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FM Demodulation

Frequency Discriminator f d , k d. f in. v out. FM Demodulation. v out. f d. f in. k d. Phase Locked Loop. “Average” Phase Error. Phase Error: v PM. LPF ~ 5 Hz. VCO f 0 , k o , D f 0. FM in. Phase Demodulation. Phase Locked Loop. v FM. LPF ~ 15 KHz. VCO

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FM Demodulation

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  1. Frequency Discriminator fd, kd fin vout FM Demodulation vout fd fin kd

  2. Phase Locked Loop “Average” Phase Error Phase Error: vPM LPF ~ 5 Hz VCO f0, ko, Df0 FMin Phase Demodulation

  3. Phase Locked Loop vFM LPF ~ 15 KHz VCO f0, ko, Df0 FMin Frequency Demodulation

  4. Slope Detection fc AM Detector vFM FMIN = fc +Df(t) Df

  5. Balanced Slope Detector fc AM Detector FMIN Df + vFM _ fc AM Detector Df

  6. Foster Seeley Discriminator Analysis High Frequencies Low Frequencies Cc Vp |VA| + V1 _ VA + Vout = |VA| – |VB| (2) _ RDC + Vp @ w _ RFC _ V2 + Cp Lp Cs Ls RDC k VB |VB| Vs = V1 – V2 V1 = Vs/2 V2 = -Vs/2 VA = Vp + V1 = Vp + Vs/2 (1a) VB = Vp + V2 = Vp – Vs/2 (1b) Redraw secondary circuit for Analysis @ w . . . .

  7. Find secondary voltage (Vs) as a function of Primary Voltage (Vp). Simplify Secondary Circuit for RF: Ls + VS _ XT/Qs + VS _ Ls RDC /2 QuXT Cs Cs + _ RDC /2 + _ Vi = Vpk(ns/np) Can neglect for high Qs Cannot neglect, or denominator will go to zero at resonance

  8. (2) Vout = |VA| – |VB| + VS _ XL/Qs Ls Cs + _ Vi = Vpk(ns/np)

  9. VB -Vs/2 Vp Vs/2 VA Vs

  10. Ratio Detector Cc Vp RDC + Vp @ w _ RFC Vout Cp Lp Cs Ls RDC k

  11. Quadrature/Coincidence Detectors FMIN vout

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