RF Module Design - [Chapter 4] Transceiver Architecture

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  1. 1. RF Transceiver Module Design Chapter 4 RF Transceiver Architectures Department of Electronic Engineering National Taipei University of Technology
  2. 2. Outline General Considerations Frequency Conversion Receiver Architectures Heterodyne Receiver Direct-Conversion Receiver (DCR) Image-Reject and Low-IF Receiver Transmitter Architectures Direct-Conversion Transmitter (DCT) Heterodyne and Sliding-IF Transmitter Open-loop and Closed-loop PLL-based Transmitter Envelope Tracking and Envelope Following Transmitter Polar Transmitter Department of Electronic Engineering, NTUT2/110
  3. 3. Front-end General Considerations TX: Adjacent channel leakage RX: Rejection of inband and out-of-band interference BPF Power Amplifier (PA) Transmitted Channel Adjacent Channels BPF Low Noise Amplifier (LNA) Bandpass Filter Response Adjacent Channel Alternate Adjacent Channel 1f f Department of Electronic Engineering, NTUT3/110
  4. 4. Interferer Suppression High linearity to accommodate interferes without experiencing compression or significant intermodulation. Filtering the interferer can relax RX linearity requirements. BPF high selectivity is required for near channel rejection. Variable BPF is required for different carrier frequencies, and it is difficult. 900 900.4 ( )MHzf 20 dB 35 dB BPF Response Hypothetical filter to suppress an interference Department of Electronic Engineering, NTUT4/110
  5. 5. Channel-Selection Filter All of the stages in the RX chain that precede channel- selection filtering must be sufficiently linear to avoid compression or excessive intermodulation Since channel-selection filtering is extremely difficult at the input carrier frequency, it must be deferred to some other point along the chain where the center frequency of the desired channel is substantially lower and hence the required filter Qs are more reasonable. Department of Electronic Engineering, NTUT5/110
  6. 6. Band-Select Filter A band-select filter selects entire RX band and reject out-of- band interferers, thereby suppressing components that may be generated by users that do not belong to the standard of interest. Trade-off between selectivity and in-band loss (higher-order filtering sections and arise NF). BPF LNADesired Channel Receive Band f f Band-selection filtering Department of Electronic Engineering, NTUT6/110
  7. 7. TX-RX Feedthrough TX leakage in a CDMA transceiver (full duplex). The RX must meet difficult linearity requirements. A BPF following the LNA can alleviate the leakage. Duplexer 20 dBm LNA PA 1 W (+30 dBm) 50 dB Duplexer LNA PA 50 dB f f TX Leakage f BPF Response BPF 10dB/div. 20 MHz/div. 1f2f TX Band RX Band 50 dB 30 dB Department of Electronic Engineering, NTUT7/110
  8. 8. Frequency Conversion (I) Recall Chapter 1 (double sideband amplitude modulation) ( ) ( )cos2m cs t A t f t=t( ) ( )BBs t A t= f f cf0 Hzcf 0 Hz USBLSB USBLSBLSBUSB cos2 cf t real signal Real signal f 0 Hz Complex conjugate USBLSB 1f1f cos2 cf t 0 Hz cfcf USBLSBLSBUSB IF cf f+c IFf fIF cf fc IFf f Double sideband (DSB) Double sideband (DSB) Department of Electronic Engineering, NTUT8/110
  9. 9. Frequency Conversion (II) Recall Chapter 1 (linear modulation) Yes, a modulated signal sm(t) is a real signal. ( ) ( ) ( ) ( )1 12 2 2 2 j t j tj f t j f tA t A t e e e e = + ( ) ( ) ( )( )1cos 2ms t A t f t t = + ( ) ( ) { }12 Re j t j f t A t e e = f 1f0 Hz1f complexcomplex real Complex conjugate ( )I t 1cos t 1sin t ( )Q t ( )ms t Real signal Complex envelope Department of Electronic Engineering, NTUT9/110
  10. 10. Frequency Conversion (III) 0 Hz 2f2f 0 Hz 2f2f USBLSBLSBUSB Real signal f 0 Hz Complex conjugate USBLSB 1f1f 1 2f f+2 1f f1 2f f2 1f f 2cos2 f t RFIF ( )I t cos IFt sin IFt ( )Q t ( )IFs t Modulated signal (real signal) f 0 Hz USBLSB IFfIFf cos2 cf t RF 0 Hz cfcf USBLSBLSBUSB IF cf f+c IFf fIF cf fc IFf f Double sideband (DSB) mixing upconversion upconversion IF LO LO By filtering, you can choose only USB or LSB transmission, which is call the single-sideband (SSB) transmission. Department of Electronic Engineering, NTUT10/110
  11. 11. Frequency Conversion (IV) 0 Hz 2f2f 0 Hz 2f2f Real signal f 0 Hz Complex conjugate 1f1f 1 2f f+1 2f f2 1f f2 1f f 2cos2 f t IFRF downconversion 2 1f f< 2f2f 0 Hz 2f2f 0 Hz 2f2f Real signal f 0 Hz 1f1f 1 2f f+2 1f f1 2f f2 1f f 2cos2 f t IFRF downconversion 2 1f f> 2f2f High-side injection Low-side injection LO LO Department of Electronic Engineering, NTUT11/110
  12. 12. Receiver Architecture Basic Heterodyne Receiver Modern Heterodyne Receiver Hetero-dyne Different-freq. Mixing Department of Electronic Engineering, NTUT12/110
