// RF Theory
Diplexer, Duplexer & Circulator
Three terms that appear constantly in RF system design and are frequently confused with each other. This page explains precisely what each component does, how they relate to one another, and where each fits in a real RF system — with block diagrams and real-world examples throughout.
// The Big Picture
Three Different Problems, Three Different Solutions
All three components — diplexer, duplexer and circulator — deal with routing RF signals between multiple ports while keeping certain paths isolated. But they solve fundamentally different routing problems.
The key insight: a duplexer is a special case of a diplexer (more on this in the Relationships section), while a circulator is a completely different physical mechanism that can sometimes perform the same function as a duplexer through an entirely different principle.
One-line definitions:
Diplexer — routes signals by frequency: "low frequencies go left, high frequencies go right"
Duplexer — lets TX and RX share one antenna: "transmit goes out, receive comes in, they never interfere"
Circulator — forces one-directional flow: "signal always flows Port 1→2→3→1, never backwards"
Diplexer — routes signals by frequency: "low frequencies go left, high frequencies go right"
Duplexer — lets TX and RX share one antenna: "transmit goes out, receive comes in, they never interfere"
Circulator — forces one-directional flow: "signal always flows Port 1→2→3→1, never backwards"
// Component 1
The Diplexer
A diplexer is a three-port passive filter network that separates or combines signals by frequency. It has one common port and two frequency-selective ports. Signals at one band go to port A; signals at another band go to port B.
How a Diplexer Works
Internally, a diplexer is two filters — typically a low-pass and a high-pass filter — sharing a common port. The filters have complementary responses: what one passes, the other blocks.
The diplexer is a purely frequency-domain component. It is completely reciprocal — it works equally well in both directions. You can split signals from the common port or combine signals from the two frequency ports.
Key Diplexer Characteristics
Ports: 3 (1 common, 2 frequency-selective)Working principle: Passive LC filter — low-pass + high-pass (or two bandpass)
Isolation mechanism: Filter stopband rejection — the two paths don't overlap in frequency
Directionality: Reciprocal — works equally in both directions
Insertion loss: 0.3–1.5 dB typical
Isolation: 30–60 dB between Port A and Port B
Where Diplexers Are Used
1. Multi-band antenna sharing: A dual-band phone carries 850 MHz and 1900 MHz on one antenna. A diplexer routes each band to its respective radio front-end.
2. Dual-band WiFi: A WiFi 6 router splits 2.4 GHz and 5 GHz at the antenna port — each radio sees only its own band.
3. Cable TV: A single coax carries both downstream broadcast (54–862 MHz) and upstream internet (5–42 MHz). Diplexers separate the two at every amplifier stage.
4. Triplexers and multiplexers: Generalisations of the diplexer concept — N bandpass filters sharing a common port for N frequency bands.
2. Dual-band WiFi: A WiFi 6 router splits 2.4 GHz and 5 GHz at the antenna port — each radio sees only its own band.
3. Cable TV: A single coax carries both downstream broadcast (54–862 MHz) and upstream internet (5–42 MHz). Diplexers separate the two at every amplifier stage.
4. Triplexers and multiplexers: Generalisations of the diplexer concept — N bandpass filters sharing a common port for N frequency bands.
Real Example — Dual-Band WiFi Router
A WiFi 6 router supports 2.4 GHz and 5 GHz on a single antenna. A diplexer with crossover at 3.5 GHz is fitted at the antenna port.
Common port: antenna
Port A (low): 2.4 GHz radio chip
Port B (high): 5 GHz radio chip
Without the diplexer, each radio's transmitter output would leak into the other radio's receiver — the 2.4 GHz receiver would hear the 5 GHz transmitter at full power.
Common port: antenna
Port A (low): 2.4 GHz radio chip
Port B (high): 5 GHz radio chip
Without the diplexer, each radio's transmitter output would leak into the other radio's receiver — the 2.4 GHz receiver would hear the 5 GHz transmitter at full power.
// Component 2
The Duplexer
A duplexer is a three-port network that allows a transmitter and a receiver to share a single antenna simultaneously, while keeping the transmitter output from desensitising the receiver. It makes full-duplex radio possible with one antenna.
How a Duplexer Works
A duplexer is essentially a diplexer where the two filtered bands are specifically the TX frequency and the RX frequency. Port A connects to the transmitter, Port B to the receiver, and the common port to the antenna.
