How do I design a diplexer or triplexer to combine multiple frequency bands into a single port?
Diplexer and Multiplexer Design
A diplexer is not simply two filters connected to a common port; the filters must be designed together as a single network. When two bandpass filters share a common port, the out-of-band impedance of each filter appears as a reactive load to the other filter at its passband frequencies. This loading changes the individual filter responses unless it is accounted for in the design.
The star junction approach connects both filters at a single point (the common port). The design process starts by designing each filter to have a specific out-of-band impedance (typically a short circuit or open circuit at the other band's center frequency). When properly designed, each filter presents an open circuit at the other's passband, allowing all signal energy to flow into the correct filter.
For a triplexer (three bands) or multiplexer (four or more), the junction design becomes increasingly complex. Manifold multiplexer designs connect all filters to a common transmission line (manifold), with each filter tuned to present the correct impedance at all other bands. ComputerOptimization is essential for multiplexer designs with four or more channels.
Frequently Asked Questions
Can I just connect two off-the-shelf filters?
Generally no. Off-the-shelf filters are designed for 50 Ω termination, not for the reactive impedance presented by another filter at the common port. The result is degraded passband performance (ripple, loss) and poor isolation. Custom-designed diplexer filters account for the mutual loading and provide optimum performance.
What is the typical isolation between diplexer ports?
Isolation depends on the frequency separation between bands and the filter selectivity. For bands separated by more than 10%: 50-70 dB isolation is achievable with moderate-order filters. For closely spaced bands (2-5% separation): 30-50 dB requires high-order filters with steep skirts.
What about channel filters in satellite payloads?
Satellite input multiplexers use manifold-coupled channel filters to split a wide transponder bandwidth into multiple channels. These are among the most demanding multiplexer designs, requiring extremely low loss, flat group delay, and precise frequency alignment across 8-24 channels simultaneously.