Introduction to Single Side Band

[Music] this is an oscilloscope display of a radio frequency carrier amplitude modulated by my voice using conventional AM techniques a by conventional AM we mean double sideband unsuppressed carrier amplitude modulation the important thing to notice here is that when the modulation is removed the radio frequency carrier remains now you're looking at a display of amplitude modulation using single sideband techniques you notice any difference the voice variations look very much the same but when we remove the modulation or when I stop talking there is no signal no carrier this difference as you will see gives single sideband a tremendous advantage over conventional AM but what is single sideband for more than 50 years AM has made voice communications possible over great distances literally everyone has made use of it and the demand on communications frequencies continues to grow but the frequency spectrum used for communications is limited over the years many types of stations for numerous purposes have come into existence such as public information and entertainment public utilities and services military and transportation private citizen services national and international services and many many others and as the population increases more people have a need to use these services and you can see what's happening the frequency spectrum is already too crowded and getting more so every day now this is a serious problem so electronic engineers turn their attention to solving it there were two possible solutions one to increase the efficient use of transmitter power so that more distance could be obtained which would enable fewer transmitters to cover more of a given area or to to narrow the frequency bandwidth required for each radio signal so that more frequency space would be available for additional stations the engineers efforts to accomplish one of the other of these objectives has resulted in a more efficient communication system it's called single sideband and it's described as a technique of selecting and transmitting only a portion of the frequencies of the conventional AM waveform now before we continue with single sideband let's take a look at AM let's summarize our knowledge of conventional amplitude modulation now you know that in an AM transmission an RF carrier is required and that for 100% modulation an audio frequency of equal peak amplitude is needed when the two frequencies mix the result is four frequencies the two originals the sum and the difference frequencies which are called the upper and lower side bands when modulating 100% each sideband voltage is only half as great as the carrier voltage the audio frequency is not developed in the output because the RF resonant circuits have a very low impedance to audio the RF carrier and its sidebands are coupled through the RF tuned circuits to the antenna so we transmit these three frequencies the carrier the upper and lower side bands in a composite waveform now let's review this composite waveform for just a moment first let's look at the frequency relationships the carrier frequency determines the location in the spectrum of the transmitted signal and when the carrier racks with the audio or the modulation frequency the two sidebands are generated now each side man is spaced the distance of the audio frequency from the carrier now let's look at the power relationships each sideband voltage is only half as great as the carrier voltage at 100 percent modulation in the formula for power as you recall P equal a squared over our voltage changes will square changes in power so if each sideband voltage is only half as great as the carrier voltage the power of each sideband must be only one fourth that of carrier power or one sixth that of total power now from what you have seen of the frequency and power relationships in the composite waveform you know that varying the modulating frequency will vary the frequency of the sidebands if the voltage of the modulating frequency is varied the power in each side band will vary accordingly so you see that the two side bands contain the same intelligence but the power is equally distributed between the two the carrier always remains constant in both frequency and amplitude now if the carrier always remains constant why does the oscilloscope show at varying in amplitude because what you see on a scope is not just the carrier the side bands are also present to show the carrier by itself while the side bands were present the display would have to look like this showing all three frequencies in their individual forms and relationships simultaneously but as you know a scope cannot show three different voltage values simultaneously it shows the instantaneous nation of these voltages when all three are in phase the resultant is large on the sidebands are in phase with each other and out of phase with the carrier the resultant is small thus you see what only appears to be the carrier varying in amplitude but back to the composite waveform let's look at it in a little different way and we begin to see some undesirable things about the conventional am signal after the sidebands are generated the carrier is not required for transmission of the sidebands a since the carrier is constant and amplitude and in frequency it contains absolutely no information at all so why transmit it yet two-thirds of the total power is wasted in transmitting a carrier thus the sidebands are being deprived of power which might increase the range of transmission and also since we have two sidebands the required receiver bandwidth allows in more atmospheric noise at the received end noise of course is proportional to bandwidth thus the wider bandwidth raises the noise level of the signal if the signal is very weak it may be lost in the noise and thus be totally unusable now we have seen that identical intelligence is present in each of the sidebands now since either