Technical Brief SWRA030 Understanding and Enhancing Sensitivity in Receivers for Wireless Applications Edited by Matt Loy Wireless Communication Business Unit Abstract This technical brief provides an overview of communication receiver sensitivity. One of the most important parameters in determining the overall performance of a communication system, receiver sensitivity translates directly into communication distance and reliability. A few receiver architectures can offer dependable communication at low cost, if proper design procedures and trade-offs are implemented. RF amplifiers, mixers, and filters are common circuit building blocks for every architecture. System performance is tied to each individual block comprising the receiver. Each circuit generates noise that degrades reception of the desired signal. Understanding noise sources and the methods of minimizing degradation allows optimal design trade-offs for a given cost. Circuit nonlinearity causes undesired signals to hinder the reception of desired signals. A low-noise system design typically does not produce the best linearity, and high linearity typically produces more noise. A thorough understanding of the receiver RF environment can help your design achieve the proper specifications for optimal noise and linearity. Contents Introduction............................................................................................................................................... 4 Receiver Architectures .............................................................................................................................. 4 Homodyne (Zero IF) Receiver......................................................................................................... 4 Heterodyne Receiver...................................................................................................................... 6 Access Methods and Modulation Schemes ................................................................................................ 7 Access Methods............................................................................................................................. 7 Modulation Schemes.................................................................................................................... 10 Receiver Building Blocks......................................................................................................................... 16 Antenna....................................................................................................................................... 17 Duplexers .................................................................................................................................... 18 Low-Noise Amplifier (LNA) ........................................................................................................... 19 RF Filters..................................................................................................................................... 19 Mixers.......................................................................................................................................... 20 Local Oscillator ............................................................................................................................ 22 IF Filter ........................................................................................................................................ 23 IF Amplifier .................................................................................................................................. 23 Detectors ..................................................................................................................................... 24 Receiver Sensitivity................................................................................................................................. 24 Noise of Two-Port Networks ......................................................................................................... 25 Noise Temperature....................................................................................................................... 29 Digital Signal Processing Solutions May 1999 Noise Figure ................................................................................................................................ 32 Cascaded Noise Temperature ......................................................................................................37 Cascaded Noise Figure................................................................................................................ 40 Lossy Two-Port Noise Figure........................................................................................................ 42 Mixer Noise Figure....................................................................................................................... 44 Minimum Detectable Signal .......................................................................................................... 46 Signal-to-Noise Ratio ................................................................................................................... 48 Sensitivity .................................................................................................................................... 50 Phase Noise ................................................................................................................................ 50 Receiver Nonlinear Performance ............................................................................................................. 51 Gain Compression ....................................................................................................................... 51 Intermodulation Distortion............................................................................................................. 54 Dynamic Range ........................................................................................................................... 60 Blocking....................................................................................................................................... 62 Spur-Free Dynamic Range ........................................................................................................... 63 Undesired Spurious Responses.................................................................................................... 64 Self-Quieters................................................................................................................................ 67 Receiver Design Trade-Offs .................................................................................................................... 67 Summary................................................................................................................................................ 70 Glossary ................................................................................................................................................. 70 References ............................................................................................................................................. 74 Figures Figure 1. Homodyne (Zero-IF) Receiver............................................................................................... 5 Figure 2. Dual-Conversion Superheterodyne Receiver ......................................................................... 6 Figure 3. User Time Slots for TDMA Example ...................................................................................... 8 Figure 4. Frequency Channels for FDMA Example............................................................................... 8 Figure 5. Modulation Schemes .......................................................................................................... 11 Figure 6. Constellation Diagram (I/Q Plot) .......................................................................................... 12 Figure 7. OQPSK State Diagram ....................................................................................................... 13 Figure 8. π/4-DQPSK State Diagram.................................................................................................. 14 Figure 9. 16QAM State Diagram........................................................................................................ 15 Figure 10. Receiver Components ........................................................................................................ 16 Figure 11. Mixer Driven by LO Signal .................................................................................................. 21 Figure 12. I/Q Detector........................................................................................................................ 24 Figure 13. Complex Source Connected to Complex Conjugate Load Termination ................................. 25 Figure 14. Thermal Noise Visualized as Signal Generators .................................................................. 26 Figure 15. Filter Response Equated to Brick-Wall Filter Response........................................................ 27 Figure 16. Two-Port Network Connection for Available Gain Definition.................................................. 29 Figure 17. Available Thermal Noise (Noise Temperature)..................................................................... 30 Figure 18. Noise Output Power Versus Source Temperature (Noise Temperature) ..............................