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TSEK38: Radio Frequency Transceiver Design Lecture 2

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TSEK38: Radio Frequency Transceiver Design Lecture 2 "2 Repetion of Lecture 1 TSEK38: Radio Frequency • Wireless communication systems today Transceiver Design • Some wireless standards Lecture 2: Fundamentals of • Communication radio examples System Design • Architectures: the big picture Ted Johansson, ISY [email protected] TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "3 "4 Lisam Course Room There is a standard for almost any need! • https://liuonline.sharepoint.com/sites/TSEK38/TSEK38_2019VT_FU Mobility Data rate Mb/s TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "5 "6 Examples of Standards 5G Standard Access Frequency Channel Frequency Modulation Rate Peak Power Scheme/Dupl band (MHz) Spacing Accuracy Technique (kb/s) (uplink) GSM TDMA/FDMA/ 890-915 (UL) 200 kHz 90 Hz GMSK 270.8 0.8, 2, TDD 935-960 (DL) 5, 8 W DCS-1800 TDMA/FDMA/ 1710-1785 (UL) 200 kHz 90 Hz GMSK 270.8 0.8, 2, TDD 1805-1850 (DL) 5, 8 W DECT TDMA/FDMA/ 1880-1900 1728 kHz 50 Hz GMSK 1152 250 mW TDD IS-95 CDMA/ FDMA/ 824-849 (RL) 1250 N/A OQPSK 1228 N/A cdmaOne FDD 869-894 (FL) kHz Bluetooth FHSS/TDD 2400-2483 1 MHz 20 ppm GFSK 1000 1,4,100 mW 802.11b CDMA/TDD 2400-2483 20 MHz 25 ppm QPSK/CCK 1, 2, 11 1 W (DSSS) Mb/s WCDMA W-CDMA/TD- 1920-1980 (UL) 5 MHz 0.1 ppm QPSK, 3840 0.125, 0.25, (UMTS) CDMA/F/TDD 2110-2170 (DL) 16/64QAM (max) 0.5, 2W LTE OFDMA (DL) 700-2700 scalable 4.6 ppm QPSK, DL/UL Scalable SC-FDMA (UL) to 20MHz /32 ppm 16/64QAM 300/75 with BW to FDD/TDD 0.1 ppm (BS) Mb/s 250 mW Source: 5G RF for dummies (Corvo) TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "7 "8 Digital Transmitter Digital Receiver Baseband! RF! Down! signal Filter LNA IF/BB! ADC Demodulator ! Converter Filter & DSP Upconverter/ RF! ADC Modulation & DSP DAC PA Filter Modulator Baseband! Carrier Gain control signal Power Carrier control Digital baseband Digital baseband section ! RF frontend: ! (compression, coding, image rejection, low section: ! RF section ! equalization, modulation, shaping) noise, gain control, (up-conversion, filtering, demodulation, down conversion, power gain and control) decoding, channel selection Tradeoff between power efficiency and spectral efficiency decompression TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "9 "10 Superheterodyne receiver Homodyne receiver (Zero-IF) • Direct conversion. • Fewer components, image filtering avoided – no IR and IF filters. • Large DC offset can corrupt weak signal or saturate LNA (LO mixes itself), notch filters or adaptive DC offset cancellation – eg. by DSP baseband control. • Flicker noise (1/f) can be difficult to distinguish from signal. • Channel selection with LPF, easy to integrate, noise-linearity-power tradeoffs are critical, even-order distortions low-freq. beat: differential circuits useful. TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "11 "12 Low-IF receiver Direct conversion transmitter • Tradeoff between heterodyne and homodyne. • Up-conversion is performed in one step, fLO = fc • DC offset and 1/f do not corrupt the signal, like in the homodyne, still • Modulation, e.g. QPSK can be done in the same process DC offset must be removed - saturation threat. • BPF suppresses harmonics • Image problem reintroduced - close image! • LO must be shielded to reduce corruption • Still even-order distortions can result in low-frequency beat: differential • I and Q paths must be symmetrical and LO in quadrature, circuits useful but not sufficient otherwise crosstalk TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "13 "14 Outline of lecture 2 Linear and Nonlinear Systems (2.1.1) • A system is said to be linear if it follows the • System Design Fundamentals! superposition rule: Gu: Chapter 2 (with re-used slides from TSEK02)! If X (t) Y (t) • 2.1 Linear systems (shortened) 1 1 • 2.2 Nonlinear issues (extended) X2(t) Y2(t) • 2.3 Noise then • 2.4 Digital Base-Band (sampling, modulation, errors, …): skipped. aX1(t) + bX2(t) aY1(t) + bY2(t) TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "15 "16 Example Time Invariance Output contains the Linear same frequency components as input, they are just stronger • Time-variant = response depends on the time of Vout = G * Vin origin. A cos ω1t GxA cos ω1t • A system is time-invariant if a time shift in