What is the difference between a diplexer and a duplexer?

A diplexer separates two different frequency bands and can be used with any two non-overlapping signals. A duplexer is a specific application of a diplexer used in FDD (frequency-division duplex) transceivers to allow simultaneous transmit and receive through the same antenna by separating the TX and RX frequency bands. The duplexer must handle the full transmit power while providing 50 to 55 dB of TX-to-RX isolation to prevent the transmitter from desensitizing the receiver. All duplexers are diplexers, but not all diplexers are duplexers.

How much guard band is needed between the two channels?

The guard band depends on the required isolation and the filter technology. Lumped-element LC diplexers on PCB need a guard band of 20 to 40 percent of the center frequency for 40 dB isolation. Ceramic resonator diplexers (used in cell phones) achieve 50 dB isolation with 3 to 5 percent guard band. Cavity diplexers for base stations achieve 60 to 70 dB isolation with 1 to 2 percent guard band but are large and heavy. BAW and FBAR diplexers used in modern smartphones achieve 55 dB isolation with guard bands as narrow as 2 percent at frequencies up to 6 GHz.

Can I build a diplexer from two filters connected together?

Not simply by connecting two filters to a tee junction. Each filter must present a reactive (not absorptive) termination to the common port at frequencies outside its passband. If Filter A's out-of-band impedance is not purely reactive, it loads Filter B's passband and degrades its insertion loss and return loss. Proper diplexer design requires complementary filter synthesis where the two filters are designed together so that at every frequency, the common port sees a matched impedance from one filter or the other (or both). This is why off-the-shelf filters rarely work as diplexers without redesign.

RF Design

Diplexer

One antenna, two radios on different bands. Without a diplexer, each radio would need its own antenna, or they would interfere with each other through the shared feedline. A diplexer solves this by placing two complementary filters at a common port: a lowpass (or bandpass) routes the lower band to one radio while a highpass (or second bandpass) routes the upper band to the other. At any frequency, the common port sees a matched load from whichever filter is passing that band, while the other filter presents a reactive termination that reflects energy back toward the correct path.

Category: RF Design

Ports: Common, Band 1, Band 2

Key Spec: Inter-band isolation (dB)

Complementary Filters at a Shared Junction

Diplexer Technology Comparison

Technology	Guard Band	Isolation	IL	Power	Size	Application
Lumped LC (PCB)	20 to 40%	30 to 40 dB	1 to 2 dB	10 W	10 × 10 mm	Low-cost IoT, ISM
Ceramic resonator	3 to 5%	45 to 55 dB	1.5 to 2.5 dB	2 W	5 × 3 mm	Smartphone front-end
BAW / FBAR	2 to 3%	50 to 60 dB	1.0 to 1.8 dB	1 W	2 × 2 mm	5G sub-6 handsets
Cavity (air)	1 to 2%	60 to 80 dB	0.3 to 0.8 dB	100+ W	200 × 100 mm	Base station, broadcast TX
Waveguide	1 to 3%	50 to 70 dB	0.1 to 0.3 dB	kW	Large	Satellite transponder

Why You Cannot Just Tee Two Filters Together

Common port matching requirement:
At each frequency f, the common port impedance Z_common(f) must satisfy:
Z_common = Z_A(f) || Z_B(f) ≈ Z₀

In the passband of Filter A: Z_A ≈ Z₀, Z_B must be high-impedance (reactive)
In the passband of Filter B: Z_B ≈ Z₀, Z_A must be high-impedance (reactive)

If Filter B presents a 50 Ω resistive load at Filter A's frequency, half the power is absorbed by Filter B instead of reaching the antenna. Complementary synthesis ensures each filter is reactive (reflective) outside its band.

Common Questions

Frequently Asked Questions

Diplexer vs. duplexer?

All duplexers are diplexers, but a duplexer specifically separates TX and RX bands in FDD transceivers. It must handle full transmit power while providing 50 to 55 dB TX-to-RX isolation to prevent receiver desensitization. A diplexer is the general term for any two-band frequency combiner.

How much guard band is needed?

Depends on filter technology. LC on PCB: 20 to 40% for 40 dB isolation. Ceramic: 3 to 5% for 50 dB. BAW/FBAR: 2 to 3% for 55 dB. Cavity: 1 to 2% for 70 dB. Tighter guard bands require higher-Q resonators.

Can I just connect two filters at a tee?

No. Each filter must be purely reactive (not absorptive) out-of-band, or it loads the other filter's passband. Diplexers require complementary filter synthesis where both filters are designed together to present a matched impedance at every frequency.

Related Terms

← Diode Detector Directional Coupler →

Filter Design

Diplexer Synthesis Tool

Specify your two passbands, required isolation, and technology, and generate the complementary filter schematic with component values for simulation.

Design a Diplexer