Why is TX-RX isolation so critical in full-duplex systems?

In FDD cellular systems, the transmitter and receiver operate simultaneously on different frequencies. A base station transmits at +46 dBm while its receiver tries to detect signals at minus 100 dBm. That is a 146 dB difference. If even a tiny fraction of the TX power leaks into the RX path, it overwhelms the desired signal. The duplexer filter must provide at least 50 dB of TX-to-RX isolation. Combined with the frequency separation (30 to 75 MHz depending on the band) and additional filtering, the total isolation typically reaches 80 to 100 dB. Even with 90 dB total isolation, the TX leakage at the receiver input is +46 minus 90 = minus 44 dBm, which is 56 dB above the desired signal and must be handled by the receiver's dynamic range.

How does poor isolation in a mixer affect receiver performance?

Mixer isolation prevents the LO signal from leaking to the RF port (where it would radiate from the antenna) and prevents the RF signal from reaching the LO port (where it could injection-lock or pull the synthesizer frequency). LO-to-RF leakage in a direct-conversion receiver is particularly problematic: the leaked LO reflects off the antenna impedance mismatch and re-enters the mixer, creating a DC offset that varies with antenna impedance and degrades EVM. A double-balanced mixer achieves 30 to 40 dB LO-RF isolation through symmetry cancellation; a single-ended mixer may achieve only 10 to 15 dB. For direct-conversion architectures, 40 dB minimum LO-RF isolation is typically required.

RF Design

Isolation

Q: What determines switch isolation?

Switch isolation is the attenuation of the signal path through the switch when it is in the off state. For a series FET switch, isolation is determined by the drain-source capacitance (Cds) of the transistor in the off state: the capacitance forms a leakage path that worsens with frequency. A GaAs pHEMT switch with Cds = 30 fF has about 25 dB isolation at 2 GHz but only 15 dB at 6 GHz. Stacking multiple FETs in series improves isolation (each stage adds its off-state attenuation) but increases insertion loss. A SOI CMOS switch stack of 8 FETs can achieve 40 dB isolation at 5 GHz. PIN diode switches achieve higher isolation (50 to 60 dB) because they have lower off-state capacitance per unit area.

A cellular base station transmits +46 dBm (40 watts) while its receiver simultaneously listens for signals at −100 dBm (0.01 picowatts). The power ratio between transmitter and desired receive signal is 146 dB: 10 trillion to one. If the duplexer filter separating TX from RX leaks even 0.001% of the transmit power into the receiver, the leakage (−4 dBm) drowns the desired signal by 96 dB. Isolation is the measure of how well a component prevents signals from appearing where they should not: TX leaking into RX, LO leaking onto the antenna, or an off-state switch port leaking to the output. It is measured in positive dB: higher is better.

Category: RF Design

Unit: dB (positive, higher = better)

Critical Spec: Duplexer TX-RX, Switch off-state

Isolation Requirements by Component

Component	Isolation Type	Typical Value	Mechanism	Consequence if Insufficient
Duplexer	TX-to-RX	50 to 55 dB	Filter rejection	RX desensitization, PA noise in RX band
SP4T switch (off arm)	Off-state	25 to 40 dB	FET Cds, stacking	Band leakage, spurious emission
Double-balanced mixer	LO-to-RF	30 to 40 dB	Balun symmetry	LO radiation from antenna
Circulator	Reverse (port 2→1)	20 to 25 dB	Ferrite non-reciprocity	PA sees reflected power
Diplexer	Band-to-band	40 to 60 dB	Filter skirt overlap	Intermodulation, blocking
MIMO antenna array	Element-to-element	15 to 25 dB	Spatial separation, decoupling	Correlation, capacity loss

Isolation (general):
Isolation (dB) = P_{source port} − P_{isolated port}

Switch off-state (capacitive leakage):
Isolation ≈ 1/(2πf·C_off·Z₀), in dB: 20·log(1/(2πfC_offZ₀))
C_off = 30 fF at 5 GHz in 50 Ω: Isolation = 20·log(1/(2π×5G×30f×50)) = 21.2 dB

Common Questions

Frequently Asked Questions

Why is TX-RX isolation critical in FDD?

TX at +46 dBm, RX detecting −100 dBm: 146 dB difference. Duplexer gives 50 dB. With filtering: 80 to 100 dB total. Even at 90 dB, TX leakage is −44 dBm, 56 dB above the desired signal. The receiver's dynamic range must handle it.

What determines switch isolation?

Off-state drain-source capacitance (C_ds). Leakage worsens with frequency. GaAs pHEMT (30 fF): 25 dB at 2 GHz, 15 dB at 6 GHz. Stacking FETs adds isolation but increases IL. PIN diodes: 50 to 60 dB (lower C_off).

How does mixer isolation affect the receiver?

LO-RF leakage radiates from the antenna (regulatory issue). In direct conversion, reflected LO re-enters the mixer creating DC offset. DBM: 30 to 40 dB. Single-ended: 10 to 15 dB. Direct-conversion needs ≥40 dB.

Related Terms

← Intermodulation Isotropic →

System Design

TX Leakage & Desensitization Calculator

Enter TX power, duplexer isolation, and RX sensitivity. Compute the leakage power at the LNA input and the required receiver dynamic range to handle simultaneous TX and RX.

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