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Digit al Parts of Receivers and Transmitters

Digit al Parts of Receivers and Transmitters. Vilmos Rösner. Digital Parts of Receivers Digital Parts of Transmitters Special Application: Bandpass Filter. 0. f. DEM. out. Antenna output: U(t) =A(t) • exp(j •(t• ω (t)+ φ (t))). modulation-> information. Problem: receiver selectivity.

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Digit al Parts of Receivers and Transmitters

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  1. Digital Parts of Receivers and Transmitters VilmosRösner

  2. Digital Parts of Receivers • Digital Parts of Transmitters • Special Application: Bandpass Filter

  3. 0 f DEM out Antenna output: U(t)=A(t)•exp(j•(t•ω(t)+φ(t))) modulation-> information Problem: receiver selectivity

  4. 0 f IF DEM out 0 f Superheterodyne Receiver(Edwin Armstrong 1918) new problem:image rejection

  5. RF RF IF IF DEM BB Frequency Ranges generally: FRF: FIF≈ 10..3 problem with tuning linearity, images... problem with image rejection →solution: multiple conversion (IF) If it is possible, one of the standard intermediate frequencies should be used.

  6. analogue world BB M/U BB ADC RF RF IF IF BB RF RF IF IF ADC BB M/U BB BB RF RF IF ADC IF M/U digital world Digital Output, Position of the ADC modem digital IF receiver “software radio”

  7. ADC in FILTER out DDC Digital IF Receiver Components • Analog to Digital Converter (ADC) • Digital Mixer (multiplier) • DigitalOscillator (NCO Numerically Controlled Oscillator) • Digital Filter DDC

  8. LUT data bus clk address PACC N set f FTW NCO • fout=FTW * fClk / 2^N • Intersil HSP45102, HSP45116A • optionally complex output • other accessories (sweep, modulation, dither) • NCO+DAC≈ Direct Digital Syntheser (DDS)

  9. Digital Filter / FIR http://www.digitalfilter.com

  10. FIR Filter Coeffitients, Window Functions rectangular window: effect of increasing taps (order) other window side lobe levelstop band attenuation main lobe widthroll-off (slope) http://en.wikipedia.org/wiki/Window_function

  11. FIR Filter Design windowing method: coefs=(ift of ideal response) * window Further methods (e.g. Parks-McClellan)

  12. Digital Filter / IIR • better resource utilization as in FIRs • oscillations may occour→not recommended http://www.bores.com/courses/intro/iir/

  13. Digital Filter/CIC (Cascaded Integrator-Comb) • mentioned as FIR filter, but it has a feedback part • stable • there is not multiplier: easy to realize http://www.us.design-reuse.com/articles/article10028.html

  14. limited band 0 -Fsa/2 +Fsa/2 1/Fsa t Fs=Fsa ignored samples Fs=Fsb=Fsa/2 1/Fsb t -Fsb/2 0 +Fsb/2 Decimation if the signal is in a limited narrow band, we can skip samples without losing information

  15. 0 -Fsa/2 +Fsa/2 -Fsb/2 0 +Fsb/2 stop band pass band 0 +Fsa/2 Fs CIC (lpf) Fs DEC Fs/DEC Decimation the decimation causes spectrum overlapping, therefore the decimation usually happens at the output of a low pass filter using Mth order CIC and M factor decimation (M=4)

  16. Functional Block Diagram of DDCs

  17. in CIC FIR out 0 Fs f DDC 0 Fs f 0 FsBB f The FIR filter is realized with several (1..16) multiplier → we can use more taps at larger decimation

  18. in in CIC DEC=16 CIC DEC=32 FIR TAPS=16 FIR TAPS=32 out out 64MHz 64MHz 4MHz 2MHz Example • Two different settings of the same DDC • The decimation plays the similar role as down conversion in the analog techique

  19. DDC FIR FilterLimits

  20. ADC usually faster ADC -> worse SNR

  21. Signal SDFR Spour 0 Fs/2 Fs 0 Fs/2 Fs quantization noise, SDFR baseband sampling: if the signal has a large and low frequency part, the overtones are in the used band IF sampling: the overtones can evade the used band, where filters can attenuate them http://www.beis.de/Elektronik/DeltaSigma/DeltaSigma.html

  22. Advantages of Digital IF • easy to reproduce • thermal stability • flexibility • the output signal can be better as at BB sampling • accurate IQ output

  23. real sampling Alias Signal -Fs/2 0 +Fs/2 ideal complex sampling Signal -Fs/2 0 +Fs/2 unbalanced complex sampling alias rejection Alias Signal -Fs/2 0 +Fs/2 AAF I AMP I CH I ADC CH Q IF AAF Q AMP Q "cos" "sin" splitter (analogue complex sampling) It is possible sampling the IQ signal on baseband, and there are dual ADCs for IQ sampling applications, but these systems have finite rejection to the fs/2 alias because of the analogue gain and phase differences and crosstalk between the two analogue paths.

  24. in INT DAC out 0 FsBB f DUC 0 Fs f 0 Fs f Digital Upconverters

  25. Interpolation • inverse operation of decimation • zero padding or repeating is applicable • we have to use LPF to remove unwanted overtones

  26. Functional Block Diagram of a DUC

  27. F P G A PFIR ADC MIX CIC PFIR CIC MIX DAC CH0 CH0 CH1 CH1 CH2 CH2 CH3 CH3 GC5016/DUC GC5016/DDC Special Application: Bandpass Filter

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