The critical specification is TX-to-RX isolation. The transmitter at +23 dBm must be prevented from reaching the receiver, which detects signals as weak as −100 dBm. Modern BAW duplexers achieve 50–60 dB isolation in a component smaller than a fingernail.
Critical Duplexer Specifications
TX insertion loss: 0.5–1.5 dB (power lost TX to antenna)RX insertion loss: 0.5–1.5 dB (signal lost antenna to LNA)
TX-to-RX isolation: ≥ 50 dB (LTE Band 5: 45 MHz duplex spacing)
TX power handling: +23 dBm (handset), +37 dBm (base station)
Technology: SAW, BAW (Bulk Acoustic Wave), or LC cavity resonator
Where Duplexers Are Used
1. Every FDD cellular handset: GSM, WCDMA, LTE, 5G NR FDD — each band requires a duplexer. A 5G phone with 15+ bands has 15+ duplexers.
2. FDD base stations: Cavity resonator duplexers handle 20–200 W TX while protecting the LNA. Physically large — the size of a shoebox.
3. Amateur radio repeaters: Transmit and receive on different frequencies simultaneously, sharing one antenna tower.
2. FDD base stations: Cavity resonator duplexers handle 20–200 W TX while protecting the LNA. Physically large — the size of a shoebox.
3. Amateur radio repeaters: Transmit and receive on different frequencies simultaneously, sharing one antenna tower.
TDD vs FDD: TDD systems (WiFi, 5G NR TDD, LTE TDD) use the SAME frequency for TX and RX, alternating in time. TDD does NOT need a duplexer — it uses a T/R switch instead. The switch connects the antenna to either the PA or the LNA, never both simultaneously.
LTE Band 5 — Isolation Requirement
TX: 824–849 MHz at +23 dBm (200 mW)
RX sensitivity: −100 dBm (10 femtowatts)
Duplex spacing: 45 MHz
Required total isolation: 23 − (−100) = 123 dB
A BAW duplexer provides ∼50 dB alone. The TX and RX filters are in series for the TX→RX leakage path, giving ≈100 dB combined. Receiver design accounts for the remaining margin through LNA compression thresholds and sensitivity specifications.
RX sensitivity: −100 dBm (10 femtowatts)
Duplex spacing: 45 MHz
Required total isolation: 23 − (−100) = 123 dB
A BAW duplexer provides ∼50 dB alone. The TX and RX filters are in series for the TX→RX leakage path, giving ≈100 dB combined. Receiver design accounts for the remaining margin through LNA compression thresholds and sensitivity specifications.
// Component 3
The Circulator
A circulator is a three-port passive device that forces RF signals to travel in only one direction around a ring. A signal at Port 1 exits at Port 2. A signal at Port 2 exits at Port 3. A signal at Port 3 exits at Port 1. No signal travels in reverse — the device is nonreciprocal.
How a Circulator Works — Ferrite Nonreciprocity
The circulator exploits the Faraday rotation effect in ferrite materials. A ferrite placed in a DC magnetic field becomes anisotropic — its permeability is direction-dependent. An RF wave travelling one way through the ferrite experiences a different phase shift than a wave travelling the opposite way. By carefully choosing the geometry, the device is made so that forward waves add constructively at the output port while reverse waves cancel.
Circulator Signal Flow
1 → 2: signal passes (insertion loss 0.2–0.8 dB)2 → 3: signal passes (insertion loss 0.2–0.8 dB)
3 → 1: signal passes (insertion loss 0.2–0.8 dB)
Reverse (1→3, 2→1, 3→2): blocked — 20–35 dB isolation
Key property: S₁₂ ≠ S₂₁ — the device is NONRECIPROCAL
Physical mechanism: Ferrite ceramic + permanent magnet bias
The Circulator as an Isolator
Terminate Port 3 with a 50 Ω resistor and the circulator becomes a two-port isolator: signals flow freely 1→2, but signals entering Port 2 are routed to Port 3 (the termination) and absorbed — they never reach Port 1. Used to protect oscillators and power amplifiers from reflections.
Where Circulators Are Used
1. PA protection (isolator): Reflected power from a mismatched antenna is routed to a 50 Ω dummy load instead of back into the PA transistors — preventing failure.
2. Radar duplexer: The transmit pulse goes Port 1→2 (to antenna). Received echoes go Port 2→3 (to receiver). TX never reaches RX. Works at ANY frequency within the band — unlike a filter duplexer that is narrowband.
