1 / 63

F1 x F2 

Sum and Mixing of Frequencies. F1 x F2 . eam=EcSin(Wct)+mEc/2Cos(Wc-Wm)t-mEc/2Cos(Wc+Wm)t. USB. LSB. Carrier. f USB = fc + fm and f LSB = fc − fm. Figure 3-8: The relationship between the time and frequency domains. Sidebands and the Frequency Domain. E max − E min. Ea =.

ima
Télécharger la présentation

F1 x F2 

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. Sum and Mixing of Frequencies F1 x F2 

  2. eam=EcSin(Wct)+mEc/2Cos(Wc-Wm)t-mEc/2Cos(Wc+Wm)t USB LSB Carrier fUSB = fc + fm and fLSB = fc − fm

  3. Figure 3-8: The relationship between the time and frequency domains. Sidebands and the Frequency Domain

  4. Emax−Emin Ea = 2 Calculatiom of modulation index by envelope m =Ea / Ec Ec = Emax - Ea

  5. Ec mEc/2 mEc/2 Fc Fc+Fm Fc-Fm BW = fUSB−fLSB=2fm Ec/2 Ec/2 mEc/4 mEc/4 mEc/4 mEc/4 -Fc Fc-Fm Fc Fc+Fm -Fc-Fm -Fc+Fm Two sided spectrum

  6. Property of Active Device

  7. Block Diagram of a Simple AM Transmitter

  8. Power relations in AM PT = (IT)2R where IT is measured RF current and R is antenna impedance PT=PC+PUSB+PLSB

  9. Power relations in AM in terms of current PT=PC+PUSB+PLSB PT = (IT)2R Pc = (Ic)2R But

  10. Transmission Efficiency Useful Power /Total power Percent efficiency

  11. Modulation by several sinewaves Two modulating signals are given by X1(t)=Em1CosWm1t X2(t)=Em2CosWm2t ec=Ec CosWct Carrier wave eam=A CosWct where A=Ec+X1(t)+X2(t) eam= (Ec+Em1CosWm1t + Em2CosWm2t) CosWct eam= Ec(1+Em1/Ec CosWm1t + Em2/ Ec CosWm2t) CosWct eam= Ec(1+m1 CosWm1t + m2 CosWm2t) CosWct eam= Ec CosWct+m1Ec/2 Cos(Wc+Wm1)t + m1Ec/2 Cos(Wc-Wm1)t+m2Ec/2 Cos(Wc+Wm2)t+m2Ec/2 Cos(Wc-Wm2)t Ec m2Ec/2 m1Ec/2 m1Ec/2 BW=2fm2 m2Ec/2 Fc-fm2 Fc Fc+fm2 Fc+fm1 Fc-fm1

  12. Total power in AM Wave= Pt=PUSB1+PUSB2+PLSB1+PLSB2 Modulation Index

  13. Power relations in AM PT = (IT)2R where IT is measured RF current and R is antenna impedance PT=PC+PUSB+PLSB

  14. Power relations in AM in terms of current PT=PC+PUSB+PLSB PT = (IT)2R Pc = (Ic)2R But

  15. Transmission Efficiency Useful Power /Total power Percent efficiency

  16. Modulation by several sinewaves Two modulating signals are given by X1(t)=Em1CosWm1t X2(t)=Em2CosWm2t ec=Ec CosWct Carrier wave eam=A CosWct where A=Ec+X1(t)+X2(t) eam= (Ec+Em1CosWm1t + Em2CosWm2t) CosWct eam= Ec(1+Em1/Ec CosWm1t + Em2/ Ec CosWm2t) CosWct eam= Ec(1+m1 CosWm1t + m2 CosWm2t) CosWct eam= Ec CosWct+m1Ec/2 Cos(Wc+Wm1)t + m1Ec/2 Cos(Wc-Wm1)t+m2Ec/2 Cos(Wc+Wm2)t+m2Ec/2 Cos(Wc-Wm2)t Ec m2Ec/2 m1Ec/2 m1Ec/2 BW=2fm2 m2Ec/2 Fc-fm2 Fc Fc+fm2 Fc+fm1 Fc-fm1

  17. Total power in AM Wave= Pt=PUSB1+PUSB2+PLSB1+PLSB2 Modulation Index

  18. Amplitude Modulators • There are two types of amplitude modulators. They are low-level and high-level modulators. • Low-level modulators generate AM with small signals and must be amplified before transmission. • High-level modulators produce AM at high power levels, usually in the final amplifier stage of a transmitter. • Modulators are class C amplifiers and at output tank circuit.

  19. Low level AM Transmitter 540KHz to 1640KHz

  20. 540KHz to 1640KHz modulator

  21. Amplifier Classes Class A - bias point is set so that the amplifier conducts through a complete cycle (360 deg) of the input waveform. This class has low efficiency (~35%) but high linearity. Class AB - bias point is set so that the amplifier conducts through at least 180 deg but less than 360deg of the input waveform. This class has better efficiency (~55%) but lower linearity. Class B - bias point is set so that the amplifier conducts through a half cycle (180 deg) of the input waveform. This class has higher efficiency (~60%),but poor linearity. Class C - bias point is set so that the amplifier conducts through less than 180 deg of the input waveform. This class has higher efficiency (~70%), but even poorer linearity

  22. Use of Tank circuit

  23. Low level Class C Grid Modulator

  24. High level Plate Modulator

  25. Low level Transistor Modulator

  26. Low level and High level AM Transmitter 540KHz to 1640KHz modulator

  27. Advantages of DSBFC = 1.Transmitters are less complex 2.Receivers are simple, detection is easy. 3.Cost efficient. Disadvantages = 1.Power wastage - carrier doesn’t carry any information and USB & LSB contains same information. 2. Needs larger Bandwidth 3. Gets affected by noise.

  28. Balanced Modulator Modulating signal • Types of AM= • DSBFC • DSBSC • SSB • ISB • VSB 180 phase shift DSBSC carrier

  29. mEc/2 mEc/2 Fc Fc+Fm Fc-Fm • DSB-SC Generation Methods • Ring Balanced Modulator • Lattice Balanced Modulator • Push pull Balanced modulator eam=mEc/2Cos(Wc-Wm)t-mEc/2Cos(Wc+Wm)t 180 phase shift Ec BW=2fm

  30. Balanced Modulator 1.Ring modulator 2.Lattice-type balanced modulator.

  31. Lattice Modulator + - - +

  32. Push Pull Balanced Modulator Drain Current inet = aem + 2becem Modulating signal Two side bands

  33. AM Waveforms

  34. mEc/2 mEc/2 mEc/2 mEc/2 Fc Fc+Fm Fc-Fm Fc Fc+Fm Fc-Fm • SSB Generation Methods • Filter Method • Phase shift method • Third method (Weaver method) BW=fm

  35. SSB Circuits Figure 4-31 An SSB transmitter using the filter method.

  36. This technique can be used at relatively low carrier frequencies. At high frequencies, the Q of the filter becomes unacceptably high. The required Q necessary to filter off one of the sidebands can be approximated by:

  37. SSB Circuits Figure 4-33 An SSB generator using the phasing method.

  38. SSB phase shift

  39. Important points= • Sharp cutoff Filters are not required • Freq Up conversion is not required • Easy to switch between sidebands. Simply change the oscillator position. • Designing a phase shift network for AF range is dificult.

  40. Weaver Method or Third method LSB

  41. Independent side band transmitter 10MHz to 30MHz

More Related