Modulasi Oleh Risanuri Hidayat modulasi Introduction Baseband signal
Modulasi Oleh Risanuri Hidayat modulasi
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 11/29/2020 modulasi 2
Isyarat Sinus 11/29/2020 modulasi 3
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) 11/29/2020 modulasi 4
Amplitude Modulation[AM 11/29/2020 modulasi 5
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 11/29/2020 modulasi 6
Percentage of Modulation 11/29/2020 modulasi 7
Percentage of Modulation 11/29/2020 modulasi 8
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. 11/29/2020 modulasi 9
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 upper sideband, 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. 11/29/2020 modulasi 10
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, f. A, beats against the carrier, f. C, to produce upper side band f. H and lower side band f. L. • The modulated carrier occupies a section of the radiofrequency spectrum extending from f. L to f. H, 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. 11/29/2020 modulasi 11
Side Bands 11/29/2020 modulasi 12
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. 11/29/2020 modulasi 13
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. 11/29/2020 modulasi 14
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) 11/29/2020 modulasi 15
DSBFC DSB-SC carrier -Fc 11/29/2020 0 +Fc modulasi 16
DSBFC Message, m(t) Message + d. c. , 1+m(t) Envelope modulated signal 11/29/2020 modulasi 17
Demodulation DSBFC Half-wave rectifier circuit + LPF R C LPF RC time constant RC 11/29/2020 f RC modulasi f 18
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 RC f • Too large a RC make it unable to follow fast fall in modulating signal envelope 11/29/2020 RC modulasi f 19
Double sideband suppressed carrier Let Message signal Carrier signal then 11/29/2020 modulasi 20
DSBSC 11/29/2020 modulasi 21
Demodulation [DSB-SC] XAM(t) message LPF Carrier replica XC(t) message DSB-SC Y(t) 11/29/2020 modulasi LPF 22
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 = k. TB 11/29/2020 modulasi 23
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 11/29/2020 modulasi 24
SSBSC DSB-SC HPF SSB-SC DSB-SC 0 SSB-SC 0 11/29/2020 modulasi 25
Demodulation [SSB-SC] XAM(t) Y(t) LPF Z(t) Carrier replica XC(t) 11/29/2020 modulasi 26
Power Relationship Root Mean Square value [rms] rms value Power into 1 ohm of resistance carrier Two sidebands 11/29/2020 modulasi 27
Power Relationship AM Let Am=1; total transmitted Power: DSBFC 11/29/2020 modulasi 28
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 11/29/2020 modulasi 29
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 11/29/2020 modulasi 30
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. 11/29/2020 modulasi 31
Vector Representation 11/29/2020 modulasi 32
Vector Representation 11/29/2020 modulasi 33
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. 11/29/2020 modulasi 34
Vector Representation 11/29/2020 modulasi 35
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Frequency Modulation[FM] The carrier frequency fi is made to vary according to the instantaneous amplitude of the message Unmodulated carrier Modulation constant, frequency deviation constant freq. fmax : max. frequency deviation, fd fc Vmax 11/29/2020 modulasi m(t) 37
FM Message Unmodulated carrier FM signal 11/29/2020 modulasi 38
FM Signal Analysis since 11/29/2020 modulasi 39
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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 11/29/2020 modulasi 41
Frequency Division Multiplexing[FDM] fc 1 LPF f 1 SSB mod fc 2 LPF f 2 MUX SSB mod O/P X(t) fc 3 LPF f 3 SSB mod Guardband To band limit each input signal to avoid interference 11/29/2020 fc 1+f 1 modulasi fc 2+f 2 fc 3+f 3 f 42
CCITT FDM Hierachy Channel 1 ch 4 k. Hz Group 12 ch 48 k. Hz Super group 60 ch 240 k. Hz Master group 300 ch 1. 2 MHz Super Master group 900 ch 3. 6 MHz 11/29/2020 modulasi 43
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