SSB VERSUS AM: TYPES OF SIDEBAND SIGNAL

1. SSB VERSUS AM: TYPES OF SIDEBAND SIGNAL • a conventional AM signal consists of a carrier or CFC, a lower sideband and an upper sideband, notice how we have shown the sidebands as ranges of frequency rather than individual frequencies. This is because a real information signal has a range of frequencies rather than just a single frequency. BW = 2fm(max) • the human speech is contained in the frequency range 300 hz to 3000 hz. Most systems that are intended to send only human voice are designed to reproduce the frequency range 300 z(fmin) to 3000 hz(fmax) 40W 10W 10W USB LSB F Fc Fc+fm(max) Fc-fm(max) Fc-fm(min) Fc+fm(min)

2. 40W 10W 10W USB LSB F Fc Fc+fm(max) Fc-fm(max) Fc-fm(min) Fc+fm(min) • EXAMPLE: • Speech is to be sent using an AM transmitter. The available carrier power is 40watt, as shown. The transmitter is operating at 100% modulation. • What is the total power? • Pt = Pc ( 1 + m2/2 ) = 60 W • b) What is the power of the information • Pinfo = Pside = Pc ( m2/2 )= 20 W • What bandwidth will be needed? • BW = 2fm(max)=6000hz=6 khz

3. 30W 30W USB LSB F Fc Fc+fm(max) Fc-fm(max) Fc-fm(min) Fc+fm(min) • DSB-SC Operation • Balanced modulator produces an AM signal with sidebands but no • carrier, this is called DSB-SC • However the resulting DSB-SC signal can no longer be properly • demodulated by a diode detector. Therefore an oscillator circuit called beat • frequency oscillator ( or BFO ) must be added to the detector circuit to • reinsert the missing carrier signal. • Since we are now transmitting additional information power, we say that we have gained a decibel power advantage over conventional AM transmitter. The decibel power advantage can be calculate: • decibel power advantage = Db=10log( (P2/P1)(BW1/BW2)2) • Where P2=new information power, P1=original information power • BW2=new bandwidth and BW1=original bandwidth

4. 30W 30W USB LSB F Fc Fc+fm(max) Fc-fm(max) Fc-fm(min) Fc+fm(min) • EXAMPLE: • If the total power of the transmittion shown is 60 watts and again human speech is to be transmitted, calculate: • the bandwidth of the DSB-SC signal • BW=unchanged • 2. the decibel power advantage of the DSB-SC signal over AM-FC signal • db = 10log(60w/20w)(6khz/6khz)2) = +4.77db ( 3 times) stronger than DSB unit. That is total power level Pt of the AM transmitter would have to be 180 watts to equal the information carrying capacity of the DSB-SC unit. This is a great power saving.

5. SSB-SC • we can improve the efficiency of transmission even further by eliminating one of the redundant sidebands. • A SSB signal has a bandwidth that is slightly less than half of a corresponding AM or DSB-SC signal 60W USB LSB F Fc Fc+fm(max) Fc-fm(max) Fc-fm(min) Fc+fm(min) • Look how the entire 60 watts is now concentrated in a much more narrow “slice” of spectrum. This characteristic gives a SSB transmitter much more “talk power” than an AM transmitter of the same power level.

6. 60W USB LSB F Fc Fc+fm(max) Fc-fm(max) Fc-fm(min) Fc+fm(min) • EXAMPLE: • if the SSB signal as shown is sending a voice signal (300hz-3khz) calculate: • The bandwidth • BW = fmax – fmin= [fc+fm(max)] – [ fc+fm(min)=fm(ma) – fm(min) • BW = 3000HZ – 300HZ =2700HZ • 2. The decibel power advantage of the SSB signal over the AM signal • dB=10log(P2/P1)(bw1/bw2)2 = 11.7 dB • Note: we are not transmitting any additional information power, we are just packing the information into a smaller bandth “space” • The power level an AM transmitter would need in order to have equivalent performance • Gp= dB=(P2/P1)(bw1/bw2)2 = (60/20)(6khz/2.7khz)2 = 14.8:1 • the AM transmitter would need 14.8 times the power of the SSB transmitter in order to have equivalent performance. That is 888 watts ( 14.8 x 60)

8. Balanced Modulators A balanced modulator is a circuit that generates a DSB signal. The output of a balanced modulator can be further processed by filters or phase-shifting circuitry to eliminate one of the sidebands, resulting in an SSB signal. Lattice Modulators. One of the most popular and widely used balanced modulators is the diode ring or lattice modulator.