1 / 20

Digital FM Demodulator

Digital FM Demodulator. m.Khafaji. Outline. Introduction to FM Modulation FM signal demodulation Quadrature-Mixer Digital Limiter Baseband Delay Demodulator Phase-Adapter Demodulator Direct Digital Synthesizers. Introduction to FM Modulation.

lupita
Download Presentation

Digital FM Demodulator

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Digital FM Demodulator m.Khafaji May 2005

  2. Outline • Introduction to FM Modulation • FM signal demodulation • Quadrature-Mixer • Digital Limiter • Baseband Delay Demodulator • Phase-Adapter Demodulator • Direct Digital Synthesizers

  3. Introduction to FM Modulation • Information signal encoded in carrier frequency (or phase) • Modulated signal is s(t)=Accos(q(t)) q(t)=2pfct+2pkfm(t)dt • Benefits: • Instantaneous frequency: fi=fc+kfm(t) • Signal robust to amplitude variations • Robust to signal reflections and refractions

  4. B2Df WBFM B2Bm NBFM .5AcJn(b) … … Ac .5Acb .5AcJn(b) f fc -4fm fc+fm fc -3fm fc+3fm fc fc -2fm fc+2fm fc+ 4fm fc-fm -.5Acb Spectral Analysis of FM • s(t)=Accos(2pfct+2pkf m(t)dt) • Very hard to analyze for general m(t). • Let m(t)=cos(2pfmt): Bandwidth fm • Using Fourier Series analysis: • Df <<fm b<<1Jn(b)0 for n>1  B 2fm=2Bm • If Df >>fm, significant components up to fc±Df.

  5. FM signal demodulation • filtering and Limiting the transmitted signal. • Differentiation to obtain the phase information in the modulated signal. • There are four ways to implement differentiation: • Differentiator and Envelope Detector • Zero Crossing Detector • Uses rate of zero crossings to estimate fi • Phase Lock Loop (PLL) • Uses VCO and feedback to extract m(t) • Phase-Shift or Quadrature Detection

  6. Quadrature-Mixer • The mixing to the baseband is carried out by the multiplication of the FM signal and a complex oscillator ejwTn and a low pass filter • The input signal is: • SFM(n) = A . cos (wTn +FFM (n)) • And the output signal of the mixer is: • Sbasis(n) = • = A/2 . cos (FFM (n)) + jA/2 . sin (FFM (n))

  7. Real quadrature-mixer The mixer can also be realized with real signals by multiplying the FM signal with a sine and cosine oscillation signal Sreal(n) = A/2 . cos (FFM(n)) Simag(n)= A/2 . sin (FFM(n))

  8. Digital Limiter • The received FM signal due to distortion in the channel is not known • However demodulators need a constant amplitude which is achieved by normalizing the magnitude of the signal

  9. Baseband Delay Demodulator • Delay demodulator needs the FM-Signal in the baseband • For this a quadrature mixing has to be done first

  10. Real Baseband Delay Demodulator

  11. Continued • An approximation of the actual phase angle is sufficient for computing arcsine function • Several popular methods include: Table look up, Taylor-series approximation, and polynomial fitting • The signal after the arcsine must be limited between −p/2 andp/2 to be clearly defined • ( )

  12. Phase-Adapter Demodulator • The signal after the arc tangent function g (n) = FFM (n) must be limited between −p/2 and p/2 to be clearly defined • For very low message frequencies, the maximal derivation will be very low and not practicable for most applications. • Therefore this demodulator is only useful for narrowband FM.

  13. Mixed Demodulator • A combination of the delay demodulator and the phase adapter demodulator.

  14. Direct Digital Synthesizers • DDSs also called Numerically Controlled Oscillators • Directly Synthesize a Selectable Output Frequency from a Clock Using Digital Techniques • Types of DDSs • Pulse Output • Sine Output • Fractional Divider • Fractional Divider Phase Interpolation • Other

  15. Sine Output DDS

  16. 5-Bit DAC 11-Bit DAC 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 dBc fo=333.25 KHz fc=1 MHz Span=10 KHz Res BW=10 Hz Typical Sine Output DDS Spectrums

  17. DDS with modulation capabilities

  18. Quadrature Outout of DDS

  19. Summery • Review of FM modulation and demodulation • Implementation of digital mixer, limiter, demodulator • Produce of quadrature signal with direct digital synthesizer

  20. References • Franz Schnyder-Christoph Haller, “Implementation of FM Demodulator Algorithms on a High Performance Digital Signal Processor” , Diploma Thesis-2002 • James Micheal Shima, “FM Demodulation Using A Digital Radio And Digital Signal Processing”, Master of Science Thesis University of Florida 1995 • Jouko Vankka,” Direct Digital Synthesizers: Theory, Design and Applications”, PhD Thesis Helsinki University of Technology • November 2000

More Related