TSEK38: Radio Frequency Transceiver Design Lecture 3: Superheterodyne TRX design

Transcription

1 TSEK38: Radio Frequency Transceiver Design Lecture 3: Superheterodyne TRX design Ted Johansson, ISY

2 2 Outline of lecture 3 Introduction RF TRX architectures (3) Superheterodyne architecture (3.1, 3.1.1) Frequency planning (3.1.2) - IF selection ( ) - Spurious analysis ( ) Design Considerations (3.1.3) Summary

3 RF transceivers main building blocks 3 frequency filters amplifiers frequency converters modulator/demodulators oscillators synthesizers ADC/DAC signal coupler/divider/combiner/attenuators switches power/voltage detectors

4 Transceiver architectures 4 Superheterodyne (Ch 3.1) Most popular (and still is), invented in Somewhat complex and limited flexibility by fixed filters Homodyne (direct conversion, zero-if)(ch 3.2) Integratabtle Flexible Low IF (Ch 3.3) to overcome some drawbacks with the homodyne IF bandpass sampling (3.4), Software-defined radio,

5 Ted's history corner 5 Armstrong invented the superheterodyne receiver in 1918

6 6 S. Maas, IEEE Microwave Magazine, Sep/Oct 2013, p. 34

7 Quality Factor (Q) 7 Quality factor of a filter is a quantitative measure of how much loss the filter exhibits Lower quality factor indicates more losses Practical filters (especially on-chip filters) have losses and therefore low Q It can be shown that the quality factor is inversely proportional to the fractional bandwidth of the filter: f = BW / fc fc is the center frequency, BW is the -3 db limit. To have a small BW at high f c, a filter with very high Q is needed

8 The SAW filter 8 SAW (surface acoustic wave) filters are electromechanical devices commonly used in radio frequency applications. Electrical signals are converted to a mechanical wave in a device constructed of a piezoelectric crystal or ceramic; this wave is delayed as it propagates across the device, before being converted back to an electrical signal by further electrodes. The delayed outputs are recombined to produce a direct analog implementation of a finite impulse response filter. This hybrid filtering technique is also found in analog sampled filters. SAW filters are limited to frequencies up to 3 GHz. Wkipedia

9 The SAW filter 9 SAW filters combine low insertion loss with good rejection, can achieve broad bandwidths and are a tiny fraction of the size of traditional cavity and even ceramic filters. Because SAW filters are fabricated on wafers, they can be created in large volumes at low cost. SAW technology also allows filters and duplexers for different bands to be integrated on a single chip with little or no additional fabrication steps. Wkipedia

10 Channel Selection re-used from TSEK02 10 Most communication systems divide the frequency band into several narrower channels. The receiver should select each channel for detection Need for very sharp filter response (high Q-filter), Need for variable filter (tunable filter). Practical filters have low Q so their fractional bandwidth cannot be reduced too much It is practically very difficult to make tunable filters

11 re-used from TSEK02 The problems of channel selection: 11 Problem: We need to limit the bandwidth for better channel selection and limit the noise (improve the SNR). Solution: Reduce the center frequency, so that much lower BW can be achieved with the same fractional bandwidth. Problem: We need to filter signals at different frequencies (channels). Solution: Use a fixed filter and move the signal frequency instead.

12 Frequency Conversion re-used from TSEK02 12 Frequency of a signal can be shifted by multiplying it with another sinusoidal signal: x(t) = Acosω in t, s(t) = cosω LO t x(t) * s(t) = ½A*cos(ω in - ω LO t) + ½A2*cos(ω in + ω LO t) multiplication is performed by a mixer Low Pass Filter removes the high frequency signal the other sinusoidal signal comes from a local oscillator

13 Frequency Conversion re-used from TSEK02 13

14 re-used from TSEK02 Heterodyne Receiver improved sensitivity By down-converting the radio-frequency signal (RF) to a lower intermediate frequency (IF), much better selectivity can be achieved and SNR is improved 14 Bandwidth of this filter determines the noise power (ktb)

15 re-used from TSEK02 Heterodyne Receiver Channel Selection By changing ω LO, different ω in will down-convert to the same IF. 15 The IF filter is always at a fixed frequency! Variable LO frequencies can be made with a synthesizer ω LO1 = ω 1 -ω IF ω LO2 = ω 2 -ω IF ω LO3 = ω 3 -ω IF

16 re-used from TSEK02 Two-step Conversion Transmitter 16 In this architecture, we intentionally do not choose carrier frequency of the quadrature modulator to be the final transmission frequency, and perform a second frequency up-conversion by ω 2 We call ω 1 the intermediate frequency (IF)

17 3.1.1 Superheterodyne configuration 17 Section (pp ) in the book gives many details on the heterodyne building block functions and design selections. Highlights: duplex/half-duplex, duplexer (FDD, TDD) receiver RF, IF, BB transmitter RF, IF, BB transmitter PA classes READ BOOK!

