What is the difference between a diplexer and a duplexer?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

A diplexer is a three-port frequency multiplexer that separates two bands (for example low-band and high-band) onto separate ports. A duplexer is a diplexer used to let a transmitter and a receiver share an antenna in frequency-division duplex (FDD) systems; it must provide high transmit-to-receive isolation so transmitter power and noise do not desensitize the receiver.

How is a duplexer specified?

Tx and Rx insertion loss (Tx loss costs PA efficiency; Rx loss adds directly to NF).
Tx-to-Rx isolation in the Rx band (suppressing Tx noise) and in the Tx band (preventing Rx overload).
Power handling and passive intermodulation.

How are they designed?

Two filters are joined at the common port through matching or phasing lines so that each filter looks like an open circuit in the other’s band. Lumped versions use complementary low-pass/high-pass filters; cellular handsets use acoustic duplexers; base stations use cavity duplexers.^[1]

What alternatives exist for time-division systems?

TDD systems (most Wi-Fi, 5G NR TDD bands) use a transmit/receive switch instead of a duplexer, since Tx and Rx do not occur simultaneously. Circulators can also separate Tx and Rx at the same frequency with limited isolation; see circulators.

Frequently asked questions

Is a duplexer always a filter?

In FDD systems yes; in radar, ‘duplexer’ can mean a circulator or T/R switch.

What isolation is typical?

Cellular duplexers are commonly designed for roughly 50–55 dB or more Tx-Rx isolation, depending on band.

Can a diplexer be lossless?

Ideally lossless; real ones have finite-Q losses.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Microwave Filters, Impedance-Matching Networks, and Coupling Structures”. G. Matthaei, L. Young, E. M. T. Jones, McGraw-Hill / Artech House. 1964; Artech reprint 1980.

Categories: Filters and passives

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