The History of Single Sideband
In the early days of radio, little was known about sidebands. The concept of an amplitude-modulated signal being a composite rather than an indivisible whole was not well defined and was a subject open to vigorous controversy. In 1914 it was established mathematically that an amplitude-modulated wave consists of a carrier and two identical sidebands which are spaced above and below the carrier by an amount equal to the modulating frequency. The following year experiments were conducted at the US Naval Radio Station in Arlington, Virginia, using very low frequencies with an antenna which was tuned to pass one sideband and attenuate the other. This tended to substantiate the concept of a composite wave and, in addition, indicated that one sideband contained all the elements necessary for voice transmission. Another investigator had found that injection of the carrier frequency at the receiver improved detection of the received signal. These discoveries paved the way for development of the concept of single-sideband transmission and reception.
1915 John R. Carson applied for a patent on his idea to suppress the carrier and one sideband. After much litigation the patent was granted in 1923. In that year the first trans-Atlantic radio telephone demonstration used SSB with pilot carrier on a frequency of 52 kc. Single sideband was used because of limited power capacity of the equipment and the narrow bandwidths of efficient antennas for those frequencies. By 1927 trans-Atlantic SSB radiotelephony was open for public service. In subsequent years the use of SSB was limited mainly to low-frequency and wire applications. This may have been due in part to a general lack of interest in spectrum-conserving techniques as increasing knowledge opened up new portions of the spectrum. In addition, early developments in FM transmission stimulated a belief that this mode might prove to be the ultimate in voice communication. The resulting slow development of SSB technology precluded practical SSB transmission and reception at high frequencies. Amateur SSB activity followed very much the same pattern. Although amateurs have been responsible in part for much of the communications pioneering done in the past, early developments in single sideband are an exception. Some activity took place about 1933, but it was nearly fifteen years later when the use of SSB began in earnest on the amateur bands .
The advent of World War II brought with it a heretofore unparalleled need for communication facilities. From necessity, advances in electronic technology progressed at a high rate. There were major breakthroughs, not only in basic knowledge, but also in manufacturing techniques. These thrusts forward, and those in the years following the war, were important factors in the development of h-f SSB communication. Developments such as highly stable variable oscillators and the mechanical filter made SSB not only practical but economical. Continued advances in technology have refined techniques to the point where SSB has become a dominant mode of radio communication.
The Conservation of RF Spectrum
The radio-frequency spectrum, once thought to be adequate for all needs, is becoming crowded. As the world's technical sophistication progresses, the requirements for rapid and dependable radio communications increase. The competition for available space in the spectrum is intense. Services which have enjoyed use of the spectrum for many years are being asked to present cogent reasons why their frequency allocations should not be changed, reduced, or even eliminated. Low-priority services and those which are wasteful of allocated spectrum space are in precarious positions. The communications needs resulting from the increasing speed and volume of air traffic, both military and commercial, are examples of the heavy demand for more voice channels .
The HF spectrum, from 2 to 30 mc, has been particularly hard pressed to provide more voice channels. Some relief has been obtained by advances in microwave technology and VHF and UHF scatter techniques. Expansions in these areas actually have improved communications for many types of services. The fact remains that an increasing number of services need the long-distance communications links obtainable only by propagation in the HF-range. During years of low sunspot activity, the problem is compounded by lowered MUF which effectively shrinks the h-f portion of the spectrum. Single sideband has demonstrated the capacity to double the number of available voice channels within a given frequency range as compared to conventional AM. At the same time it has proved more dependable. Continuing refinements in SSB technology, such as increased stability of oscillators, further reductions in voice-channel width, and improved linearity in amplifiers, have widened the performance gap. There is good reason to believe that the advantages of SSB will continue to be needed as the frontiers of VHF and UHF propagation are rolled back. Present developments in these portions of the spectrum have attracted the attention of great numbers of users and potential users of radio communications.
