TSEK38: Radio Frequency Transceiver Design Lecture 6: Receiver Synthesis (I) Ted Johansson, ISY ted.johansson@liu.se
Systematic Receiver Synthesis (1) 4.1 Introduction 4. Sensitivity, Noise Figure Receiver Densensitization Mismatch between antenna and Rx 4.3 Intermodulation characteristics
4.1 Introduction 3 Many different ways to realize the receiver, and somewhat fewer for transmitter. TDD half-duplex: GSM, WLAN, DECT,, 5G, FDD full-duplex: WCDMA, LTE, Usually different frequency bands for RX and TX. The TX signal may be 10 db stronger than the RX signal. => FDD receivers are generally trickier to implement because of TX leakage into RX.
Introduction 4 Key design parameters, RX: sensitivity: overall noise figure (4.) intermodulation: 3rd order distortion, IP3 (4.3) single-tone desensitization (4.4, not!) adjacent/alternate channel selectivity: channel filter, phase noise, (4.5) interference blocking: channel filter, phase noise, (4.5) dynamic range: AGC, ADC (4.6). Book: Full-duplex, but can be applied half-duplex (some not relevant for half-duplex).
4. Sensitivity and NF 5 Sensitivity = weakest detectable signal to obtain a minimum SNR for achieving required BER. Directly related to the overall NF of the RX. Varies because: signal modulation and characteristics, signal propagation in the channel, external noise level. Calculation of sensitivity (pp. 30-3).
4..1 Reference sensitivity and NF 6 1 Sin,min PNi (Nref) SNRin (CNRin) Receiver Front-End ADC To demodulator SNR > SNRmin Sensitivity Ref noise Sin,min BERmax SNRmin 10log(kT0 * BW) Receiver input signal: Ps = kt0 * BW * FRx * SNRmin Sin,min = 10log (Ps,min) = (4..4) -174 dbm/hz + 10log(BW) + NFRx + SNRmin NF Rx = SNR in SNR min = S in,min (-174dBm/Hz + 10logBW) SNR min (4..7)
re-used from TSEK0 7 BER vs SNR SNR BER BER depends on SNR of the received signal, i.e. the signal after the receiver block. More complicated modulation schemes require higher SNR for the same error (trade off between BW and BER) It may be possible to correct errors with advanced Forward Error Correction (FEC) Coding (reduce BER for the same SNR) TSEK0 Radio Electronics 017/Ted Johansson
re-used from TSEK0 8 E b /N 0 vs. Signal-to-Noise Ratio A better measure of signal-to-noise ratio for digital data is the ratio of energy per bit transmitted (EB) to the noise power density (N0). SNR (a quantity which can be measured) is related to E b /N 0 (an artificial quantity used in comparisons) by SNR= Signal Power Noise Power = E b R b N 0 B R b B is the spectral density (bitrate / bandwidth). TSEK0 Radio Electronics 017/Ted Johansson
BER versus CNR in demodulation 9 Very dependent on modulation and demodulator implementation 10-3 (C/N) = Psig/N = (EbR)/(N0BW) (C/N) = Eb/N0 * R/BW R/BW 0.5-1.5 (typically) QPSK Eb = energy per transmitted byte N0= noise power density R = bit rate BW = channel bandwidth CNRmin = (Eb/N0)dB + 10log(R/BW)
10 p. 104 Examples of PNi (reference noise): GSM: -174+10log(00 khz) = -11 dbm Vn = 6.3 μv BT: -174+10log(1 MHz) = -114 dbm Vn = 14.1 μv WCDMA: -174+10log(5 MHz) = -107 dbm Vn = 31.6 μv
Reference sensitivity and NF 11 3 GSM (calc or from the table): Sin,min= -10 dbm, Nref = -11 dbm, BERmax= 4 10 - (4 %) => SNRmin = 6.5 + 10*log(71/50) 6.8 db => NFRx = -10- (-174-54) - 6.8 = 11 db specify NFRx = 7 db - 3 db margin because of ~3 db loss in TDD switch and RF filter, - 1 db additional margin.
4.. Noise Figure of Cascaded Stages 1 Gain is power gain, which depends on the impedance of each stage.
Cascaded Noise Figure 13
Cascaded Noise Figure 14
15 4..3 Receiver desensitization, TX leakage We want to find the receiver desensitization caused by TX leakage into the receiver band for an FDD TRX. This emission noise passes through the duplexer. It can be modelled as: Approach #1: Equivalent Duplexer Noise Figure, Approach #: Equivalent Antenna Temperature.
