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TSEK38 2019 L6 Handouts.Pdf "2 Lecture schedule w4: TSEK38: Radio Frequency • Le1: Introduction (Ch 1) • Le2: Fundamentals of RF system modeling (Ch 2) Transceiver Design • Le3: Superheterodyne TRX design (Ch 3.1) Lecture 6: Receiver Synthesis (I) w6: • Le4: Homodyne TRX design (Ch 3.2) • Le5: Low-IF TRX design (Ch 3.3) Ted Johansson, ISY • Le6: Systematic synthesis (calculations) of RX (Ch 4) [email protected] w7: • Le7: Systematic synthesis (continued) • Le8: Systematic synthesis (calculations) of TX (Ch 5) w8: • Le9: Systematic synthesis (continued) TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "3 "4 Lab schedule Repetition of lectures 3 - 5 w6: • We: Lab1a (after Le6): 15-19 (ASGA) • Heterodyne Architecture • Th: Lab1b: 17-21 (SOUT) w7: • Homodyne Architecture • We: Lab1c (after Le8): 15-19 (SOUT) • Low-IF Architecture • Th: Lab1d: 17-21 (EGYP) Instructions: • One long lab (4 x 4 h). • Lab manual in the Lisam Course Documents/2019/Lab folder. • Supervision (Ted) available at the times above. • To pass: complete and document the exercises in the lab manual, go through with Ted. TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "5 "6 Heterodyne with analog IF architecture FDD, one antenna, shared LO1 Heterodyne receiver • Careful frequency planning required (IF frequencies). • Many possible intermodulation effects must be considered. IFA SAW VGA LNA LPF ADC IR IF • Pulling of LO by TX avoided. Filter Filter LPF ADC • Trade-off between sensitivity and selectivity in RX reduced by 2nd stage. • LO in first stage can be shared, giving different IFTX and IFRX. B I Q LO2R • Trade-off between sensitivity and linearity and power in RX. LO1 VGA in IQ paths! Duplexer avoided • Most of RX gain at IF or BB (after removing blockers). B LO2T • Today heterodyne architecture mostly for TX rather than for RX since IR filters I Q difficult to integrate. LPF DAC RF Filter PA Driver VGA VGA LPF DAC SAW RF, IF filters and duplexer not integrated → Most of gain at IF (75 %) and RF. matching issues. IF gain is more power efficient TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "7 "8 Homodyne receiver (Zero-IF, direct conversion) Low-IF receiver • Fewer components, image filtering avoided – no IR and IF filters • Tradeoff between heterodyne and homodyne • Large DC offset can corrupt weak signal or saturate LNA (LO mixes itself), notch filters or adaptive DC offset cancellation – eg. by DSP baseband • Blocking profile dictates IF = BW/2. control • DC offset and 1/f do not corrupt the signal (cf. homodyne). • IP2 of mixer critical • Image problem reintroduced - close image! • Flicker noise (1/f) can be difficult to distinguish from signal • Still even-order distortions can result in low-freq. beat: differential • Channel selection with LPF, easy to integrate, noise-linearity-power tradeoffs circuits useful but not sufficient 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 "9 "10 Close-image problem IQ imbalance and Image rejection • IQ imbalance causes image crosstalk and degrades image rejection. Tough requirements for IQ match if image is large, otherwise signal strongly corrupted PSignal IR =10log PImage fIF = # BW typical 1+ 2(1+δ )cosε + (1+δ )2 IR =10log 1− 2(1+δ )cosε + (1+δ )2 δ - amplitude (gain) imbalance! IR is partly insensitive,! ε - phase imbalance e.g. for 0.3d B gain imbalance, IR=30dB keeps up to 2° phase imbalance TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "11 "12 Direct conversion transmitter Systematic Receiver Synthesis (1) • Up-conversion is performed in one step, fLO = fc • 4.1 Introduction • Modulation, e.g. QPSK can be done in the same process • 4.2 Sensitivity, Noise Figure • BPF suppresses harmonics • LO must be shielded to reduce corruption • Receiver Desensitization from transmitter • I and Q paths must be symmetrical and LO in quadrature, otherwise emissions crosstalk • Mismatch between antenna and Rx • 4.3 Intermodulation characteristics • Phase noise degradation TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "13 Introduction "14 4.1 Introduction • Many different ways to realize the receiver. • Key design parameters, RX: Somewhat fewer for transmitter. • sensitivity: overall noise figure (4.2) • TDD half-duplex: GSM, WLAN, DECT, …, 5G, • intermodulation: 3rd order distortion, IP3 (4.3)! also IP2 • FDD full-duplex: WCDMA, LTE, … • single-tone desensitization (4.4, not!) • Usually different frequency bands for RX and TX. • adjacent/alternate channel selectivity: channel filter, • The TX signal may be 120 dB stronger than the