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Lecture 2: RF Issues for Software Radios RF Engineering for the DSP Engineer. TOPICS RF Receiver Chain RF Transmitter Chain A Quantitative perspective of noise and distortion Overcoming RF limitations with DSP. What You’ll Learn. Role of RF in SDR SDR RF Structures

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Lecture 2: RF Issues for Software Radios RF Engineering for the DSP Engineer


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    1. Lecture 2:RF Issues for Software RadiosRF Engineering for the DSP Engineer TOPICS RF Receiver Chain RF Transmitter Chain A Quantitative perspective of noise and distortion Overcoming RF limitations with DSP

    2. What You’ll Learn • Role of RF in SDR • SDR RF Structures • Common Performance Metrics • SDR Amplifiers and Overcoming RF Problems with DSP • Impact of MEMs

    3. Role of RF in SDR • Why are we focusing first on Hardware for a Software radio? • Radio may be defined digitally, but the real world is analog.

    4. Generic Transmitter Antenna Digital Input Digital to Selection and Conversion Analog Amplification Conversion transmitter section

    5. Generic Receiver Antenna Digital Output Analog - to - Selection Conversion Digital Conversion RF Front End

    6. Purpose for the RF • Extract a desired low level signal: 10-16 to 10-3 Watts. • Reject out of band noise and interference • Convert signal’s center frequency to a range compatible with the A/D. • Modulate, amplify, and filter signal for transmission. • Minimize additive noise and distortion

    7. Goals of RF in SDR (1/3) • Support MBMMR – Multi-band, multimode radio • Operate over numerous bandwidths, frequencies, waveforms • Implies wideband operation, but… • Produce highest quality signal for baseband processing • Reject out-of-band noise and interference • Implies narrowband operation…

    8. Goals of RF in SDR (2/3) • Facilitate recovery of weak signals in presence of strong interferers • Wide Dynamic Range • Implies High Quality Components (Means high cost) • Practical Considerations • Low Cost • Low Power • Small Form Factors

    9. Goals of RF in SDR (3/3) • Have your cake and eat it too… • However, complementing hardware with software changes the RF “cake” equation

    10. RF View of Radio Systems (1) • Antenna Design • Receiver Design • Topology of Receivers • Component Issues • Special capabilities • Transmitter Design • Topology of Transmitters • Component Issues • Special capabilities

    11. RF View of Radio System (2) • Noise and Distortion Characterization • Noise • Non-linear distortion • Compensation for distortion • Power Supply and Consumption Considerations

    12. Key Antenna Issues • Most antennas support bandwidths on the order of 10% of the carrier frequency, • Multimode radios 900 MHz to 2 GHz are difficult to support with the same antenna • Impendence, hence matching can vary with the environment • Form factor is important for handsets

    13. Key Antenna Design Issues (2) • Gain versus directionality trade-off • Sensitivity to coverings (e.g., hand) • Diversity design -- multiple antennas and smart antenna algorithms • Radiating head

    14. Key Receiver Parameters • Sensitivity: • Defines the weakest signal that a receiver can detect and is usually determined by the various noise sources in the receiving system. • Selectivity: • The ability of the receiver to detect the desired signal and reject others. • Spurious Response: • The spurious response is a receiver’s freedom from interference due to these internally generated signals or their interaction with external signals. • Stability: • Receiver gain & frequency change with temperature, time, voltage, etc.

    15. Characteristics that Determine Suitability of Receiver Topologies (2) • Dynamic Range: The difference in power between the weakest signal that the receiver can detect and the strongest signal that can be supported (either in band or out of band) by the receiver without detrimental effects

    16. Dynamic Range Constraints Spurious Signal Limited Noise Limited Bit Error Rate (BER) Usable Dynamic Range Received Signal Power

    17. Factors Impacting Dynamic Range A/D Converter Constraints Range Selected Dynamic Range AGC and Power Control Analog Front End Constrains

    18. Various Types of Receivers- Tuned Radio Frequency Receiver Input signal level may span 100 dB in dynamic range To A/D Converter LNA BPF AGC RX filter Not very practical

    19. Tuned Radio Frequency Receiver Disadvantages • Not as practical • Extreme demands on RF and A/D • Tunable very narrowband filters • Dynamic range of LNA • Advantages • Few analog components • Isolation problems minimized

