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Software Defined Radio Lec 3 – RF Front-End for SDR

Software Defined Radio Lec 3 – RF Front-End for SDR . Sajjad Hussain, MCS-NUST. Outline for Today’s Lecture. RF implementation issues Purpose of RF Front End Dynamic Range RF Receiver Front-End Topologies Enhanced Flexibility of RF Front-End with SDRs

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Software Defined Radio Lec 3 – RF Front-End for SDR

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  1. Software Defined Radio Lec 3 – RF Front-End for SDR Sajjad Hussain, MCS-NUST

  2. Outline for Today’s Lecture • RF implementation issues • Purpose of RF Front End • Dynamic Range • RF Receiver Front-End Topologies • Enhanced Flexibility of RF Front-End with SDRs • Importance of Components in Over All performance • Noise and Distortion in RF • ADC/DAC distortion • Use of MEMS for Flexible RF design

  3. Purpose of RF FrontEnd • RX – • Filtering of Unwanted Signals • Down-conversion • Amplification • ADC • TX – • DAC • Up-conversion • Power Amplification • Bandwidth Limitation • Need to have a balance between different components  ADC and RF dynamic range • DSP engr. must be aware of limitations of RF frontend to compensate them in DSP

  4. SDR – RF FrontEnd

  5. RF Front End- Basic Functions • Objectives of RF Frontend are • Reject as many undesired signals as possible • Amplification of the ‘deisred’ signal to the range of ADC with minimal distortion • Minimize AWGN • Achieve a dynamic range which is compatible to that of ADC • Must separate the desired signal (-70 to -130 dBm) from the background RF environment (0 to -20 dBm)  sets the system SNR • Achieving adequate dynamic range is one of the central issues in RF design • Overall sys. must have considerable dynamic range to accommodate high power background signals to low power desired signals • The wider the BW, more the interference and noise and hence difficult to achieve high dynamic range

  6. Dynamic Range • Key design challenge in RF front-end • Measure of highest and lowest level signals that can be simultaneously accommodated by radio • Strong relationship b/w battery consumption and dynamic range  important tradeoff for mobile systems • Limited by physical characteristics of various components • Improvements • Variable approaches to RF part design (proper selection of components and good circuit design techniques) • DSP algorithms after ADC • Good initial RF designs  low interference and dynamic range constraints in subsequent systems • Dynamic Range • Lower bound -- AWGN sources (thermal noise, ADC quantization, jitter etc) • Upper bound – interference (co-channel, adjacent channel, self-induced etc.) • Attenuation of high-level interference signals to avoid non-linearities  presence of low-level desired signals • Presence of DC bias

  7. Dynamic Range

  8. RF Receiver Front-End Topologies • A no. of different RF front-end topologies exist each with its own advantages and disadvantages • Most common • Dual Conversion • Single Conversion • Tuned Radio Freq. receivers • Selection of a topology depends on • Sensitivity • Selectivity • Stability • Dynamic Range • Spurious Response • Scalability

  9. TRF

  10. RF RX Topologies - TRF • TRF (Tuned Radio Freq.) topology • BPF, LNA, AGC • BPF – filter quality factor of 107 for 30 kHz signal at 900 MHz with 60dB attenuation for channel 60 kHz away • ADC directly samples RF input • Constraints in practical TRF transceiver  • ADC for high freq. signals • high power consumption with high sample-rate • Requires high dynamic range of 100 dB for wide BW • Extreme demands on tunable RF filter to remove interferences in the dynamic range • Advantage  minimal no. of analog parts required

  11. Single-Mixer

  12. RF RX Topologies – Direct Conversion • Homodyne, Zero-IF • Single mixing stage – direct conversion to baseband • Channel selection and ADC at baseband • Mixers have high power consumption • Low power consumption for 1-stage mixing can be traded-off for high dynamic range • Isolation b/w Local Oscillator (LO) and input ports is desirable but difficult to achieve • Capacitive coupling • Tracking and feedback of DC error • Error due to non-matching of I and Q branch phase and amplitudes • Eliminate with DSP • Disadvantage as compared with TRF

  13. Homodyne

  14. RF RX Topologies – Heterodyne Receiver • Most common RF Front-end for radios • Freq. translating the incoming signal to an IF that is fixed and independent of Fc • BW<IF<Fc  superheterodyne • Good RF components availability for IF Freqs. • At times, two stages of downconversion  relaxed filtering requirements – low Filter Quality Factor • Disadvantage – larger circuit and higher power consumption • I and Q branches must be matched to prevent distortion

  15. Hetererodyne Receivers – Image Problem • Downconversion also leads to upconversion of a part of band • To mitigate this, an image filter precedes the mixer to suppress the low-freq interfering band – 60-80 dB attenuation • Careful freq. planning to relieve filter req.

  16. RF RX Topologies – Comparison • Tradeoff of sensitivity vs. selectivity • TRF receiver  more suitable for SDR • Filter requirements make the multi-mode operation difficult • Retuning • Complex interaction between multiple RF components • Simpler the RF chain  more predictable response after re-tuning • Factors to consider : channel spacing, freq. plan, spurious response, total gain etc.

