RF Passives › Diplexers

Diplexer Designer

Design LC diplexers for sharing a common antenna port between two bands. Computes lowpass and highpass filter component values, crossover frequency, isolation between ports, frequency response and a component table with E12 rounded values.

Lowpass Branch (Port 2 — Low band)
MHz
Highpass Branch (Port 3 — High band)
MHz
Common Parameters
Ω
Diplexer design rule: Set fLP = fHP for a contiguous diplexer (total power conservation). Set fHP > fLP for a guard band between the two bands — the gap provides extra isolation but some frequencies are unserved.
Diplexer principle:
Port 1 (common) connects to both LP and HP filter networks in parallel. At low frequencies, LP passes and HP blocks. At high frequencies, HP passes and LP blocks. At the crossover frequency, both filters present −3 dB — total power is conserved (S21²+S31²≈1).

Isolation between ports 2 and 3:
At fLP, port 3 sees LP stopband ≈ −40dB (order 5, 2×fc) and vice versa. Actual isolation = LP stopband + HP stopband at the frequency of interest.
Component Values
ComponentBranchIdeal valueE12 standardElement
Diplexer Frequency Response
LP port (S21) HP port (S31) −3 dB level
Performance Summary
Crossover & Isolation
LP −3 dB cutoff—
HP −3 dB cutoff—
Crossover frequency—
Port isolation at fLP—
Port isolation at fHP—
LP stopband at fHP—
HP stopband at fLP—
Guard band—

LC Diplexer Design Guide

A diplexer is a three-port passive network that allows two frequency bands to share a common antenna port. The lowpass port passes signals below the crossover frequency; the highpass port passes signals above it. At the crossover frequency, both ports present −3 dB insertion loss and total power is conserved. Diplexers differ from duplexers in that they split by frequency, not by direction (transmit/receive).

Diplexer vs Duplexer

A diplexer separates two frequency bands on one antenna. A duplexer separates transmit and receive paths on the same frequency (or adjacent frequencies), using tight bandpass filters or circulators. For example, a WiFi/Bluetooth diplexer splits 2.4 GHz and 5 GHz signals, while a cellular duplexer separates uplink and downlink within the same band.

Filter Order and Isolation

Higher filter order gives steeper roll-off and better port isolation, at the cost of more components and higher insertion loss in the passband. For most applications, order 3 (3 components per branch) gives adequate isolation if the two bands are well separated. Order 5 is recommended when bands are close together or when >40 dB isolation is required. The isolation between ports 2 and 3 at any frequency equals the LP stopband attenuation plus the HP stopband attenuation at that frequency.

Component Tolerances

Diplexer performance is sensitive to component tolerance, especially near the crossover frequency. Use 1% or 2% tolerance capacitors and inductors for the crossover components. Standard value (E12/E24) components will shift the crossover frequency slightly — the E12 rounded values shown in this calculator indicate the resulting frequency shift. For millimetre-wave diplexers above 10 GHz, use coupled microstrip or waveguide structures instead of lumped LC.