  13. 13. Basic Heterodyne Receivers (I) Translating the desired channel to a much lower center frequency to permit a channel-selection filtering with a reasonable Q. inin 0 LOLO 0 Downconversion by mixing RF input in Mixer 0 cos LOA t vLPF IF Output in LO in LO + 0 in LO +in LO Filtered-outFiltered-out LO ( ) ( ) 1 1 cos cos cos cos 2 2 in LO in LO in LOt t t = + + Low freq.High freq. Two IF frequencies: Department of Electronic Engineering, NTUT13/110
  14. 14. Basic Heterodyne Receivers (II) Use of LNA to reduce noise Variable IF: Constant IF: Mixer 0 cos LOA t vLPF IF Output RF input LNA IFj RFj LOf f f= (Constant LO freq. and variable IF freq.) IF RFj LOjf f f= (Variable LO freq. and constant IF freq.) Precise LO freq. and steps provided by a frequency synthesizer Constant IF approach is more common to simplify the design of IF path; e.g., it does not require a variable-frequency channel selection filter. LO Department of Electronic Engineering, NTUT14/110
  15. 15. Basic Heterodyne Receivers (III) Constant-LO downconversion mixing Constant-IF downconversion mixing 1RFf f f f 1LOf IFf0 1RFf f f f LOf 1IFf0 2RFf f f f 2IFf0 LOf 2RFf f f f IFf0 2LOf Department of Electronic Engineering, NTUT15/110
  16. 16. While each wireless standard impose constrains upon the emissions by its own users, it may have no control over the signals in other bands. The image power can therefore be much higher than that of the desired signal, requiring proper image rejection. Image Problem in Heterodyne RX cos LOt vLPF Desired signal Image in im IF IF IF LO High-side injection ( ) ( )cosd d dA t t t + ( ) ( )cosim im imA t t t + ( ) ( ) ( ) ( ) ( ) ( ) ( ) 1 1 cos cos 2 2 IF d LO d LO d d LO d LO dx t A t A t t A t A t t = + + + ( ) ( ) ( ) ( ) ( ) ( ) 1 1 cos cos 2 2 im LO im LO im im LO im LO imA t A t t A t A t t + + + + Department of Electronic Engineering, NTUT16/110
  17. 17. Downconverted Spectrum (I) 12 1+ 2+0 0 LO+LO 0 12 1+ 2+0 0 LO+LO 0 Downconversion for 1LO < Downconversion for 2 1LO > > Department of Electronic Engineering, NTUT17/110
  18. 18. Downconverted Spectrum (II) 12 1+ 2+0 0 LO+LO 0 12 1+ 2+0 0 LO+LO 0 Downconversion for 2 1LO > > Downconversion for 2LO > 1 2 2 LO + = Department of Electronic Engineering, NTUT18/110
  19. 19. The most common image rejection approach is to precede the mixer with an image-rejection filter. The filter exhibits a relatively small loss in the desired band and a large attenuation in the image band, two requirements that can be simultaneously met if 2IF is sufficiently large. A filter with high image rejection typically appears between the LNA and the mixer to lower the noise contribution to the RX NF (The NF increases while the filter precedes the LNA). Image Rejection Image Reject Filter in im 2 IF cos LOt v Image Reject Filter LNA Department of Electronic Engineering, NTUT19/110
  20. 20. Image Rejection v.s. Channel Selection Image Reject Filter in im 2 IF cos LOt v Image Reject Filter LNA v Channel Select Filter Desired channel Interference Image Channel Select Filter (high-Q needed) IF 0 0 IF in im 2 IF High IF Low IF If the IF is high, the image can be suppressed but complete channel selection is difficult, and vice versa. Department of Electronic Engineering, NTUT20/110
  21. 21. Image Noise Increases Noise Figure Even in the absence of interferes, the thermal noise produced by the antenna and the LNA in the image band arrives at the input of the mixer. The thermal noises in the desired channel and image band are downconverted to IF (unless the LNA has a limited bandwidth to suppresses the noise in the image band). LO LNA in Thermal Noise LO in 2 in LO Department of Electronic Engineering, NTUT21/110
  22. 22. Dual IF Receiver (I) The concept of heterodyning is extended to multiple downconversions, each followed by filtering and amplification, to resolve the trade-off between image rejection and channel selection. This technique performs partial channel at progressively lower center frequencies, thereby relaxing the Q required of each filter. 1LO vBPF2 LNA vBPF1 vBPF3 2LO vBPF4 Band Select Filter Image Reject Filter RF Mixer MX1 Channel Select Filter IF Mixer MX2 Channel Select Filter IF Amp. A C E GB D F H Department of Electronic Engineering, NTUT22/110
  23. 23. Dual-IF Receiver (II) 1LO vBPF2 LNA vBPF1 vBPF3 2LO vBPF4 Band Select Filter Image Reject Filter RF Mixer MX1 Channel Select Filter IF Mixer MX2 Channel Select Filter IF Amp. A C E GB D F H Department of Electronic Engineering, NTUT Desired Channel Image fA C E G B D F H BPF1 BPF2 Image Image BPF3 BPF4 f f f f f f f 23/110
  24. 24. Mixing Spurs (I) In practice, the mixing is the multiplication of the RF input by all harmonics of the LO. Thus the RF mixer produces components at and IF mixer, where m and n are integers. For the desired signal, only is of interest. But if an interferer, , is downconverted to the same IF, it corrupts the signal; this occurs if . 1in LOm 1 2in LO LOm n 1 2in LO LO int int 1 2 1 2LO LO in LO LOm n = Department of Electr