A diplexer is a design that receives an RF signal, splits that signal into two distinct paths, and ultimately routes it to two ports. This function is typically accomplished using a combination of high-pass and low-pass filters, as well as channelized structures, to achieve the isolation required for effective signal separation. When two signals reach the common input port, the lower-frequency signal sees a low-loss pathway through the low-pass channel but encounters a high-impedance, high-rejection barrier at the high-pass channel. Likewise, the higher-frequency signal is rejected by the low-pass channel and directed through the high-pass channel.
By effectively isolating the signals and the two channels, a diplexer ensures that nearby transmitters do not leak RF energy into one another, preventing signal distortion. The sharp rejection outside the intended passbands ensures that out-of-band signals are blocked, minimizing system interference. One of the primary challenges in diplexer design is achieving proper impedance matching at the common input point. If the two filter circuits that separate the frequency bands are not precisely matched, signals will encounter an impedance mismatch and be reflected back toward the source, resulting in standing waves, increased reflections, and unwanted insertion loss.
Visit our filter customization tool to optimize your demanding diplexer requirements. Our extensive library of low-loss designs can reduce the need for additional gain, supporting SWaP-constrained applications. Spectrum engineers apply innovative techniques to deliver high-performance solutions.
Role of Diplexers
Diplexers are often the simplest and most efficient way to combine or separate two bands that share a common antenna or signal path. Rather than routing transmit and receive signals through entirely separate connectorized hardware paths, a diplexer allows engineers to merge them at a single RF junction, reducing component count, weight, and complexity without sacrificing performance or isolation between the two RF paths.
This makes diplexers a common choice where size and weight are important but two signals still need to share a port, such as dual-band antenna feeds, combined transmit and receive front ends, or RF systems that must pass one frequency while rejecting another nearby frequency. Because each channel is specifically engineered for high rejection outside its own passband, the two signals remain separated even though they share the same physical input connector and enclosure.
Mixed Topologies
The core design challenge in a diplexer is ensuring both filters perform together within a shared enclosure without degrading RF performance. The lowpass and highpass sections must be matched at the common port so that neither channel presents an impedance mismatch. A mismatch can result in reflected RF power, standing waves, and added loss where signals exit the enclosure.
Because of this, Spectrum Control designs the filter topology to match each channel’s requirements rather than using a single approach. For example, RF performance may be optimized using a compact lumped-element section on one side and a higher-Q cavity section on the other, depending on which channel requires tighter rejection or lower loss. The result is a diplexer optimized for the frequency and performance requirements of each channel rather than a generic two-filter design.
Novel Packaging
Creative layouts set some diplexer manufacturers apart. This Spectrum Control design required bottom-mounted terminations for precise alignment with the customer’s existing ports. Rather than redesigning the board layout around a standard side connector, Spectrum Control adapted termination geometries to mate directly with existing port locations.
Adapting the interface simplified integration while maintaining RF performance and minimizing system redesign. This flexibility matters in diplexer applications, where housings often fit within tight spaces alongside other RF hardware. RF performance can be correct in theory, but a diplexer that doesn’t physically fit the footprint still fails the application.
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Spectrum Control engineers use suspended substrate topologies for ultra-broadband diplexer applications. These designs bridge the gap between larger cavity filters and microstrip designs, which typically have higher loss. By suspending the PCB in air between two metal ground planes, the design uses air as the primary dielectric, reducing loss and improving RF performance. Air’s lower dielectric constant also reduces sensitivity to dielectric variation across temperature—a key advantage in diplexers, where both channels must remain precisely tuned. This stability helps prevent changes in electrical length that can shift isolation between signal paths at the shared junction. |
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Diplexer objectives often extend beyond passing desired signals to rejecting high-power unwanted tones. Spectrum Control addresses this challenge with a pole-placed high-pass/low-pass notch design that delivers high rejection while maintaining low insertion loss in the desired passbands. Placing the rejection pole near the passband edge sharply suppresses unwanted tones without affecting the band of interest. This is critical when channels are closely spaced and added loss can affect isolation. Strong rejection also helps prevent high-power unwanted signals from driving components into nonlinear operation, which can generate harmonics or intermodulation products that degrade passband performance. |
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Spectrum Control high-performance diplexers use cross-coupling techniques, routing coupling paths between non-adjacent resonators to create transmission zeros at targeted frequencies. This approach sharpens the rejection slope at the passband edge—precisely where a diplexer’s two channels meet—while improving out-of-band rejection. By achieving sharper filtering without additional sections, cross-coupling can also help minimize overall size and insertion loss while maintaining strong isolation between channels. |