3. VNA: Circulators separate the incident and reflected waves at each port, enabling S-parameter measurement.
4. Balanced amplifier: Protects hybrid couplers from reflected power during amplifier failure.
2. Radar duplexer: The transmit pulse goes Port 1→2 (to antenna). Received echoes go Port 2→3 (to receiver). TX never reaches RX. Works at ANY frequency within the band — unlike a filter duplexer that is narrowband.
3. VNA: Circulators separate the incident and reflected waves at each port, enabling S-parameter measurement.
4. Balanced amplifier: Protects hybrid couplers from reflected power during amplifier failure.
Circulator limitations: Requires a ferrite material biased by a permanent magnet — making it bulky, expensive ($5–$500+), temperature-sensitive, and impossible to integrate into CMOS. At handset frequencies, BAW duplexer filters are preferred. Circulators dominate in military radar, satellite and high-power industrial RF where wideband, direction-agnostic isolation is essential and size/cost are secondary.
// The Key Question
Relationships — Is One a Subset of Another?
Is a Duplexer a Subset of a Diplexer?
Yes — a duplexer is a special-purpose diplexer. Every duplexer is a diplexer, but not every diplexer is a duplexer.
// The Subset Relationship
A diplexer is the general class: any three-port frequency-routing device. A duplexer is a diplexer with a specific purpose — its two ports are assigned to a transmitter and a receiver, and it is specified for the TX-to-RX isolation required to protect the receiver from the transmitter.
Analogy: All squares are rectangles but not all rectangles are squares. All duplexers are diplexers but not all diplexers are duplexers.
Analogy: All squares are rectangles but not all rectangles are squares. All duplexers are diplexers but not all diplexers are duplexers.
Can You Build a Duplexer from a Diplexer?
Yes — assign the low-band port to TX and the high-band port to RX. If the diplexer's isolation and power handling meet the duplexer requirements, it functions as a duplexer. In fact, a duplexer IS a diplexer specified and marketed for the TX/RX sharing purpose. The design optimisation differs: duplexers additionally optimise TX power handling, RX noise contribution, and maximum TX-to-RX isolation.
Is a Circulator a Duplexer? Can It Function as One?
A circulator is not a duplexer by definition — it operates on a completely different principle (ferrite nonreciprocity rather than frequency-selective filtering). However, a circulator can functionally perform duplexer duties.
// Circulator Wired as a Duplexer
Port 1 → Transmitter Port 2 → Antenna Port 3 → Receiver (LNA)
TX signal (1→2) reaches the antenna cleanly. Received signals (antenna → Port 2, exits Port 3) reach the LNA cleanly. TX signal never reaches the LNA (1→3 is blocked). This is exactly duplexer behaviour.
Key advantage: The circulator's isolation is NOT frequency-dependent — it works simultaneously at any frequency within its operating band. A filter duplexer only works at its designed TX/RX frequencies. This makes circulators the natural choice for wideband or frequency-agile systems (radar, EW).
Key limitation: Circulators provide only 20–35 dB isolation. Cellular FDD needs 50–70 dB. Insufficient alone — a circulator plus an additional bandpass filter can reach 55 dB, but this combination is larger and costlier than a single BAW duplexer.
TX signal (1→2) reaches the antenna cleanly. Received signals (antenna → Port 2, exits Port 3) reach the LNA cleanly. TX signal never reaches the LNA (1→3 is blocked). This is exactly duplexer behaviour.
Key advantage: The circulator's isolation is NOT frequency-dependent — it works simultaneously at any frequency within its operating band. A filter duplexer only works at its designed TX/RX frequencies. This makes circulators the natural choice for wideband or frequency-agile systems (radar, EW).
Key limitation: Circulators provide only 20–35 dB isolation. Cellular FDD needs 50–70 dB. Insufficient alone — a circulator plus an additional bandpass filter can reach 55 dB, but this combination is larger and costlier than a single BAW duplexer.
