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Modulación Analógica (AM-FM) (Parte II)

Modulación Analógica (AM-FM) (Parte II). Cx Eléctricas 08 – E.Tapia. FM Discriminator: S2N. Cont’. The carrier power has noise quoting effect in FM Recall that The average signal transmitted power is k f 2 P. How can we improve S2N in FM?. The conclusion.

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Modulación Analógica (AM-FM) (Parte II)

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  1. Modulación Analógica (AM-FM) (Parte II) Cx Eléctricas 08 – E.Tapia

  2. FM Discriminator: S2N

  3. Cont’ • The carrier power has noise quoting effect in FM • Recall that • The average signal transmitted power is kf2P

  4. How can we improve S2N in FM?

  5. The conclusion • FM provides a mechanism for the exchange of improved noise performance by increased transmission bandwidth • FM can also reject other FM signals closed to the carrier frequency provided interferent signal are weaker w.r.t. the target FM input

  6. Threshold Effect in FM • Assumption • Carrier to Noise ratio at the discriminator input >> 1 • Violation to this assumption • FM receiver breaks. From breaks to sputtering sounds. The formula does not hold.

  7. No signal but Noise • Ac >> nI , nQ • Ac << nI , nQ • P1 noves to the origin and random phase is observed is around

  8. Alternatevely • Clicks are heard after the low pass filter

  9. Threshold Effect • As  is decreased the rate of clicks grows • Rate of clicks is high threshold occurs

  10. Designing an FM System • Given D () • Compute BT • Given BT and N0 (Noise power per unit bandwidth) • Determine AC to keep above the threshold

  11. FM Threshold Reduction • FM demodulator with negative feeback (FMFB) or PLL

  12. FM Threshold Reduction The VCO output The phase comparator output

  13. FM Threshold Reduction (cont)

  14. FM Threshold Reduction (cont)

  15. FM Threshold Reduction (cont)

  16. Linear Model of the PLL-FM Demodulator

  17. PreEmphasis - Deemphasis • Pre at transmitter • De- at the receiver

  18. Pre-emphasis & De-emphasis • Pre at transmitter • De- at the receiver

  19. Conclusions

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