sideband carries the same intelligence transmitting both side bands is a waste of frequency space since the carrier and one of the side bands are not required for transmission why don't we eliminate them and use their power to boost the remaining side ban well let's see what would happen we eliminate one sideband we would narrow the required frequency spectrum by 1/2 and we'd still have the same intelligence in the remaining sideband and by putting the power from the removed sideband into the remaining sideband total sideband power is restored and is contained in just one of the side vans well so far the sideband energy has only been rearranged with no resulting power gained in the information being transmitted however putting the carrier power into the remaining sideband would boost the sideband power tremendously now all of the transmitter power is located in one sideband now you recall that in conventional am at 100% modulation one sideband contains only one sixth of the total transmitted power thus if all the power were placed in one sideband its level would be increased six times this increases the power and this sideband by a factor of six but it only increases the intelligence power by three since part of the power was already intelligence power in the sideband that was removed so by taking our conventional AM wave and just rearranging the power quantities through circuit changes our transmitter is now effectively three times more powerful than before using the same total transmitter power or to look at it another way a single sideband transmitter requires only one third the power to equal the performance of a conventional AM transmitter now this is a tremendous advantage in itself but let's look a little further when we have two sidebands the receiver bandwidth required is twice that needed or single-sideband reception giving us a lower signal-to-noise ratio and now that we have eliminated one sideband the receiver bandwidth may be narrowed considerably and now what happens to the signal-to-noise ratio the noise originally passed by the wider bandwidth has been reduced in the signals a single sideband receiver now this means that with the same amount of received signal and only half the noise level the signal-to-noise ratio has also been doubled so our receiver has given us an effective gain of two this has increased the system's effectiveness by a factor of six eliminating one sideband reduces the used frequency space by half this increases the number of frequency channels available and shifting the power of the carrier to the remaining sideband tripled the effective power of the transmitter reducing the bandwidth of the receiver doubled the signal-to-noise ratio which effectively doubled the gain of the received signal so a transmitter gain of three a receiver gain of two gives us an overall system gain of six over the conventional AM system this is the theory of single sideband let's now compare it in an actual operating condition thus far ideal transmitters receivers and propagation conditions have been assumed when considering range versus power the frequency of operation must also be considered extremely high frequencies are limited to line-of-sight regardless of power thus increasing power above a certain level with not greatly increase the effective range beyond the line-of-sight transmission capability in military applications single sideband is normally operated at frequencies which allow long-range propagation by way of the ionosphere which is radically different from the ideal conditions previously considered under average long-range propagation conditions selective fading is likely to exist and it is far more damaging to conventional AM signals then to the single sideband signal selective fading is caused by a combination of signals arriving at the receiver over two or more propagation paths of different lengths resulting in phase distortion or cancellation of sideband information under conditions of extreme fading single sideband communication has been accomplished when conventional AM systems of similar sideband power were completely out of service so we see that the requirements of a single sideband transmitter must be elimination of the carrier elimination of one sideband and now let's take a look at the way this is done looking at the simplified block diagram of a conventional AM transmitter let's change it to operate single sideband of course we use a crystal oscillator of low frequency RF to generate a stable carrier when we modulate it with an audio frequency the two signals are mixed together in a balanced modulator which is designed to cancel out the carrier and filter the modulating frequency in its output and produce only the two side bands the upper and the lower we then insert a filter which will pass only one of the side bands and of course we can select either one and will reject the other one thus we have only one side ban to be transmitted with all of the transmitters power at this point if we desire to operate at a frequency other than the frequency of the sideband we can change the sideband frequency by mixing it with another RF signal in a balance mixer and selecting the sum frequency we now have the sideband raised to a predetermined higher frequency containing the same recoverable data as the original and by this method we can transmit a single sideband of any frequency with the full power of the transmitter now let's make a quick comparison of this single sideband waveform with a conventional AM wave as we have already shown you this is the single sideband signal let's really appreciate the difference between the two by again looking at an AM wave here is an AM signal but I'm not talking you're looking at the RF carrier wasted energy when I speak I modulate the carrier the apparent changes in amplitude of the carrier are caused by the phase relationships of the side bands generated at this time we're transmitting