    20. Direct Conversion Receiver I Input signal level may span 100 dB in dynamic range LNA BPF BPF AGC LO ADC 90o RX filter RX filter Q • Advantages • Fewer parts • Analog Images Eliminated • Conceptually simple • Possibly lower power consumption • Disadvantages • High isolation needed in mixer between LO and LNA input • eg, signal = -116 dBm, LO = 5dBm, thus isolation >> 120 dB • Phase noise of LO critical • DC offset at A/D substantial and dynamic • Balanced mixers needed • Second order distortions occur in band

    21. Direct Conversion Transmitter Architecture Spurs typically 60 dB down or more I Binary Data Source Frequency Source (DDS or Programmable VCO) Power Amplifier BPF BPF Q I/Q LO source RF VCO • Advantage • Conceptually simple filter requirements • Low complexity • Circumvents image problems • Disadvantage • Not as practical because of isolation problems • Balance in I&Q • Consistent performance over wide band

    22. Multiple Conversion Receiver I LNA BPF BPF LO BPF AGC BPF 90o Image filter Image filter RX filter Q IF LO IF LO • Advantages • More isolation than direct conversion or single conversion (due to distributing gain into to sections) • Better rejection of ACI • Better gain possible through distributed amplificaiton • Disadvantages • More parts • More power consumption? Multiple conversion possible, but not as common

    23. IF and Low -IF Conversion Receiver I ADC LNA BPF LO BPF AGC BPF 90o Image filter RX filter Q IF LO • Advantages • More isolation than direct conversion – less DC offset • Lower parts count than dual conversion Analog Digital • Disadvantages • Gain is limited • More image rejection required over direct conversion

    24. Review of the Mixing Process Amplitude Desired signal Adjacent channel interference -  136MHz 136MHz -d -1 1 d Amplitude Desired signal downconverted Adjacent channel interference upconverted by the mixer Desired signal corrupted by interference -  -1-68=-wd+68 1+68=wd-68 -d-68 -1+68 1-68 d+68

    25. Two Stage Transmitter BPF Power Amplifier I Binary Data Source Frequency Source (DDS or programmable VCO) BPF BPF Q RF VCO RF VCO • Advantage • Better isolation • Disadvantages • More parts and higher cost • Higher power consumption

    26. Transmitter Component Issues (1/2) • Transmit IF VCO • Phase Noise Provides Significant Modulation to Narrowband Signals • Up-Converter • Linearity to Reduce Spurious Products • Modulator • Balance Between I&Q Required to Keep Distortion (Sidebands) Down • Variable Gain Amplifier • Linearity and Fidelity

    27. Transmitter Component Issues (2/2) • Transmit Filters • Must Prevent PA Transmitter Noise Leakage (supplement duplexer) • Low Loss required • Power Amplifiers • Cost - especially for base stations • Spurious response (source of interference) • Packing to handle heat • Low distortion traded for power efficiency traded for bandwidth (in practice only about 25% of the battery is effectively used during the talk time

    28. Key Receiver Component Issues (1/2) • Duplexers • full duplex, e.g., AMPS is difficult and expensive • half duplex, e.g., GSM still some difficulty in integration • duplexers that work both for TDMA and FDMA • Low noise amplifier (LNA) • trade off in gain, noise, power consumption, and dynamic range (noise figure is ratio of output SNR to input SNR) • low power consumption needed

    29. Key Receiver Component Issues (2/2) • RX filter • Initial BPF after antenna • rejects out of band interference • helps isolate of the tx and rx • Image Reject BPF before mixer • protects mixer from interference • suppresses spurious signals generated by mixer impacting LNA • RF Mixer • Spurious response • LO drive level • too high -- power consumption issue • too low -- more harmonic distortion • Isolation between RF, IF, and LO ports limits post mixing harmonics • Harmonics due to mixer and LNA non-linearities may end up in the IF pass band after mixing

    30. Impact on Constellation Due to Imperfect Mixing

    31. Key Receiver Design Issues: AGC (1) • Intermediate Frequency (IF) filter sets noise bandwidth of the Receiver • Implementation impacted by cost, signal loss, and adjacent channel rejection

    32. Key Receiver Design Issues: AGC (2) • Automatic Gain Control (AGC) • Placement for minimal noise (after IF for constant noise figure) • Large dynamic range to match the A/D dynamic range • Response time of AGC loop is critical for min. distortion and maximum dynamic range

