duplexer
A duplexer is a component that allows a radio device to transmit and receive signals simultaneously using a single shared antenna, by separating the outgoing and incoming signals so the strong transmitted signal doesn't overwhelm the receiver trying to pick up a much weaker incoming one.
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**Duplexers exist because a radio device transmitting and receiving at the same time over a shared antenna faces a fundamental problem, its own outgoing transmit signal is enormously stronger than the weak incoming signal its receiver is trying to detect, and without separation the transmitter would simply overwhelm the receiver.** Since the transmit and receive signals typically differ in frequency or occur at different moments in time, a duplexer exploits that difference to route the strong outgoing signal to the antenna for transmission while simultaneously routing the much weaker incoming signal from the antenna to the receiver, keeping the transmitter's overwhelming power from swamping the sensitive receiver circuitry.
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| Aspect | Separate TX/RX antennas | Duplexer with shared antenna |
|---|---|---|
| Number of antennas needed | Two | One |
| Simultaneous transmit and receive | Naturally possible | Enabled through signal separation |
| Physical device size and cost | Higher, extra antenna | Lower, one antenna shared |
| Common use | Space-unconstrained systems | Compact devices like cellular handsets |
**Duplexers commonly rely on frequency-division duplexing, using separate frequency bands for transmit and receive, or time-division duplexing, using the same frequency but alternating rapidly between transmit and receive time slots, as their separation mechanism.** Because frequency-division duplexing keeps transmit and receive signals apart by giving each a dedicated frequency band, while time-division duplexing keeps them apart by giving each a dedicated moment in time on a shared frequency, different wireless standards choose between these two duplexing approaches, and duplexer implementation details differ correspondingly based on which approach a given standard uses.
**Duplexers used in frequency-division duplexing systems rely on precise filtering to isolate the transmit and receive frequency bands, since even a small amount of transmit signal leaking into the receive path can significantly degrade receiver performance given the large power difference involved.** Because the transmit signal can be many orders of magnitude stronger than the receive signal the duplexer needs to protect, frequency-division duplexers use carefully designed filters providing very high isolation between the transmit and receive frequency bands, since even tiny amounts of transmit signal leakage can overwhelm the comparatively minuscule receive signal.
**Duplexers are a standard component in modern cellular handsets and other compact wireless devices, since these devices need to support simultaneous transmission and reception while keeping the number of antennas and overall device size as small as possible.** Because cellular communication generally requires a device to be able to transmit and receive at essentially the same time while also staying compact and cost-effective, duplexers have become an essential, standard component enabling that simultaneous full-duplex operation through a single shared antenna in the vast majority of modern cellular devices.
Read the duplexer through a two-way-street lens: much like a well-designed two-way street lets traffic flow in both directions on the same road without collisions by keeping each direction in its own lane, a duplexer lets transmit and receive signals share the same antenna without the strong outgoing traffic overwhelming the delicate incoming traffic.