RF Transceivers


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A presentation towards explaining conecpt of RF Transceivers

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RF Transceivers

  1. 1. RF TRANSCEIVERS Presentation by: - Ritul Sonania 2005H124416 BITS-Pilani 24 April 2007
  2. 2. Contents <ul><li>Introduction to RF Transceivers </li></ul><ul><li>Considerations </li></ul><ul><li>Various Architectures </li></ul><ul><li>Characterization of RF Transceivers </li></ul><ul><li>Philips GSM TRx </li></ul><ul><li>References </li></ul>
  3. 3. Introduction <ul><li>RF Section – analog, high frequencies </li></ul><ul><li>Baseband Section - mostly digital today </li></ul><ul><li>(DSP), low frequencies </li></ul>
  4. 4. Ultimate objective <ul><li>Single-chip transceiver </li></ul><ul><li>• Minimum external components </li></ul><ul><li>• Inductors and capacitors integrated on chip </li></ul>
  5. 5. Considerations <ul><li>Limited Spectrum allocation </li></ul><ul><ul><li>IS-54 = 30 kHz, GSM = 200 kHz </li></ul></ul><ul><ul><li>Limited information rate, so coding, compression and BW efficient modulation is needed. </li></ul></ul><ul><li>Interference </li></ul><ul><ul><li>BPF order needed to select a channel (KHz) is very high (10 7 ) </li></ul></ul><ul><li>Dynamic range </li></ul><ul><ul><li>100 dB required </li></ul></ul><ul><ul><li>MDS in mVs. </li></ul></ul><ul><ul><li>Large signals may be experienced so AGC is required. </li></ul></ul><ul><li>Power Amplifiers </li></ul><ul><ul><li>Consumes a lot of power and switching them on/off is required. </li></ul></ul><ul><ul><li>Large current drawn from them causes battery voltage to change and creates noise so noise immunity is required. </li></ul></ul>
  6. 6. Rx Architectures <ul><li>Heterodyne Receivers </li></ul><ul><li>Concept :- </li></ul>
  7. 7. <ul><li>Suffers from image interference. </li></ul><ul><li>Image can be suppressed by image reject filter. </li></ul><ul><li>But IF can not be too large and too small </li></ul><ul><li>Some constraints are to be taken in to accout i.e. High IF means greater image suppression and Low IF means great suppression of nearby interferers. </li></ul><ul><li>So Choice of IF is a trade off between selectivity and sensitivity. </li></ul><ul><li>Possible solution to this is to use a Dual IF topology. </li></ul>
  8. 8. Dual IF topology( Heterodyne)
  9. 9. Direct Conversion (homodyne) Receiver <ul><li>Also called as Zero IF Architecture </li></ul><ul><li>Frequency of LO and RF signal is same. </li></ul><ul><li>So fewer components, image filtering avoided – no IR and IF filters </li></ul><ul><li>For PM and FM downconversion requires quadrature outputs to avoid loss of information. </li></ul>
  10. 10. Homodyne contd. <ul><li>Issues affecting:- </li></ul><ul><ul><li>Large DC offset can corrupt weak signal or saturate LNA (LO mixes itself), its called as self-mixing. Possible Soln. DC-free encoding and exploiting idle time intervals. </li></ul></ul><ul><ul><li>Flicker noise (1/f) be difficult to distinguish from signal </li></ul></ul><ul><ul><li>(because working on zero frequency. </li></ul></ul><ul><ul><li>Channel selection with LPF, easy to integrate, (noise-linearity-power tradeoff are critical) </li></ul></ul>
  11. 11. Homodyne contd. <ul><li>Qudrature generation is implemented by shifting LO phase by 90 degrees and this causes I/Q Mismatch while quadrature conversion phase and gain error occurs resulting in bad constellations resulting in wrong recovered information signal. </li></ul><ul><li>Possible solution – use of signal processing techniques. </li></ul>
  12. 12. Digital IF Receiver <ul><li>Second set of mixing is done in efficient manner using digital techniques. </li></ul><ul><li>Most critical thing is ADC, resolution required for performing is >14 bits. </li></ul><ul><li>This architecture is not yet fully implemented and is currently under research. </li></ul>
  13. 13. Transmitters <ul><li>Performs modulation, upconversion and power amplification. </li></ul><ul><li>Less architectures because noise, interference and band selection are relaxed in transmitters. </li></ul><ul><li>RF/Base band Interface </li></ul>
  14. 14. Transmitter <ul><li>Direct Conversion- </li></ul><ul><li>Called direct conversion because LO and carrier frequency is same. </li></ul><ul><li>Baseband signal is generated in tx itself so mixer noise is less critical. </li></ul><ul><li>Maximum Power transfer using matching network. </li></ul><ul><li>Drawback- </li></ul><ul><li>Injection Pulling or Injection Locking </li></ul>
  15. 15. Direct conversion contd.
  16. 16. Direct conversion contd. <ul><li>Injection pulling can be reduced if PA output spectrum is sufficiently higher or lower the LO frequency. </li></ul><ul><li>This needs use of a offset oscillator in order to add or subtract the LO signal. </li></ul><ul><li>Here carrier frequency is f1 +f2 and far away from f1, f2. </li></ul>
  17. 17. Transmitters contd. (4) <ul><li>Two Step transmitter- </li></ul><ul><ul><li>LO pulling problem is removed </li></ul></ul><ul><ul><li>First BPF suppresses IF signal harmonics </li></ul></ul><ul><ul><li>Second BPF removes unwanted sidebands </li></ul></ul><ul><ul><li>I & Q matching is superior so less cross talk. </li></ul></ul>
  18. 18. Characterization of TRx <ul><li>As such it depends on 100s of tests </li></ul><ul><li>Sensitivity and Dynamic Range- </li></ul><ul><ul><li>MDS of -120 dBm for GSM </li></ul></ul><ul><ul><li>SNR 9 -12 dB </li></ul></ul><ul><ul><li>BER 10 -3 </li></ul></ul><ul><ul><li>C/(N+I) >9 dB </li></ul></ul><ul><li>Unwanted Emissions </li></ul><ul><ul><li>Signals radiated by antenna must comply with FCC </li></ul></ul><ul><ul><li>For this a modulation mask is provided , </li></ul></ul><ul><ul><li>Also ACP (Adj. Chl. Power) is set as <-26dBc for IS-54 </li></ul></ul>
  19. 19. Transceiver implementation
  20. 20. Case Study <ul><li>Philips GSM TRx </li></ul>
  21. 21. <ul><li>VCO 1.3 GHz </li></ul><ul><li>VLO 2 800Mhz and then its divided by 2 to avoid self mixing. </li></ul><ul><li>Dual band 1800 MHz as well as 900 MHz </li></ul><ul><li>Fabricated using 13GHz Bi CMOS technology </li></ul><ul><li>Current drawn 50mA during reception </li></ul><ul><li>Current drawn 105mA during transmission. </li></ul><ul><li>Power supply 2.7 Volts . </li></ul>
  22. 22. References <ul><li>“ RF Microelectronics”, Behzad Razavi </li></ul><ul><li>“ Radio Frequency Integrated Circuits and Technologies”, Springer </li></ul><ul><li>RF Transceiver Front Ends, Philips Semiconductors CTO/AST </li></ul><ul><li>UAA3537EDGE Transceiver, www.semiconductors.philips.com </li></ul>