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Modula si

Modula si. Oleh Risanuri Hidayat. Introduction. Baseband signal = electrical replica of the message itself, such baseband signal is not suitable for transmission over the transmission medium Carrier signal = another electrical signal is used to carry the baseband signal

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Modula si

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  1. Modulasi Oleh Risanuri Hidayat

  2. Introduction • Baseband signal = electrical replica of the message itself, such baseband signal is not suitable for transmission over the transmission medium • Carrier signal = another electrical signal is used to carry the baseband signal • Modulation = Process that modify carrier signal according to the input signal • Modulation leads to frequency “translation” • Modulation Method AM, FM, PAM, PCM, etc. • Reason for modulation : for ease of radiation/reception, for frequency translation to assigned band, and for multiplexing modulasi

  3. Isyarat Sinus modulasi

  4. Amplitude Modulation[AM] • Amplitude of a sinusoidal carrier is made to change according to the “instantaneous” value of message • In general, the modulating signal such as voice or music is a complex waveform consist of bands of frequency, thus the modulated AM wave consists of two sidebands for frequency • Great disadvantage: In the a-m receiver, interference has the same effect on the r-f signal as the intelligence being transmitted because they are of the same nature and inseperable. • There are various forms of AM 1. Double sideband - suppressed carrier [DSB-SC] 2. Single sideband - suppressed carrier [SSB-SC] 3. Double sideband - full carrier [DSB-FC] (envelope AM) modulasi

  5. Amplitude Modulation[AM modulasi

  6. Percentage  of  Modulation • In amplitude modulation, it is common practice to express the degree to which a carrier is modulated as a percentage of modulation.  • When the peak-to-peak amplitude of the modulationg signal is equal to the peak-to-peak amplitude of the unmodulated carrier, the carrier is said to be 100 percent modulated.  • The actual percentage of modulation of a carrier (M) can be calculated by using the following simple formula M = percentage of modulation • M= ((Emax - Emin) / (Emax + Emin)) * 100 • where Emax is the greatest and Emin the smallest peak-to-peak amplitude of the modulated carrier.  • For example, assume that a modulated carrier varies in its peak-to-peak  amplitude from 10 to 30 volts.  • M = ((30 - 10) / (30 + 10)) * 100 = (20 / 40) * 100 = 50 percent.  • This formula is accurate only for percentages between 0 and 100 percent modulasi

  7. Percentage  of  Modulation modulasi

  8. Percentage  of  Modulation modulasi

  9. Percentage  of  Modulation • This results in a distorted signal, and the intelligence is received in a distorted form.  • Therefore, the percentage of modulation in a-m systems of communication is limited to values from 0 to 100 percent. modulasi

  10. Side  Bands • When the outputs of two oscillators beat together, or hetrodyne, the two original frequencies plus their sum and difference are produced in the output.  This heterodyning effect also takes place between the a-f signal and the r-f signal in the modulation process and the beat frequencies produced are known as side bands.  • Assume that an a-f signal whose frequency is 1,000 cps (cycles per second) is modulating an r-f carrier of 500 kc (kilocycles). The modulated carrier consists mainly of three frequency components: the original r-f signal at 500 kc, the sum of the a-f and r-f signals at 501 kc, and the difference between the a-f and r-f signals at 499 kc.  • The component at 501 kc is known as the uppersideband, and the component at 499 kc is known as the lower side band.  Since these side bands are always present in amplitude modulation, the a-m wave consists of a center frequency, an upper side-band frequency, and a lower side-band frequenmcy.  modulasi

  11. Side  Bands • The carrier with the two sidebands, with the amplitude of each component plotted against its frequency, is represented in figure.  • The modulating signal, fA, beats against the carrier, fC, to produce upper side band fH and lower side band fL.  • The modulated carrier occupies a section of the radio-frequency spectrum extending from fL to fH, or 2 kc.  • To receive this signal, a receiver must have r-f stages whose bandwidth is at least 2 kc.  When the receiver is tuned to 500 kc, it also must be able to receive 499 kc and 501 kc with relatively little loss in response. modulasi

  12. Side  Bands modulasi

  13. Side  Bands • The audio-frequency range extends approximately from 16 to 16,000 cps.  • To accommodate the highest audio frequency, the a-m frequency channel should extend from 16 kc below to 16 kc above the carrier frequency, with the receiver having a corresponding bandwidth.  • Therefore, if the carrier frequency is 500 kc, the a-m channel should extend from 484  to 516 kc.  (Double Side Band) • This bandwidth represents an ideal condition; in practice, however, the entire a-m bandwith for audio reproduction rarely exceeds 16 kc. • For any specific set of audio-modulating frequencies, the a-m channel or bandwidth is twice the highest audio frequency present. modulasi

  14. Side  Bands • The r-f energy radiated from the transmitter antenna in the form of a modulated carrier is divided among the carrier and its two side bands.  With a carrier componet of 1,000 watts, an audio signal of 500 watts is necessary for 100-percent modulation.  Therefore, the modulated carrier should not exceed a total power of 1,500 watts.  The 500 watts of audio power is divided equally between the side bands, and no audio power is associated with the carrier. • Since none of the audio power is associated with the carrier component, it contains none of the intelligence.  From the standpoint of communication efficiency, the 1,000 watts of carrier-component power is wasted.  Furthermore, one side band alone is sufficient to transmit intelligence.  • It is possible to eliminate the carrier and one side band, but the complexity of the equipment needed cancels the gain in efficiency. modulasi