its input -ω1 ω1 Both inputs are amplified by results in the same time shift in its output. Linear the same amount If y(t) = f [x(t)] Vout = G * Vin then y(t-τ) = f [x(t-τ)] A cos ω1t GxA cos ω1t B cos ω t GxB cos ω t 2 2 • LTI: Linear time-invariant, y(t-τ) = L * [x(t-τ)]. ! Input signals are stronger now Different tones are A time shift in the input causes the same time shift Non-linear amplified differently in the output. Vout = G(Vin) * Vin New frequencies • Examples: filters, isolators, duplexers, linear K1 cos (2ω1-ω2 ) t A’ cos ω1t A’’ cos ω t appear at the output 1 amplifiers. B’ cos ω2t B’’ cos ω2t K2 cos (2ω2-ω1 ) t TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "17 "18 Time domain vs. frequency domain representation (2.1.2) Example – effect of filtering Time domain Frequency domain Smoother and wider in time domain • The two representations are related through Fourier transformation and both contain the same information TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "19 "20 Bandwidth and frequency dependence Fundamentals of System Design (Ch 2) • Many different bandwidth definitions: • 2.1 Band-pass and low-pass RF system • most common is ”3 dB”: range over which the representation gain drops by less than 3 dB. • 2.2 Nonlinear issues (extended) • 2.3 Noise • Many systems have frequency dependent transfer functions. • Amplitude response is usually what we look at. • But also phase response is usually a nonlinear function of the frequency, caused by varying delay in the circuit, and causing linearity degradation as well as the amplitude. TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "21 "22 Non-linear issues (p. 29-) x(t) y(t) Effects of Nonlinearity F(x) • Static model • Consider a nonlinear memoryless system y(t) = F(x(t)) y(t) x(t) x 2 (t) x 3 (t) nonlinear 2 3 = α 0 +α1 +α 2 +α 3 x(t) y(t)= #0 + #1 Vin + #2 V in + #3 V in+... 3rd order polynomial • Let us apply a single-tone (A cosωt) to the input and Linear part approximation adequate calculate the output: for weak nonlinearities in x(t) Acos t RF blocks. = ω 2 2 3 3 y(t) = α1 Acosωt +α 2 A cos ωt +α 3 A cos ωt • Dynamic model α A2 ⎛ 3α A3 ⎞ α A2 α A3 y!(t) = F y(t), x(t) (eg. RF amplifier) 2 A 3 cos t 2 cos 2 t 3 cos3 t ( ) = + ⎜α1 + ⎟ ω + ω + ω 2 ⎝ 4 ⎠ 2 4 Also polynomial approximation preferred for F() if weakly nonlinear.! Volterra series especially for PA analysis to include memory effects. Second Third DC Fundamental Harmonic Harmonic TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "23 "24 Effects of Nonlinearity Gain (1dB) Compression (p. 32) • Observations: • Eventually at large enough signal levels, output power – Fundamental amplitude reduced by gain compression, does not follow the input power, #1*#3<0. – Harmonics: not of major concern in a receiver (out of band), but critical in transmitters. – Even-order harmonics result from #j with even j, and same for odd. Can be avoided by differential architecture – nth harmonic grows in proportion to An n x(t–) =AmplitudeAcosωt of nth harmonic grows in proportion to A in The P-1dB point correlates well to 2 2 3 3 loss of linear behavior, getting out- y(t) = α1 Acosωt +α 2 A cos ωt +α 3 A cos ωt of-spec in standards (EVM, ACPR, etc.) so for linear applications, α A2 ⎛ 3α A3 ⎞ α A2 α A3 2 A 3 cos t 2 cos 2 t 3 cos3 t operation beyond this point is = + ⎜α1 + ⎟ ω + ω + ω 2 ⎝ 4 ⎠ 2 4 useless. TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "25 "26 Desensitization (p. 32) Desensitization • At the input of the receiver, a strong interference may exist close to the desired signal. • Assume x(t) = A1cos$1t + A2cos$2t ! where A1 is the desired component at $1, A2 the interferer at $2. • When in compression • For A1 << A2:! • The small signal is superimposed on the large signal (time domain). If the large signal compresses the amplifiers, it will also affect the small signal. If #1#3 < 0, the receiver may not sufficiently amplify the small signal A1 due to the strong interferer A2. The gain may even drop to zero. • Also called "blocker". Creates problems when trying to keep the number of filters low. TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "27 "28 Cross-modulation (p. 32) Cross-modulation example • Any amplitude variation (AM) of the strong interferer A2 will also appear on the amplitude of the signal A1 at the desired frequency and distort the signal.
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