The amateur service in particular has felt the pressure of increasingly crowded conditions on the h-f amateur bands. New licenses are being granted at a rate far in excess of that experienced in the past. Although organized amateurs have presented a strong case for the retention of amateur frequencies, future changes could result in reduced rather than expanded allocations. Therefore, continued overcrowding appears inevitable. The use of SSB has done much to alleviate congestion in the phone bands, but there is room for further improvement. The exclusive use of SSB is desirable. This, coupled with intelligent operating practices to minimize distortion products, will result in a greater number of satisfactory contacts within the limited spectrum available.
Single sideband in general practice, and as referred to in this book, actually is single sideband with suppressed carrier. Carrier- suppression techniques reduce the level of the carrier but do not eliminate it. This is a useful concept, since the carrier frequency is a reference around which the sidebands are produced in the transmitter and from which the nature of a single-sideband signal may be defined. To visualize carrier suppression, consider an SSB transmitter rated at 100 watts output. With 50 dB of carrier suppression, the output power contained in the carrier is 0.001 watt. This is a very small fraction of total output power, but it is finite.
In essence, a single-sideband signal is an AM signal with the nonessential elements effectively removed. Figure 1a and Figure 1b illustrates the spectrum occupied by 3700 kHz AM and SSB signals, each modulated by a single 1 kHz tone. The AM signal consists of a carrier and two identical sidebands which are spaced above and below the carrier by an amount equal to the frequency of the modulating tone. It is the sidebands which contain the intelligence of the signal.
The SSB signal contains the same elements, but there is a marked difference in the relative amplitudes of these components. The carrier and upper sideband (USB) are suppressed to very low levels. In terms of the relative power contained in each, compared to the lower sideband (LSB), these components in effect have been eliminated. The lower sideband, so called because it is lower in frequency than the carrier, is shown amplified to a level approximately equal to that of the AM carrier. The resulting SSB signal is referred to in terms of the desired sideband and, in this case, is an LSB signal. The suppressed sideband also is referred to as the unwanted sideband. The amount of carrier and unwanted sideband suppression generally are expressed as power ratios (dB) relative to peak level of the desired sideband.
The term ,sideband inversion" is used to indicate that, in the lower sideband, the highest modulating frequencies become the lowest output frequencies. This is illustrated in figure 2. Assume that a carrier wave, fc, is modulated by two audio frequencies, af1 and af2.
The resulting upper and lower sidebands each contain both of the modulating frequency components. The upper-sideband components consist of the sum of each modulating frequency plus the carrier frequency, and no inversion takes place. The lower-sideband components consist of the carrier frequency minus each of the modulating frequencies, and they become inverted. Inversion occurs in any frequency-translating process when the mixing frequency is higher than the signal frequency and the difference products are selected in the output circuit. This principle can be used for sideband switching in both transmitters and receivers, since by this means an upper-sideband signal is converted to a lower-sideband signal or vice versa. There are several variations in the types of SSB emission, however, the application of these methods to amateur radio is for the most part infrequent. Transmission of a pilot carrier is sometimes used when precise demodulation of the received signal is required. The pilot carrier is suppressed, but not to the extent prevalent in amateur practice. In one method of reception, the pilot carrier is separated from the sideband information, amplified in a separate channel, and re-inserted at the detector. Another method is to use the pilot carrier as received to derive error signals for automatic-frequency control of a receiver oscillator. In the double-sideband (DSB) system, both sidebands are transmitted, but the carrier is suppressed, unless one sideband is highly attenuated in the receiver, detection of these signals requires that the locally inserted carrier be precise with regard to both frequency and phase. This is in contrast to detection of a conventional SSB signal wherein the phase of the locally inserted carrier is unimportant. Actually, the frequency may be in error by 100 cycles or more without serious loss of intelligibility. In some commercial and military systems various multiplexing techniques are used to increase the information-carrying capacity of an SSB signal. In some systems both sidebands are used to increase the number of separate channels provided by a single transmitter. In a transmitter of given power-handling capacity, the use of multiplex and multichannel facilities reduces the amount of output power available for each channel, because total power input must not exceed the peak capability of the transmitter.