phase noise, (4.5) RX signal. • interference blocking: channel filter, phase noise, (4.5) • FDD receivers are generally trickier to implement • dynamic range: AGC, ADC (4.6). because of TX leakage into RX, VCO pulling from LO, ... • Book: Full-duplex, but can be applied half-duplex (some not relevant for half-duplex). TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "15 "16 4.2 Sensitivity and NF AWGN • Sensitivity = weakest detectable signal to obtain a Additive white Gaussian noise (AWGN) is a basic noise minimum SNR for achieving required BER. model to mimic the effect of many random processes that • Simple model: noise at input is white noise. Directly occur in nature: related to the overall NF of the RX. • Additive because it is added to any noise that might be intrinsic to the information system. • Varies because: signal modulation and • White refers to the idea that it has uniform power across characteristics, signal propagation in the channel, the frequency band for the information system. It is an external noise level. analogy to the color white which has uniform emissions at • In this book: Carrier-to-Noise Ratio (CNR) for the all frequencies in the visible spectrum. analog signal. SNR is for the digital BB. But we will • Gaussian because it has a normal distribution in the time use S and SNR! domain with an average time domain value of zero. • Calculation of sensitivity and NF: pp. 230-232. [Wikipedia] TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "17 "18 Eb/N0 vs. Signal-to-Noise Ratio BER vs SNR SNR BER • A better measure of signal-to-noise ratio for digital data is the ratio of energy per bit transmitted (Eb) to the noise • BER depends on SNR of the power density (N0). received signal, i.e. the signal after the receiver block. • SNR (a quantity which can be measured) is related to E /N (an artificial quantity used in comparisons) by • More complicated modulation b 0 schemes require higher SNR for SignalPower E R the same error (trade off between SNR = = b b BW and BER) NoisePower N0 B • It may be possible to correct errors ! with advanced Forward Error ! Correction (FEC) Coding ! Rb is the spectral density (bitrate / bandwidth). (reduce BER for the same SNR) B TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "19 "20 BER versus SNR in demodulation 2 BER vs. Eb/N0 Dependent on modulation and demodulator implementation. 10-3 (S/N) = Psig/N = (EbRb)/(N0B)! (S/N) = Eb/N0 * Rb/B Rb/B $ 0.5 - 1.5 (typically) QPSK Eb = energy per transmitted byte N0= noise power density Rb = bit rate B = channel bandwidth p. 92 For spread-spectrum (e.g. CDMA) ! SNRmin = (Eb/N0)dB + 10log(Rb/B) Rb/B<<1 => SNRmin < 0 TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "21 "22 4.2.1 Reference sensitivity and NF 1 Reference sensitivity and NF Input signal: Min (S/N)o required for BER for the sensitivity level = (S/N)min = SNRmin. Smin: Sensitivity! => Sensitivity = Smin[dB] = 10log(PS,min) = -174 + 10log(B) + NFRX + SNRmin (4.2.4) PNi (Nref): integrated thermal noise power within Rx BW.! SNRin (CNRin): signal-to-noise => NFRX [dB] = SNRin - SNRout = SNRin - SNRmin = ! Smin - (-174 dBm/Hz + 10log(B)) - SNRmin (4.2.7) Output signal: Receiver! ADC To demodulator Front-End SNR > SNRmin Calculation flow: 1. BER requirement. Reference noise or noise floor Receiver noise factor: FRX = (S/N)i / (S/N)o = (PS/PNi) / (S/N)o (4.2.1) 2. Use graph to convert to Eb/N0. 3. Calculate SNRmin. Integrated thermal noise power within Rx BW: PNi = k * T0 * B (4.2.2) 4. Calculate Smin (sensitivity). Called Reference noise. Ground floor for noise. 5. Calculate NF. => RX input signal power: Ps = kT0 * B * FRX * (S/N)o (4.2.3) TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "23 "24 Reference sensitivity and NF Reference sensitivity and NF 3 GSM (calc or from the table): Smin= -102 dBm, Nref = -121 dBm, BERmax= 4&10-2 (4 %) p. 104 => SNRmin = 6.5 + 10*log(271/250) $ 6.8 dB (book 8 dB)! ! => NFRx = -102 - (-174 - 54) - 6.8 = 11 dB (total max noise) Specify NFRx = 7 dB: - 3 dB margin because of ~3 dB loss in TDD switch Examples of PNi (reference noise): ! and RF filter, GSM: -174+10log(200 kHz) = -121 dBm → Vn = 6.3 %V! BT: -174+10log(1 MHz) = -114 dBm → Vn = 14.1 %V! - 1 dB additional margin.! WCDMA: -174+10log(5 MHz) = -107 dBm → Vn = 31.6 %V TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson TSEK38 Radio Frequency Transceiver Design 2019/Ted Johansson "25 "26 4.2.2 Cascaded Noise Figure Noise Figure of Cascaded Stages Gain is power gain, which depends on the impedance of each stage.
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