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Communication System

TX/RX, Single Antenna (SPDT), Dual Stage Conversion

TX/RX SINGLE ANTENNA (SPDT) — DUAL STAGE CONVERSION RECEIVE PATH ANTENNA SPDTSwitch RX LNALow Noise BPF MIXER 11st Down LO 1 IF AMP+ Filter MIXER 22nd Down LO 2 BASEBANDRX Output TRANSMIT PATH BASEBANDTX Input MIXER 31st Up DRIVERAMP MIXER 42nd Up BPF PAPower Amp TX Amplifier Mixer Filter/Switch Antenna Power Amp Baseband I/O
Component Descriptions

Signal Chain Walkthrough

This architecture replaces the analog circulator with a high-speed SPDT (Single Pole, Double Throw) switch for TX/RX antenna sharing. The system operates in time-division mode, alternating between transmit and receive.

SPDT Switch

A GaAs MMIC SPDT switch routes the antenna between the TX and RX paths. Insertion loss is typically 0.5-1.5 dB with isolation of 30-50 dB. Switching speed is 2-10 ns, enabling fast TX/RX transitions. The switch provides better isolation than a circulator at the cost of half-duplex operation.

Receive Chain

Identical to the circulator dual-stage architecture: LNA, BPF, 1st mixer (with LO1), IF amplifier/filter, 2nd mixer (with LO2), baseband output. The dual-conversion approach provides excellent image rejection and selectivity.

Transmit Chain

Baseband input is upconverted in two stages, filtered, amplified by a driver stage, and sent to the PA. The PA output feeds the SPDT switch, which routes it to the antenna during transmit intervals.

Typical Specifications

Component Specifications

ComponentParameterTypical Value
SPDT SwitchInsertion Loss0.5 - 1.5 dB
SPDT SwitchIsolation30 - 50 dB
SPDT SwitchSwitching Speed2 - 10 ns
LNANoise Figure0.5 - 3.0 dB
LNAGain15 - 25 dB
MixerConversion Loss5 - 8 dB
PAOutput Power+20 to +40 dBm
PAPAE20 - 50%
Design Note: SPDT-based architectures provide higher TX/RX isolation than circulators (30-50 dB vs. 20-30 dB) but require time-division duplexing (TDD). The switch insertion loss adds directly to both the receiver noise figure and transmitter output power budget. Choose low-loss GaAs MMIC switches for optimal performance.
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