intelligence but look at the wasted power in the carrier thus the reason for single sideband only when we transmit intelligent information do we have a transmission all of the power of the transmitter is directed to the intelligence the band width is much less therefore the signal-to-noise ratio at the received end is higher and of course our transmitter efficiency is maximum all of the transmitter power is delivered to the intelligence this then is the big difference between single sideband and ordinary amplitude modulation we have accomplished our purpose to transmit a single sideband signal all of the transmitter power in the intelligence but will a conventional receiver receive this different kind of signal let's look at a typical AM receiver and see the incoming sideband signal is our F our RF amplifiers of course will receive this signal and amplify it and it can be mixed to produce an if' signal the if' signal will be amplified as usual but what happens at the detector you recall what a detector does it rectifies and filters the if' signal and detects the amplitude variations but in a single sideband signal modulated with a constant tone the single sideband would have constant amplitude the detector would rectify it filter it and out would come pure DC no intelligence so what must be done remember that the differences between the conventional AM and the single sideband waveforms were due to the absence of a radio frequency carrier so if we insert an RF oscillator let's call it a beat frequency oscillator or BFO to generate a constant frequency 1 kilohertz below the if' signal like the original carrier and mix it with the if' the result is of course four frequencies the if' the BFO signal an upper and lower sideband or a constant carrier with side bands of the audio difference the resulting phase relationships restore the original envelope variations and we can easily detect our audio all we did was reverse the process of the transfer this is called reference frequency generation the standard audio circuits will now select and amplify the audio signal or the audio amplitude variations the original intelligence that we have transmitted thus we have created a single sideband receiver can be readily seen that a single sideband receiver will be much more difficult to tune than a conventional AM receiver and it must be of course very very stable the BFO must be precisely adjusted to simulate the removed carrier any drift at any time will cause the audio output to be distorted so as we have seen the obvious requirements of the single sideband receiver are reference frequency generation and excellent frequency stability now let's quickly summarize the entire single sideband system to make sure everything fits together the transmitter generates a stable low frequency RF carrier which we modulate with an audio frequency the balance modulator mixes the two and creates the sidebands it also eliminates the carrier in the audio frequencies passing on only the sideband frequencies a side-bend filter is designed that selects one sideband and rejects the other in the balance mixer the sideband is mixed with another RF signal to produce a sideband of a higher operating frequency and the power amplifier amplifies and transmits this signal a single sideband transmitter must accomplish carrier elimination and rejection of one sideband the transmitted signal contains only one side band with all the power of the transmitter this increases the effective transmission three times over an equivalent conventional AM transmitter the reduced receiver bandwidth doubles the signal-to-noise ratio resulting in an overall system gain of six it also decreases the required spectrum occupancy this increases the number of stations that can now operate in the whole frequency spectrum at the receiver the sideband is processed through the standard RF amplifier mixer and I have stages just as in a conventional AM receiver the detector has been replaced by a B fo and the balanced mixer which inserts a reference frequency to mix with the single sideband if' this restores the AM wave shape since the phase relationships now produce the original audio variations and of course the audio variations are selected and amplified by standard audio circuits so the requirements of a single sideband receiver are reference frequency generation and excellent frequency stability and that is single sideband in a nutshell and we have seen that the characteristics of a single sideband system compared with a conventional AM system are more efficient use of available power much greater reliability at long range under adverse conditions now our bandwidth and reference frequency generation in the receiver we know our single sideband system has some disadvantages we have seen that these are high frequency stability requirements which of course means more complex circuits which in turn may result in higher cost of equipment and maintenance but let's compare the practical application of conventional AM and single-sideband for short range communications for the general public it would be impractical to have all of the public turn in there am equipment for perhaps more expensive single sideband equipment which might require more skill in tuning and higher maintenance costs so conventional AM will be with us for quite some time in many applications but for uses requiring long-range communications in ever increasingly crowded frequency bands the operational advantages of single sideband far exceed its possible increased cost single sideband is rapidly growing in use especially in the military so it's your job as an electronics technician to meet the challenge of both conventional a.m. and single sideband [Music]

Channel: John Mayer

Views: 60,986

Keywords: Single Sideband, RF Transmissions, Modulation Techniques, High Frequency Modulation Techniques

Id: -EaHZqsmnxI

Length: 26min 2sec (1562 seconds)

Published: Fri May 17 2019