AM Modulation PDF

LEC 04 – Amplitude Modulation FOEC - Lec03 1 Modulation FOEC - Lec03 2 Modulation ▪ Modulation is the process of imposing information contained in a lower- frequency electronic signal (modulating signal) into a higher-frequency signal (carrier). ▪ Why use c...

LEC 04 – Amplitude Modulation FOEC - Lec03 1 Modulation FOEC - Lec03 2 Modulation ▪ Modulation is the process of imposing information contained in a lower- frequency electronic signal (modulating signal) into a higher-frequency signal (carrier). ▪ Why use carriers? ▪ Assume that person A wants to send an audio signal (20Hz-20kHz). ▪ Why use higher frequencies as carriers? ▪ Antenna length is typically length = wavelength/4. FOEC - Lec03 3 AM Introduction + Modulation Index FOEC - Lec03 4 Amplitude Modulation FOEC - Lec03 5 Amplitude Modulation FOEC - Lec03 6 Amplitude Modulation ▪ Remember our mixing principle! FOEC - Lec03 7 Amplitude Modulation ▪ Remember our mixing principle! 𝑚𝑉𝑐 𝑚𝑉𝑐 𝑣 = 𝑉𝑐 cos 2𝜋𝑓𝑐 𝑡 + cos 2𝜋(𝑓𝑐 −𝑓𝑚 )𝑡 + cos 2𝜋(𝑓𝑐 +𝑓𝑚 )𝑡 2 2 FOEC - Lec03 8 Modulation Index and Percent of Modulation ▪ For undistorted AM to occur, the modulating signal voltage Vm must be less than the carrier voltage Vc. ▪ It is important to determine the modulation index (also known as modulating factor, modulating coefficient, degree of modulation). 𝑉𝑚 𝑚= 𝑉𝑐 ▪ m should be a number between 0 and 1. Ideal condition of AM is when Vm = Vc. FOEC - Lec03 9 Vmax and Vmin FOEC - Lec03 10 Vmax and Vmin 𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛 𝑉𝑚 = 𝑝𝑒𝑎𝑘 𝑜𝑓 𝑚𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑠𝑖𝑔𝑛𝑎𝑙 2 𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛 𝑉𝑐 = (𝑝𝑒𝑎𝑘 𝑜𝑓 𝑐𝑎𝑟𝑟𝑖𝑒𝑟 𝑠𝑖𝑔𝑛𝑎𝑙) 2 FOEC - Lec03 11 Modulation Index using Vmax and Vmin 𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛 𝑉𝑚 = 𝑉𝑚𝑎𝑥 + 𝑉𝑚𝑖𝑛 FOEC - Lec03 12 Overmodulation and Distortion ▪ m should be a number between 0 and 1. If m is greater than 1, overmodulation occurs. Overmodulation introduces distortion of the envelope. FOEC - Lec03 13 Modulation Index Sample Problem ▪ Suppose that on an AM Signal, Vmax = 6V and Vmin = 1.2V. What is (1) m, (2) Vm, and (3) Vc? FOEC - Lec03 14 Sidebands and the Frequency Domain FOEC - Lec03 15 Sideband Calculations ▪ Using the mixing principle, two new frequencies are generated with AM. These are called the upper sideband and the lower sideband. 𝑓𝑈𝑆𝐵 = 𝑓𝑐 + 𝑓𝑚 𝑓𝐿𝑆𝐵 = 𝑓𝑐 − 𝑓𝑚 FOEC - Lec03 16 Sideband Calculations ▪ Using the mixing principle, two new frequencies are generated with AM. These are called the upper sideband and the lower sideband. 𝑓𝑈𝑆𝐵 = 𝑓𝑐 + 𝑓𝑚 𝑓𝐿𝑆𝐵 = 𝑓𝑐 − 𝑓𝑚 FOEC - Lec03 17 Time Domain Representation of AM FOEC - Lec03 18 Frequency Domain Representation of AM FOEC - Lec03 19 Time/Frequency Domain Representation of AM FOEC - Lec03 20 Sideband Calculation Sample Problem ▪ Voice frequencies occur in the 300- to 3000- Hz range. What is the (1) f_usb and (2) f_lsb if a carrier frequency of 2.8MHz is used to transmit voice frequencies? What is the (3) total bandwidth? ▪ **take the sum and difference of the carrier frequency and the maximum modulating frequency. FOEC - Lec03 21 Sideband Calculation Sample Problem FOEC - Lec03 22 AM Power FOEC - Lec03 23 AM Signal ▪ The AM signal is really a composite of several signal voltages, namely, the carrier and the two sidebands, and each of