Relationship summary:
Duplexer ⊂ Diplexer — duplexer is a special case of diplexer
Circulator ≠ Diplexer — completely different physical category
Circulator can function as a duplexer in wideband/radar applications
Circulator ≠ Duplexer — different physics, different tradeoffs, not interchangeable in general
Duplexer ⊂ Diplexer — duplexer is a special case of diplexer
Circulator ≠ Diplexer — completely different physical category
Circulator can function as a duplexer in wideband/radar applications
Circulator ≠ Duplexer — different physics, different tradeoffs, not interchangeable in general
// Side by Side
Full Comparison Table
| Property | Diplexer | Duplexer | Circulator |
|---|---|---|---|
| Ports | 3 | 3 | 3 (standard) |
| Separation principle | Frequency (filter) | Frequency (filter) | Direction (nonreciprocity) |
| Physical mechanism | LC resonators, SAW, BAW | LC, SAW, BAW, cavity | Ferrite + permanent magnet |
| Reciprocal? | Yes — bidirectional | Yes — bidirectional | No — one direction only |
| Frequency-dependent? | Yes — strongly | Yes — strongly | No — any freq in band |
| Typical isolation | 30–60 dB port-to-port | 50–70 dB TX-to-RX | 20–35 dB reverse |
| Insertion loss | 0.3–1.0 dB | 0.5–1.5 dB | 0.2–0.8 dB |
| Power handling | Low–medium (up to 5 W) | High (up to 200 W cavity) | Very high (kW possible) |
| Size | Very small (SMT, mm) | Small to large | Medium–large (magnet) |
| Cost | Low ($0.50–$5) | Medium ($2–$50) | High ($5–$500+) |
| CMOS integration? | Partial | No | No |
| Typical use | Multi-band sharing, cable TV | FDD cellular, repeaters | Radar, PA protection, VNA |
| Subset of? | General class | Subset of diplexer | Independent category |
// Real-World Context
All Three in One System
A modern cellular base station uses all three components simultaneously — each solving a different problem in the same signal chain.
① Diplexer: The single antenna feed splits into Band 1 (2100 MHz) and Band 3 (1800 MHz) paths. Both bands share the same antenna without interfering.
② Duplexers: Each band's path splits into TX and RX simultaneously. The Band 1 duplexer routes uplink signals (from phones) to the LNA and downlink signals (to phones) from the PA — simultaneously on the same frequency band.
③ Circulators (inside PA isolator blocks): If the antenna presents a poor impedance due to ice, nearby metal or a cable fault, reflected power travels back toward the PA. The circulator routes this to a 50 Ω dummy load — preventing catastrophic transistor failure.
② Duplexers: Each band's path splits into TX and RX simultaneously. The Band 1 duplexer routes uplink signals (from phones) to the LNA and downlink signals (to phones) from the PA — simultaneously on the same frequency band.
③ Circulators (inside PA isolator blocks): If the antenna presents a poor impedance due to ice, nearby metal or a cable fault, reflected power travels back toward the PA. The circulator routes this to a 50 Ω dummy load — preventing catastrophic transistor failure.
// Quick Reference
Summary — Three Questions
Q: Is a duplexer a type of diplexer?
Yes. A duplexer is a diplexer specifically assigned to TX and RX frequencies, with a TX-to-RX isolation spec to protect the receiver. All duplexers are diplexers. Not all diplexers are duplexers.
Yes. A duplexer is a diplexer specifically assigned to TX and RX frequencies, with a TX-to-RX isolation spec to protect the receiver. All duplexers are diplexers. Not all diplexers are duplexers.
Q: Is a circulator a type of diplexer or duplexer?
No — different physics entirely (ferrite nonreciprocity vs frequency-selective filtering). However, a circulator can functionally perform duplexer duties, especially in wideband and radar applications where filter-based duplexers won't work across the full band.
No — different physics entirely (ferrite nonreciprocity vs frequency-selective filtering). However, a circulator can functionally perform duplexer duties, especially in wideband and radar applications where filter-based duplexers won't work across the full band.
Q: When do you use each?
| Situation | Use | Reason |
|---|---|---|
| Multi-band antenna sharing (different frequencies) | Diplexer | Frequency routing only needed |
| FDD cellular — TX and RX at fixed different frequencies | Duplexer | High isolation, small, cheap |
| TDD — TX and RX on same frequency, time-alternating | T/R switch | No frequency difference to exploit |
| Radar — TX and RX on same or wideband frequency | Circulator | Frequency-agnostic isolation |
| PA protection from antenna mismatch reflections | Circulator (isolator) | Routes reflected power to dummy load |
| Wideband / frequency-hopping duplex (SDR, EW) | Circulator | Works across full band simultaneously |
| Very high power TX with high isolation | Cavity duplexer or circulator+filter | Both handle high power |