4..3.1 Duplexer noise figure 16 16 Tx channel Rx channel Tx noise emission BW
Duplexer noise figure 17 4 Antenna port: (4..4) Noise at Rx port: gant_rx = antenna insertion gain
Duplexer noise figure 18 Noise factor from antenna to Rx (4..4 / 4..5): Noise figure from antenna to Rx:
Duplexer noise figure 19 5 NF of the duplexer can be largely degraded by TX leakage. Example: N G Tx Dup NF db Dup db = 130dBm/Hz, =.5dB = 10log 10 G 44dB Dup N A+ 174dBm/Hz db Tx db 10 10 + 10 = If Tx is off, then NFDup =.5 db A = 4.4dB
4..3. Antenna Temperature 0 Different antenna temp compared to TRX temp effective noise temperature (4..3b) is basically the same as (4..6b)
Comparison: NF of TDD and FDD Rx 1 6 TDD: FDD: NFBPF+Switch = LBPF+Switch= -GBPF+Switch NFDup = LDup = -GDup NF NF Rx Rx = 8dB, LBPF + Switch =.5dB TDD: db NF = L + NF F-E =5.5 db BPF + Switch db F E NF NF F Rx Rx Dup = = 8dB, = 4dB F Dup L F + G Dup F E Dup db 1 =.5dB FDD: NF F-E =5 db
4..4 Mismatch between antenna and Rx VSWR = Voltage standing wave ratio Impedance mismatch Causes standing waves and high voltage peaks Two waves, forward and reflected: Special case when load RL is purely resistive:
Mismatch between antenna and Rx 3 7 F F Rx Rx SNR = SNR V = 1+ in out n = 1+ + Ra I 4kTR a ( V + R I ) n n V a Ra n Antenna V R Ra a V n I n Noisy Rx Noiseless Rx V out when uncorrelated R F F opt = V Rx,min Rx = n I n = 1+ R F Rx,min = 1+ = n n n R R G n R + R R n opt a G n ( G G ) a R R a opt + opt R R opt a F Rx F = 1+ Rx,min 1 R R For R a /R opt = (3) NF min = 3 db, NF Rx = 3.5 db (4.3 db) NF min = 6 db, NF Rx = 6.8 db (7.8 db) a opt + R R opt a
4.3 Intermodulation characteristics 4 4.3.1 Intermodulation products and intercept points
Intermodulation characteristics 5 IM3 is the main problem, close to the carrier. IIP 3 = (3I in - IM3)/ = Iin + 3/ (4.3.10) IM 3 = 3I in - IIP 3 Δf I in IP3 test Δf f Rx IM is a minor problem, except for the direction conversion receiver. IIP = Iin - IM [dbm] (4.3.9) f Rx Δf < BW/ f int IP test IM = I in - IIP,Rx 4.3. Cascaded IP Can be rather complicated when frequency selectivity and matching are considered.
Intermodulation characteristics (4.3.3) 6 The RX linearity - cascaded IIP for the whole receiver - is the main cause of intermodulation distortion + LO phase noise + Pin ( S / N) min =, Pin > Pin, min ( + 3dB) N + N ( F -1) in in N + = Nref + PIM # PIM# is the cascaded IIPRX (mostly IIP3,RX and IIP,RX). Nother = other noise sources, usually assumed <= -100 dbm ref N Rx Other (book p. 58: SNR defined at Smin_ref + 3 db) P in PIM # = - FRxNref - ( S / N) min N Rx noise floor Other Headroom for P IM# limited by SNR min and N Other esp. LO phase noise + spurious Additionally P IM limited by self-mixing of the test signal (interferer) as discussed for Zero- IF Rx
Intermodulation characteristics Nin estimation N + in = Nref + PIM # N Other P in PIM # = - FRxNref - ( S / N) min N Other 7 8 I in = -38 dbm S in = -86 dbm BW = 1.5 MHz SNR min = 8 db NF Rx = 10 db 10log 10 P IM # dbm 10 = 10log 10 + 10 S in N SNR 10 Other dbm 10 min 10 174+ 10log BW + NF 10 Rx 94.5dBm + headroom for extra noise and distortion
IP3/IP estimation 8 8 I in = -38 dbm S in = -86 dbm BW = 1.5 MHz SNR min = 8 db NF Rx = 10 db P bl = -30 dbm P IM # dbm N IIP Other dbm = 10log 10 S in = 100dBm SNR 10 3( 38) ( 96) min 3, Rx = = 10 P 174+ 10log BW + NF 10 IM # dbm 9dBm Rx 10 96dBm N Other dbm 10 When all distortion is attributed to IP3. IIP IIP 3, Rx, Rx = = 3( 38) ( 99) = 7.5dBm ( P 3) ( 99) = 33dBm bl When distortion is attributed both to IP3 and IP by half each.
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