these signals produces power in the antenna. ▪ The total transmitted power PT is simply the sum of the carrier power Pc and the power in the two sidebands PUSB and PLSB FOEC - Lec03 24 AM Power FOEC - Lec03 25 AM Power ▪ Vc and Vm are peak values of the carrier and modulating sine waves. But for power calculations, the rms values must be used for the voltages FOEC - Lec03 26 AM Power ▪ Vc and Vm are peak values of the carrier and modulating sine waves. But for power calculations, the rms values must be used for the voltages FOEC - Lec03 27 AM Power FOEC - Lec03 28 AM Power Sample Problem ▪ For example, if the carrier of an AM transmitter is 1000 W and it is modulated 100 percent (m = 1), what is (1) the total AM power (2) total sideband power (3) power at one sideband (4) the ratio of the total sideband to the total AM power FOEC - Lec03 29 AM Power Sample Problem ▪ For example, if the carrier of an AM transmitter is 1000 W and it is modulated 100 percent (m = 1), what is (1) the total AM power = 1500 W (2) total sideband power = 500 W (3) power at one sideband = 250 W (4) the ratio of the total sideband to the carrier power = 50% When m = 1, the total sideband power is one-half that of the carrier power. FOEC - Lec03 30 AM Power Sample Problem ▪ For example, if the carrier of an AM transmitter is 1000 W and it is modulated 100 percent (m = 0.75), what is (1) the total AM power (2) total sideband power (3) power at one sideband (4) the ratio of the total sideband to the total AM power FOEC - Lec03 31 AM Power Sample Problem ▪ For example, if the carrier of an AM transmitter is 1000 W and it is modulated 100 percent (m = 1), what is (1) the total AM power = 1500 W (2) total sideband power = 500 W (3) power at one sideband = 250 W (4) the ratio of the total sideband to the carrier power = 1/2 (5) the ratio of the total sideband to the total AM power = 2/3 When m = 1, the total sideband power is one-half that of the carrier power. FOEC - Lec03 32 Single Sideband Modulation FOEC - Lec03 33 SSBs ▪ In amplitude modulation, two-thirds of the transmitted power is in the carrier, which itself conveys no information. The real information is contained within the sidebands. ▪ One way to improve the efficiency of amplitude modulation is to suppress the carrier and eliminate one sideband. The result is a single- sideband (SSB) signal. SSB is a form of AM that offers unique benefits in some types of electronic communication FOEC - Lec03 34 DSB Signals ▪ The first step in generating an SSB signal is to suppress the carrier, leaving the upper and lower sidebands. This type of signal is referred to as a double-sideband suppressed carrier (DSBSC or DSB) signal. The benefit, of course, is that no power is wasted on the carrier. FOEC - Lec03 35 SSB Signals ▪ In DSB transmission, since the sidebands are the sum and difference of the carrier and modulating signals, the information is contained in both sidebands. ▪ One sideband can be suppressed; the remaining sideband is called a single- sideband suppressed carrier (SSBSC or SSB) signal. ▪ Spectrum space only occupies one half of that of the AM and DSBSC. ▪ Power previously devoted to the carrier and the other sideband can be channeled into a single sideband. ▪ Since SSB signals occupy a narrower bandwidth, the amount of noise is reduced. FOEC - Lec03 36