18 READ BOOK! Superheterodyne, full-duplex TRX 18

19 Superheterodyne with analog IF architecture FDD, one antenna, shared LO1 19 LNA IR Filter IFA SAW IF Filter VGA LPF LPF ADC ADC Duplexer LO1 B B I Q LO2R LO2T I Q VGA in IQ paths avoided PA RF Filter SAW Driver VGA VGA LPF LPF DAC DAC RF, IF filters and duplexer not integrated matching issues. Most of gain at IF (75 %) and RF. IF gain is more power efficient

20 Digital IF Architecture FDD, one antenna, shared LO1 20 LNA IR Filter IFA SAW IF Filter VGA BPF ADC Duplexer LO1 B I Q LO2R B LO2T I Q PA RF Filter SAW Driver VGA VGA BPF DAC IQ mismatch avoided by digital IF but ADC/DAC claim more power. ADC needs larger DR and must be more linear. Final filtering also digital.

21 Typical macrocell basestation architecture 21 RX SAW+LP Noise Cancellatio n port Digital and DCDC LNA1 RF SAW Step Att LNA2 Splitter Mixer LC-LP DVGA LC-BP ADC Div Output Div Input Optional RX LO Synth Optional LC-LP DVGA LC-BP ADC LNA1 RF SAW Step Att LNA2 Switch Mixer SAW+LP Noise Cancellation port TX TX Synth Ant A Ant B Power Combiner Switch Noise Cancellation ports Isolator 20 db Coupler Ter m 30 db Coupler Doherty PA Switch Driver Feedback RX DVGA IQ Mod Σ LP DAC DAC ADC I Q Duplexer Isolator Ter m 20 db Coupler 30 db Coupler Doherty PA Driver DVGA Σ IQ Mod LP DAC DAC I Q TX Synth

22 3.1.2 Frequency planning, IF selection Considerations: Tx and Rx bands and IF Tx leakage and Rx in-band jamming IF/2 problem Multiband TRX constraints 22

23 Tx and Rx bands and IF 23 Down-link (basestation, BS, forward link) and up-link (mobile terminals, UE, reverse link) frequency band and channelization.

24 Frequency band allocation 24

25 Frequency band allocation, 3GPP 25

26 Flexible spectrum in LTE 26

27 re-used from TSEK02 Choice of Intermediate Frequency 27 By lowering the signal frequency to an intermediate frequency (IF), we can reduce the bandwidth and therefore improve the SNR. The lower we chose this intermediate frequency, the better performance we can get. What limits us from choosing very low IF?

28 Image Frequency re-used from TSEK02 28 A closer look at the down-conversion process: We need an ω LO which is ω IF away from the desired signal This ω LO may down-convert signals to the same ω IF (image may come from another users, system, etc.)

29 Image Reject Filtering re-used from TSEK02 29 Note that the image reject filter is at high frequency, i.e. has limited selectivity

30 Image Rejection Ratio re-used from TSEK02 30 Image Rejection Ratio, IRR = (Power of the received signal)/(power of the image signal) at the IF port Since IRR is a ratio, it is often expressed in db.

31 Trade-off in choice of IF High IF substantial rejection of the image re-used from TSEK02 31 Insufficient filtering from adjacent signals Low IF Effective filtering of from adjacent signals Insufficient image rejection

32 Frequencies in a heterodyne TRX 32 LO (UHF) reference oscillator 2 or more LO signals (VHF) 2 or more IF signals RF reception signal (weak) RF transmission signal (strong) + mixing product and harmonics => IF must be carefully chosen!

33 READ BOOK! Criteria for IF selection, full duplex 1. If sharing LO for TX and RX: TX and RX will get different IF! Receiver: high selectivity IF BPF (SAW) is used. Transmitter: not so critical, SAW not necessary. 33 TX RX B up B S B down Common LO1 f LO1 IF Rx B up = B down = B a IF Tx IF TX - IF RX = B a + B S

34 READ BOOK! TX leakage and RX in-band jamming TX band B S RX band Strong in-band interferer (blocker) ftx fblocker TX channel leakage to RX RX channel TX leakage through duplexer can be mixed with the blocker in Rx and fall in the IF band. To prevent in-band jamming: f TX - f Blocker IF RX. In practice: IF RX > 2B a +B S or IF RX < B S.

