Frequency Modulation - PowerPoint PPT Presentation

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Frequency Modulation

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  1. Frequency Modulation Analog and Digital Communications Autumn 2005-2006 CS477: Analog and Digital Communications

  2. Spectral Analysis of FM • Exponential modulation is non-linear • Bandwidth of the transmitted signals depends upon the amplitude of the information signal • Hard to analyze with Fourier transforms • Narrowband approximation makes analysis a little easier  NBFM and NBPM • For narrowband approximation to hold: CS477: Analog and Digital Communications

  3. NBFM and NBPM Consider the bandpass transmitted signal: For narrowband approximation: CS477: Analog and Digital Communications

  4. NBFM and NBPM Thus, for NBFM and NBPM: CS477: Analog and Digital Communications

  5. Tone Modulation Thus, for NBFM and NBPM: CS477: Analog and Digital Communications

  6. Tone Modulation with NB CS477: Analog and Digital Communications

  7. Tone Modulation with NB CS477: Analog and Digital Communications

  8. NOTE: and are periodic Tone Modulation with WB Try to write their Fourier series expansion! CS477: Analog and Digital Communications

  9. where and Tone Modulation with WB CS477: Analog and Digital Communications

  10. All values in the line spectrum are scaled by Tone Modulation with WB (Do Example 5.1-2 and Exercise 5.1-3 in text) CS477: Analog and Digital Communications

  11. Observation for falls off quickly for  Consider components only for Significant frequency components lie within Observation for is insignificant for  Consider components only for Significant frequency components lie within FM Bandwidth Consider Tone Modulation CS477: Analog and Digital Communications

  12. depends upon the allowable distortion level Usually approximated by For worst case, use: What is the corresponding value of NO! A smaller may result in larger FM Bandwidth: Single Tone Is this the maximum value? CS477: Analog and Digital Communications

  13. Use For practical systems : FM Bandwidth: Generic Signal Estimate BW using worst case tone modulation! Define Deviation ratio: Approximations: Carson’s rule: Carson’s rule is applied for extreme values of deviation ratio. It underestimates the BW for practical systems. (Do Example 5.2-1 and Exercise 5.2-1 in text) CS477: Analog and Digital Communications

  14. Transmission BW: Summary CS477: Analog and Digital Communications

  15. Generating NBFM Signals • Direct method • Use a voltage-controlled oscillator • Output frequency is proportional to input voltage • Use information signal as the input voltage • Restricted to low power applications • Most circuits unstable at carrier frequency • Not suitable for commercial FM broadcast • Require sophisticated feedback control CS477: Analog and Digital Communications

  16. (NBPM) 900 NBPM Modulator NBFM Output Input Generating NBFM Signals Indirect method: (NBPM/NBFM) CS477: Analog and Digital Communications