    33. Digital AGC To Software Receiver Input Signal A/D Converter Amp Gain Control Energy Detector Slew Gain Factor Mapping D/A Converter Inactive Mode Selector  + Tracking - Reference Level

    34. AGC Modes Reference Level Slew Mode Slew Mode Low amplitude level High amplitude level Input Signal Level Tracking Mode Tracking Mode AGC Inactive Zone

    35. Key Transmitter RF Design Issues • Power Efficiency • Modulation Accuracy and Linearity • Spurious Signal Reduction • SNR of Transmitted Signal • Power Control Performance • Output Power Level

    36. Transmitter Component Issues: Ocsillator & Mixer • Transmit IF VCO • noise floor • power consumption • phase noise provides significant modulation to narrowband signals • Up-Converter • Linearity to reduce spurious products • Noise floor • Power consumption

    37. Key Transmitter Component Issues: Modulation • Modulator • balance between I&Q required to keep distortion (sidebands) down • Noise figure • Power consumption • Variable Gain Amplifier • Linearity and fidelity • Noise figure

    38. Transmitter Component Issues: Transmit Filters • Transmit Filters • Isolation of transmitter noise from PA leaking into the receiver (supplement duplexer) • low loss required

    39. Transmitter Component Issues: Power Amplifier (1) • Power Amplifier (very critical) • Cost - especially for base stations • Noise floor • Spurious response (source of interference)

    40. Transmitter Component Issues: Power Amplifier (2) • Packing to handle heat • Low distortion traded for power efficiency traded for bandwidth (in practice only about 25% of the battery is effectively used during the talk time)

    41. General Performance Metrics • Noise Characterization and Figure • Spurious Free Dynamic Range • Blocking Dynamic Range • Intermod • Power Consumption

    42. Noise Characterization (1) • Noise is introduced into resistive components due to thermal actions. • where k is Boltzman’s constant (1.38.10-23 J/K), T is the temperature in Kelvin, R is component resistance (in ohms), and B is the bandwidth in Hz.

    43. Noise Characterization (2) • Antenna is the first and the base line source of noise for which other noise sources are compared. • Thermal noise and quantization noise introduced by the A/D

    44. Noise Figure • Noise Figure (NF) measure the amount of noise an element (or elements) adds to a signal. NF = SNRin/SNRout where SNRin is the input SNRout and is the device output SNR. • Active Components The manufacturer of a device usually supplies a noise figures for equals the loss of the passive components.

    45. Using the Noise Figure (1/2) • It is possible to provide an equivalent system wide noise figure NFtotal that relates the noise back to the antenna. (equation 1) • Here NFi represents the noise figure at the ith stage and Gi represents the gain at the ith stage (units are linear).

    46. Using Noise Figure (2/2) • Given a component with a noisy input having noise power Pi-1 (dBm), gain Gi (dB) and noise figure NFi (dB) the output noise power Pi(dBm)is given by Pi (dBm) = Pi-1 (dbm) + NFi(dB) + G (dB) Units are linear unless proceeded by (dB) or (dBm).

    47. Example NF Calculations (1/2) NF3 =2 dB G3=10 dB NF2=2 dB G2=-2dB NF4=6 dB To Next IF Chain Cable, G1=-3dB From Anttenna BPF X LNA LO The total noise figure equals 5.975 .

    48. Example Noise Calculations (2/2) Does ordering of the components yield optimal NF? NF3 =2 dB G3=10 dB NF2=2 dB G2=-2dB • the total noise figure equals 3.6. • In the system, the LNA has the biggest impact on the noise figure (because of its high gain) • In general, it best to have higher gain components (like the LNA) located as early as possible in the RF chain. NF4=6 dB To Next IF Chain Cable, G1=-3dB From Anttenna BPF X LNA LO

    49. Calculating Sensitivity (1) • Sensitivity of the receiver to achieve a minimal signal-to-noise ratio SNRmin is defined as S dBm = Noise floor dBm + SNRmin dB where Noise floor dBm = 10 log (kTB) + NFtotal dB = 10 log(kT) dB + NFtotal dB + 10 log(B) dB and B is the end of system bandwidth and NF is the overall system noise figure.

    50. Calculating Sensitivity (2) • For room temperature, the sensitivity becomes S dBm = -174 dBm/Hz + NF dB + 10 log(B) + SNRmin • A good conservative practice keeps the noise floor due to analog components lower than the noise introduced by the A/D converter.