How does a duplexer differ from a diplexer?

A duplexer separates Tx and Rx signals that are in nearby frequency bands (e.g., Band 1: Tx 1920-1980 MHz, Rx 2110-2170 MHz). It requires extreme isolation (>50 dB) because the Tx power (+30 to +43 dBm) is 120-150 dB above the Rx sensitivity level (-100 to -120 dBm). Even 50 dB isolation leaves the Rx seeing Tx leakage at -20 to +3 dBm, which must be further filtered. A diplexer separates two widely spaced frequency bands (e.g., 900 MHz and 2.4 GHz) and typically needs only 30-40 dB isolation because the signals are from different systems. Duplexers require much more sophisticated filters (cavity, BAW, or high-order ceramic).

What types of duplexer technology exist?

Cavity duplexers: air-cavity resonators with Q factors of 5,000-20,000. Lowest insertion loss (0.3-0.8 dB), highest power handling (>100W), best isolation (>60 dB). Large size. Used in base stations and repeaters. Ceramic duplexers: dielectric resonators in ceramic blocks. Q = 500-2,000. IL = 1-2 dB. Moderate power (1-10W). Compact. Used in small cells and infrastructure. BAW (Bulk Acoustic Wave) duplexers: thin-film resonators using acoustic waves. Q = 500-1,500. IL = 1.5-3 dB. Low power (<1W). Very compact (2x2mm). Used in smartphones and IoT. SAW duplexers: surface acoustic wave. Similar to BAW but lower frequency range. Being replaced by BAW above 2 GHz.

Why is duplexer insertion loss critical?

Duplexer IL affects both the Tx and Rx paths. On the Tx side: 1 dB of duplexer IL means the PA must produce 1 dB more power, increasing DC power consumption by approximately 25% (at typical PA efficiency). For a 20W PA, that is an extra 5W of DC power. On the Rx side: the duplexer IL adds directly to the receiver noise figure. 1 dB duplexer IL = 1 dB worse NF = 1 dB less sensitivity. This reduces the cell radius by approximately 10% in a cellular system. In a base station with 0.5 dB duplexer IL, switching to a 1.5 dB duplexer would reduce coverage area by approximately 20% and increase electricity costs. This is why base stations use large, expensive cavity duplexers.

Passive Components

Duplexer

Q: Why is duplexer insertion loss critical?

Duplexer IL affects both the Tx and Rx paths. On the Tx side: 1 dB of duplexer IL means the PA must produce 1 dB more power, increasing DC power consumption by approximately 25% (at typical PA efficiency). For a 20W PA, that is an extra 5W of DC power. On the Rx side: the duplexer IL adds directly to the receiver noise figure. 1 dB duplexer IL = 1 dB worse NF = 1 dB less sensitivity. This reduces the cell radius by approximately 10% in a cellular system. In a base station with 0.5 dB duplexer IL, switching to a 1.5 dB duplexer would reduce coverage area by approximately 20% and increase electricity costs. This is why base stations use large, expensive cavity duplexers.

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A Duplexer enables simultaneous transmit and receive on one antenna using paired bandpass filters with >50 dB Tx-to-Rx isolation. The Tx power (+30 to +43 dBm) is 120-150 dB above Rx sensitivity, making isolation critical. Technologies: cavity (base station, 0.3-0.8 dB IL), ceramic (small cell), BAW/SAW (smartphone, 1.5-3 dB IL). Essential for FDD cellular systems.

Category: Passive Components

Isolation: >50 dB

IL: 0.3-3 dB (by tech)

Understanding Duplexers

Full-duplex communication requires simultaneous transmission and reception on a single antenna. The duplexer makes this possible by routing the transmit signal to the antenna while directing the received signal from the antenna to the receiver, all with enough isolation to prevent the powerful Tx signal from overwhelming the sensitive Rx. This is one of the hardest filtering challenges in RF because the frequency bands are often only 20-80 MHz apart while the power levels differ by over 100 dB.

Duplexer Isolation Budget

Duplexer Technology Comparison

Technology	Q Factor	IL	Isolation	Power	Size	Application
Cavity	5,000-20,000	0.3-0.8 dB	>60 dB	>100W	Large	Base station
Ceramic	500-2,000	1-2 dB	>50 dB	1-10W	Medium	Small cell
BAW	500-1,500	1.5-3 dB	>50 dB	<1W	2x2mm	Smartphone
SAW	300-1,000	2-4 dB	>45 dB	<0.5W	2x2mm	IoT, <2 GHz
MEMS tunable	200-800	2-4 dB	>40 dB	<1W	3x3mm	Multi-band Rx

Key Equations

Insertion loss:
IL = −20log|S₂₁| dB

Return loss:
RL = −20log|S₁₁| dB

VSWR from Γ:
VSWR = (1+|Γ|)/(1−|Γ|)

Comparison

Technology	IL	Isolation	Size	Application
Cavity	0.3–1.0 dB	60–80 dB	Large	Base station
Ceramic	1.5–3.0 dB	50–60 dB	Medium	Handset (4G)
BAW/FBAR	1.0–2.0 dB	55–65 dB	Tiny	5G handset
SAW	2.0–3.5 dB	45–55 dB	Tiny	Low-cost
Switched	0.5–1.0 dB	30–40 dB	Small	TDD systems

Common Questions

Frequently Asked Questions

Duplexer vs. diplexer?

Duplexer: same-band Tx/Rx separation, >50 dB isolation, nearby frequencies. Diplexer: two widely spaced bands, 30-40 dB isolation. Duplexers need much more sophisticated filters because bands are close and power levels differ by 120+ dB.

Technologies?

Cavity: highest Q (5000-20000), lowest IL, largest size. Base stations. Ceramic: moderate Q, compact. Small cells. BAW: smartphone-sized (2x2mm), 1.5-3 dB IL, replacing SAW above 2 GHz. MEMS tunable for multi-band.

IL impact?

Tx: 1 dB extra = 25% more PA power. Rx: 1 dB = 1 dB worse NF = 10% less cell radius = 20% less coverage area. This is why base stations use large, expensive cavity duplexers.

Related Terms

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