systems with high-frequency interconnects using printed filters, the bandwidth will typically be limited for many systems. Finding the right mix of off-the-shelf components and printed components can help designers achieve their performance targets while reducing part count in the final assembly.
When you’re ready to design your printed RF filter elements on a PCB, make sure you use the complete set of design features in OrCAD from Cadence to specify your design requirements and create your PCB layout. OrCAD includes the industry’s best PCB design and analysis software with flex PCB design tools. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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