  15. Double sideband Full carrier • Full AM contains TWO sidebands, hence it is known as Double Sideband Full Carrier [DSB-FC] • Information is carried by two (duplicating) sidebands [as such one is redundant] • Hence, it is possible to transmit with only one of the sidebands which is known as Sinble Sideband • Envelope AM or Full-AM requires two times bandwidth of SSB-AM • Full-AM wasteful on part of transmitting power, but requires simple demodulation circuit on the receiver side (e.g. in case of millions receivers of broadcasting radio) modulasi

  16. -Fc +Fc 0 DSBFC DSB-SC carrier modulasi

  17. Message, m(t) Message + d.c., 1+m(t) Envelope modulated signal DSBFC modulasi

  18. Half-wave rectifier circuit + LPF C R LPF RC time constant RC f RC f Demodulation DSBFC modulasi

  19. RC f RC f Diagonal Clipping • In the design of an envelope detector, the RC time constant of the LPF is a critical parameter • Too small a value of RC time constant results to too much ripple • Too large a RC make it unable to follow fast fall in modulating signal envelope modulasi

  20. then Double sideband suppressed carrier Let Message signal Carrier signal modulasi

  21. DSBSC modulasi

  22. XAM(t) message LPF Carrier replica XC(t) Y(t) LPF Demodulation [DSB-SC] message DSB-SC modulasi

  23. Single sideband suppressed carrier • The main advantages of SSB-SC are 1. Only half of the bandwidth is required, hence the effective channel capacity is doubled 2. Smaller transmitter results from suppressing the carrier (containing 66.7% of the power), and one other sideband (another 16.7%) 3. Better SNR [Signal to Noise Ratio] Note : Smaller the bandwidth - Higher SNR Remember!! Noise Power : Pn = kTB modulasi

  24. SSBSC • Converting DSB-SC to SSB-SC can be achieved in a number of ways 1.By Filtering The high-pass filter must change from Full attenuation to Zero attenuation over a range of carrier frequency, hence the carrier frequency can be kept reasonably low 2.Mixer Due to the limitation of real filters available, in practice two frequency translations are necessary to obtain SSB-SC at the desired tramsmitter frequency modulasi

  25. DSB-SC SSB-SC HPF DSB-SC 0 SSB-SC 0 SSBSC modulasi

  26. XAM(t) Z(t) Y(t) LPF Carrier replica XC(t) Demodulation [SSB-SC] modulasi

  27. rms value Power into 1 ohm of resistance carrier Two sidebands Power Relationship Root Mean Square value [rms] modulasi

  28. Let Am=1; total transmitted Power: Power Relationship AM DSBFC modulasi

  29. Power Relationship AM • Double Sideband Suppressed Carrier has the potential to save up to 66.7% of power ((Ptotal-Pcarrier)/Ptotal) • Single Sideband Suppressed Carrier can save up to 83.3% of power (100 – 16.7). That is one sideband contains 16.7% of the transmitting power modulasi

  30. Phase  Modulation • the frequency or phase of the carrier can be varied to produce a signal bearing intelligence.  • The process of varying the frequency in accordance with the intelligence is frequency modulation, and the process of varying the phase is phase modulation.  • When frequency modulation is used, the phase of the carrier wave is indirectly affected.  Similarly, when phase modulation is used, the carrier frequency is affected modulasi

  31. Phase  Modulation • The starting point for measuring time is chosen arbitrarily, and at 0 time, curve A has some negative value.  If another curve B, of the same frequency is drawn having 0 amplitude at 0 time, it can be used as a reference in describing curve A. modulasi

  32. Vector Representation modulasi

  33. Vector Representation modulasi

  34. Vector Representation • For each cycle of the modulating signal, the relative phase of the carrier is varied between the values of (f+Df) and (f-Df).  • These two values of instantaneous phase, which occur at the maximum positive and maximum negative values of modulation, are known as the phase-deviation limits.  • The upper limit is +Df; the lower limit is -Df. modulasi

  35. Vector Representation modulasi

  36. modulasi

  37. Modulation constant, frequency deviation constant freq. Unmodulated carrier fmax : max. frequency deviation, fd fc m(t) Vmax Frequency Modulation[FM] The carrier frequency fi is made to vary according to the instantaneous amplitude of the message modulasi

  38. Message Unmodulated carrier FM signal FM modulasi

  39. since FM Signal Analysis modulasi

  40. modulasi

  41. Multiplexing • Multiplexing is a process of combining serveral information channels so as to share a common Transmission Channel, without mutual interference • FDM [Frequency Division Multiplexing] is a method of multiplexing based on frequency translation consideration • TDM [Time Division Multiplexing] is another mean of multiplexing based on time allocation consideration modulasi

  42. fc1 SSB mod SSB mod SSB mod LPF f1 fc2 MUX LPF f2 O/P X(t) fc3 LPF f3 Guardband To band limit each input signal to avoid interference f fc1+f1 fc2 fc2+f2 fc3 fc3+f3 fc1 Frequency Division Multiplexing[FDM] modulasi

  43. CCITT FDM Hierachy Channel 1ch 4kHz Group 12ch 48kHz Super group 60ch 240kHz Master group 300ch 1.2MHz Super Master group 900ch 3.6MHz modulasi

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