What Is a Diplexer?
A diplexer is a 3-port frequency-selective device that combines or separates signals in two different frequency bands on a common antenna port. Unlike a duplexer (which separates TX and RX for the same frequency band), a diplexer separates signals by frequency, such as routing 2.4 GHz Wi-Fi and 5 GHz Wi-Fi to a single antenna.
Diplexer vs. Duplexer vs. Multiplexer
| Device | Ports | Separation Method | Application |
|---|---|---|---|
| Diplexer | 3 | Frequency (LPF + HPF) | Dual-band antenna sharing |
| Duplexer | 3 | Frequency (BPF TX + BPF RX) | FDD TX/RX in same band |
| Triplexer | 4 | Frequency (3 BPFs) | 3 frequency bands on one antenna |
| Multiplexer | N+1 | Frequency (N BPFs) | N band combining |
Diplexer Architecture
A diplexer typically consists of a low-pass filter (LPF) path and a high-pass filter (HPF) path both connected to the common (antenna) port. The junction must present the correct impedance to both filters simultaneously. The design challenge is creating a junction network that allows each filter to see its proper termination impedance without being loaded by the other filter.
Key S-Parameter Specs
| Parameter | Definition | Typical Spec |
|---|---|---|
| Insertion loss (each path) | S21 or S31 in respective passband | 0.5–2 dB |
| Port isolation | S23 between two frequency ports | >25 dB |
| Common port match (S11) | Antenna port return loss | >15 dB in both bands |
| Stopband rejection | S21 of LPF path at HPF frequencies | >30 dB |
| Transition bandwidth | Frequency gap between bands | 10–20% of center freq |
Verifying a Diplexer with RF View
Load the diplexer's .s3p file in RF View:
- S21 (port1 to port2, LPF path): Low insertion loss in lower band, high rejection in upper band
- S31 (port1 to port3, HPF path): Low insertion loss in upper band, high rejection in lower band
- S11 (common port): Good match (<−15 dB) at both band frequencies
- S23 (between frequency ports): High isolation (>25 dB) — confirms the two bands don't interfere
The S21 + S31 sum at any frequency should be close to 0 dB (all power goes to one port or the other), minus insertion loss.