  17. Enhanced Flexibility of RF with SDR • Ways in which software based tuning of RF components can be incorporated in classical RF chain • Mixers – Biasing and phase distortion can be run-time tuned by software • Amplifiers – Sophisticated power management strategies in software – Cyclic On/Off for TDMA systems • DSP based diversity combining • Software based IQ extraction and channelization

  18. Importance of Components on Overall performance • Important for the DSP engineer to understand basic radio components and associated distortion  tradeoff exists in complexity of RF and baseband chain • Antennas : • Underrated component in the overall link • Much of the link gain can be gained or lost in the selection of antenna • For multi-mode support of SDR, antenna design is of crucial importance

  19. Importance of Components on Overall performance • Antennas : • Most antennas support BW of about 10% of Fc • Hard to support of multiple cellular (900 MHz and 2 GHz) with single antenna • Several antennas may be required  increase in size

  20. Components - Duplexers • Duplexers/Diplexers • RF filters adding isolation between transmitting and receiving band – several orders of difference b/w power of TX and RX  expensive devices • Challenges for SDR – duplexers + diplexers

  21. Importance of Components on Overall performance • RF Filters : • Used for rejecting out-of-band interference • Also help isolate the receiver from transmitter • Should have small noise, low loss, provide selectivity without compromise on BW • Low Noise Amplifier : • Boosts power in compatible range of other components • Should maximize gain -> tradeoff with power consumption • Induction of limited noise -> first stage in the RF chain ultimately sets the noise performance of the system • Image Reject and IF Filters - • Induction of low noise because of amplification in downstream

  22. Importance of Components on Overall performance • RF Mixer : • Used for down-conversion and can be a major source of inter-modulation distortion – Non-linear device • Increasing LO power can reduce non-linearity at the cost of increased power consumption • Local Oscillator : • Should have good tuning range and low phase noise • Multiplying a received signal by a noisy LO is equivalent in the frequency domain of convolving their two spectra, producing a widened resulting signal spectrum

  23. AGC • AGC • Ensures that signal has a voltage compatible with that of ADC input range • In some cases, it is advantageous to implement AGC as series of amplifiers strategically placed in the circuit with gain that can be turned on or off via software to keep circuit operating at ideal power levels for variable range and types of signals • Difficult to use in wide (multi-) band systems • Weak signals in noise and clipping of strong signals

  24. AGC Circuit Compression Ratio (M)= change in input level in dB/ change in output level in dB

  25. Digital AGC and Operating Modes

  26. ADC • ADC • Most difficult component to select and places the most constraints on system design – biggest power consumer in RX • If perfect ADCs available, TRF architecture would be chosen • Generally tradeoff b/w sampling-rate, dynamic-range, ADC resolution and power consumption • Currently SDR implementations are for base-station applications because of high power consumption of the ADC

  27. TX Architectures • Tends to be less complex than RX • High power consumption in talk mode • Dual conversion TX is more practical due to better isolation properties at cost of more expense and power consumption • Use of complex signal processing techs.

  28. TX Components on Overall performance • RF and IF VCOs : -- good phase noise characteristics + los power consumption • Mixer / Upconverter : -- good linear characteristics to reduce spurious products • IQ Modulators - should be well matched to avoid constellation distortion • Power amplifier - should be wideband, linear and low noise • tradeoff between linearity and power-efficiency • AMPS (Class B – 60%), IS-95 (Class AB- 30-45%), GSM (Class BC – 40-50%) • In practice less than 25% battery power is effectively used during transmission • In full duplex system, leakage of TX noise in RX circuitry – sensitivity reduced

  29. Noise and Distortion in RF Chain • limiting factors for a transceiver performance  quantifying noise and distortion is necessary to quantify transceiver performance • Noise Characterization • Source 1 - Thermal noise in resistive components • Antenna represents first source of noise • Source 2- ADC (thermal + quantization noise) • Error because of finite precision binary representation

  30. Noise Figure • Noise Figure (NF) describes how much noise is added by different elements of receiver chain • Most common definition  NF = SNRin/SNRout • NF provides indication how device degrades SNR. • Device manufacturers supplies the NF • NF total can be calculated by referring all the NFs back to the antenna • Once total NF is determined, sensitivity level of the RX can be determined for a minimal SNR • Keep analog components noise contribution less than ADCs noise contribution is a good practice S (dBm)=Noise floor (dBm)+SNRmin(dB) Noise floor (dBm)=10log(kTeB)+NFtotal dB

  31. Noise Figure – Components Placement Best to have LNA placed as early in the system as possible because of its high gain

  32. Noise and Distortion in RF Chain • Distortion Characterization • Distortion occurs because of non-linearities in system • It takes the form of harmonics • Cross-modulation distortion • Weak signal and strong interferer enters a non-linearity – amplitude variations • Inter- modulation distortion • Multiple-signals in the non-linear device interact in a mixing process to create signals at sum freqs.

  33. Distortion - ADC-DAC distortion • ADC/DAC introduce both noise (thermal/ quantization) and distortion • Distortion due to aperture jitter • If the signal level exceeds max level of ADC – results in non-linear distortion  requirement of AGC

  34. Distortion - Pre-distortion • Predistortion: • for good spectral efficiency, pulse shaping is used  non-constant envelope • Class A amplifiers with high linear range but low efficiency • class AB,B,C amplifiers with better power efficiency bu non-linearities  spectral broadening • Predistortion is done before power amplification to avoid the spectral broadening such that output of amplifier is the ideal output • Analog /digital (better-tunable)

  35. Pre-distortion

  36. Use of MEMS in RF • Micro-mechanical components to add flexibility and low loss at RF • Use of miniaturized/micro mechanical devices • Low loss wide bandwidth switches • Variable capacitors/inductors/varactors/ high quality factor filters/ tuners / reconfigurable antennas • High level of sophistication and reliability with low cost and power consumption because of IC based fabrication • a solution for needed flexibility in RF • Reduces interference = dynamic range requirements on components  low power

  37. MEMS

  38. Conclusion • SDRs must consider the impact of imperfections in RF • Limitation removal by downstream DSP • Better interference removal filters • pre-distortion for non-linear power amps • software flexibility of gains • tradeoff of sampling-rate with resolution for ADC etc. • Adaptive filtering of harmonics • Bottleneck of RF can be removed by early sampling but constrained by ADC tech. – cost/power

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