35 3. IF/2 problem interferer READ BOOK! 35 RX IF RX /2 f Rx (f RX + f LO1 )/2 f LO1 IF RX /2 IF RX Mixing of 2nd harmonics: 2 f LO1-2 (f RX + f LO1 )/2 = IF RX (LO 50% duty cycle to avoid 2 nd harmonic) Downconversion to IF Rx /2 and subsequently 2nd order distortion less harmful because of IF filter. IF/2 interferer must be suppressed: IF RX /2 >> B a

36 4. Multiband TRX constraints READ BOOK! 36 For multiband TRX, IFRX must be max of the standards covered while TX IF follows IFTX = IFRX + Ba + BS. Same IF filters can be used unless channel BWs are very different. TX frequency can interfere with IF and fall in the RX band of another system on the same mobile platform. Very weak 274 GPS PCS CDMA Tx { IF TX < 274 MHz or IF TX > 336 MHz} and IF RX = IF TX - B a B S = IF TX 80MHz

37 5. Multiband TRX constraints Another system should not be an image READ BOOK! 37 PCS CDMA RX 410 Bluetooth IF RX < 205 MHz Sharing LO1 by different systems with common IFRX (frequency division by 2) Cellular CDMA RX: PCS CDMA: flo1 = 2 (869+IFRX) 2 (894+IFRX) flo1 = (1930+IFRX) (1990+IFRX) LO1 tuning range: Δf = Max { 2 (894+IF RX ), (1990+IF RX )} - Min { 2 (869+IF RX ), (1930+IF RX )}

38 Frequency planning, spurious analysis f Sp = m f A ± n f B m, n = 1,2,3,4,. f har = m f A of any two strong signals, esp. TX, TX IF, LO, etc. Also harmonics of LO2, LO3, IFTX, and fref Preferably spurs should not fall in: RX band Image band IF/2 band TX band LO band Other bands to be protected, e.g. GPS

39 39 Example: Cellular CDMA TRX (full-duplex) IFRX = MHz IFTX = IFRX+Ba+BS = MHz fsp1 = 3fTX - 7IFTX fsp2 = 3fLO1-5fLO2 Those spurious response lines do not intersect with the TRX tuning line. Should they do, then the RX signal would be corrupted. Here they are rather weak inband interferers.

40 Example: Cellular CDMA TRX (full-duplex) 40 fsp3 = 7fLO3-2fLO1 This spurious response line coincides with the TRX tuning line. The spur can mix with the RX signal to produce IF component, but it should not be harmful

41 Example: Cellular CDMA TRX (full-duplex) 41 f IF/2 = (f RX +f LO1 )/2 = f TX +B a +B S +IF RX /2 f Sp4 = 2f Tx - 3IF Tx Does not interfere with the IF/2 band

42 READ BOOK! Design considerations (pp ) Receiver Trade-off between sensitivity and selectivity (filters). Trade-off between sensitivity, linearity, and power. SFDR is a joint measure of noise and linearity (IP3), also Q = IIP3-NF. Most of Rx gain at IF. Blocking determined by selectivity, phase noise, spurs of the synthesizer (IF and BB filters important), and linearity.

43 Design considerations READ BOOK! Transmitter Trade-off between power efficiency and spectral efficiency. Power, power tolerance and control, 2-3 db back-off, PA linearization. Pulse shaping and filtering (to limit ACPR and spurious). Modulation accuracy (EVM, ρ, phase error). Effect of phase noise, filter group delay, and LO leakage.

44 Design considerations RX dynamic range RX signal range, in-band blocking. Best use of ADC, max RX gain. READ BOOK! 44 AGC needed (mainly at IF/BB), transient response constraints. TX dynamic range CDMA systems need large DR 70dB (near-far effect) Other systems need much less DR 30dB Gain control mainly at IF for power savings

45 READ BOOK! Summary, Heterodyne Architecture 45 Careful frequency planning required (IF frequencies). Many possible intermodulation effects must be considered. Pulling of LO by TX avoided Trade-off between sensitivity and selectivity in RX reduced by 2nd stage. LO in first stage can be shared, giving different IFTX and IFRX. Trade-off between sensitivity and linearity and power in RX. Most of RX gain at IF or BB (after removing blockers). Today heterodyne architecture mostly for TX rather than for RX since IR filters difficult to integrate.

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