1 / 14

Analog Communications

Analog Communications. Amplitude Modulation. Double Side Band Suppressed Carrier. Contents. Theory Implementation Transmitter Detector Synchronous Square Power analysis Summary. Carrier. USB. LSB.  c -  m  c  c +  m. Single frequency.

galena-cabrera
Download Presentation

Analog Communications

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. Analog Communications Amplitude Modulation T.Srinivasa Rao

  2. Double Side Band Suppressed Carrier T.Srinivasa Rao

  3. Contents • Theory • Implementation • Transmitter • Detector • Synchronous • Square • Power analysis • Summary T.Srinivasa Rao

  4. Carrier USB LSB c - m c c + m Single frequency Double Side Band Suppressed Carrier From AM spectrum: • Carries signal c carries no information m. • Carries signal consumes a lot of power more than 50% Question: Why transmit carrier at all? Ans: Question: Can one suppress the carrier? Ans.:Yes, just transmit two side bands (i.e DSB-SC) But what is the penalty? System complexity at the receiver T.Srinivasa Rao

  5. upper side band lower side band DSB-SC - Theory General expression: Let k1 = 1, C = 0 and c = 0, the modulated carrier signal, therefore: Information signal m(t) = Am cos 2πfmt Thus T.Srinivasa Rao

  6. B = 2m Notice: No carrier frequency DSB-SC - Waveforms Mixer (Multiplier) T.Srinivasa Rao

  7. Ac (1+ 0.5 m(t) cos 2 π fct 0.5 m(t) + Carrier Ac cos 2πfct DSB-SC Acm(t) cos 2 π fct - -0.5 m(t) Ac (1- 0.5 m(t) cos 2 π fct DSB-SC - Implementation • Balanced modulator AM modulator + AM modulator • Ring modulator • Square-law modulator T.Srinivasa Rao

  8. Low pass filter DSB-SC Local oscillator c(t) = cos 2πfct information Low pass filter m 2c-m 2c+m DSB-SC - Detection • Synchronous detection Multiplier Message signal • Condition: • Local oscillator has the same • frequency and phase as that of the • carrier signal at the transmitter. high frequency T.Srinivasa Rao

  9. Low pass filter DSB-SC Multiplier Message signal Local oscillator c(t) = cos(ct+) information Low pass filter m 2c-m 2c+m DSB-SC - Synchronous Detection • Case 1 - Phase error • Condition: • Local oscillator has the same • frequency but differentphase • compared to carrier signal at the • transmitter. high frequency T.Srinivasa Rao

  10. In-phase yip(t) 0.5Ec m(t) cos  X DSB-SC LPF Ec cos (ct+) Vphase(t) Phase discriminator VCO 90o phase shift Ec sin (ct+) 0.5Ec m(t) sin  X. LPF yqp(t) Quadrature-phase Phase Synchronisation -Costa Loop Recovered signal • When there is no phase error. The quadrature component is zero • When 0, yip(t) decreases, while yqp(t) increases • The out put of the phase discriminator is proportional to  T.Srinivasa Rao

  11. Low pass filter DSB-SC Multiplier Message signal Local oscillator c(t)=Eccos(ct+) information Low pass filter m 2c-m 2c+m DSB-SC - Synchronous Detection • Case 1 - Frequency error • Condition: • Local oscillator has the same • phase but different frequency • compared to carrier signal at the • transmitter. high frequency T.Srinivasa Rao

  12. g(t) Squaring circuit g =x2 Band pass filter C 2c DSB-SC - Square Detection y(t) DSB-SC Si(t)  by 2 Regenerated carrier z(t) Multiplier Message signal Low pass filter g(t) = Si2(t) = B2 cos2mt cos2ct = B2 (½ + ½ cos 2 mt )(½ + ½ cos 2 ct ) = B2/4 [1 + ½ cos 2(c + m)t + ½ cos 2(c - m)t + cos 2mt + cos 2 ct ] y(t) = B2/4 cos 2wct z(t) = B2/4 cos wct T.Srinivasa Rao

  13. DSB-SC - Power • The total power (or average power): • The maximum and peak envelop power T.Srinivasa Rao

  14. DSB-SC - Summary • Advantages: • Lower power consumption • Disadvantage: • - Complex detection • Applications: • - Analogue TV systems: to transmit colour information • - For transmitting stereo information in FM sound broadcast • at VHF T.Srinivasa Rao

More Related