Comparison of SSB with AM
Further examination of figure 1 provides an insight into the significance of relative power distribution among the components which make up AM and SSB signals. As previously stated, the desired intelligence is transmitted in the sidebands. Notice, however, that in a conventional AM signal considerably more power is contained in the carrier than is present in either sideband. Furthermore, the remaining power content of the signal is split equally between two sidebands which contain identical information. In contrast, suppression of the carrier and one sideband in an SSB signal allows a concentration of power in the remaining sideband. The result is that SSB makes more effective use of amplifier input capability.
The use of SSB makes other significant economies possible. At higher power levels in particular, plate modulators and power supplies for AM transmitters are relatively expensive. A considerable cost reduction in these areas is possible with SSB, because the high-level modulator and the power supply load imposed by the need to amplify a high-energy carrier are eliminated. Since speech input signals are intermittent in nature, further savings can be made in the cost of power supply components for an SSB transmitter. Consequently, high-power amateur SSB transmitters are less expensive than comparable AM transmitters.
Signal-To-Noise Comparison of SSB and AM
The relative performance of SSB and AM systems can be evaluated by comparing the transmitter power required by each to produce a given signal-to-noise (s/n) ratio at the receiver under ideal propagating conditions. This comparison is useful since, neglecting frequency response, the s/n ratio determines the intelligibility of a received signal.
Figure 1 shows the power spectrum for an AM transmitter rated at one unit of carrier power. With 100-percent sine-wave modulation, such a transmitter produces 1.5 units of RF power. The additional 0.5 unit of power is furnished by the modulator and is distributed equally between the two sidebands. This AM transmitter is compared with an SSB transmitter rated at 0.5 unit of peak-envelope power (PEP). Peak-envelope power is defined as the RMS power developed at the crest of the modulation envelope.
When the RF signal is demodulated in the AM receiver an audio voltage develops which is equivalent to the sum of the upper- and lower-sideband voltages, in this case 1 unit of voltage. This voltage represents the output from a diode detector as normally used for AM reception. Such detection is called coherent detection because the voltages of the two sidebands are added in the detector. When the RF signal is demodulated in the SSB receiver, an audio voltage of 0.7 unit develops which is equivalent to the transmitted upper-sideband signal. This signal normally is demodulated by re-inserting the carrier at the detector. In a superheterodyne receiver, the received signal is translated to a new frequency before audio detection takes place. Therefore the re-inserted carrier must be at a frequency which maintains approximately the same frequency relationship to the sideband as in the transmitted signal. If a broadband noise level is chosen as 0.1 unit of voltage per 6 kc bandwidth, the AM bandwidth, the same noise level is equal to 0.07 unit of voltage per 3 kc bandwidth, the SSB bandwidth. These values represent the same noise power level per kc of bandwidth, that is, 0.12 divided by 6 is equal to 0.072 divided by 3. The s/n ratio for the AM system is 20 log s/n in terms of voltage, or 20 dB. For the SSB system the s/n ratio is also 20 dB. Therefore the 0.5 power unit of rated PEP for the SSB transmitter produces the same signal intelligibility as the 1 power unit of rated carrier power for the AM transmitter .
In summary it can be stated that, under ideal propagating conditions but in the presence of broadband noise, an SSB signal and an AM signal provide equal s/n ratios at the receiver if the total sideband power contained in each of the signals is equal. This means that, to perform under these conditions as well as an SSB transmitter of given PEP rating, an AM transmitter requires twice that figure in carrier power rating.
Antenna Voltage Comparison of SSB and AM
The use of multiband antennas is rather common on the amateur bands. Frequently these antennas employ resonant tuned circuits or traps, to disconnect portions of the antenna on certain bands, thus providing multiple resonant frequencies. Several types of triband beam antennas are typical examples. In operation these traps are subjected to high voltages which are functions of antenna input power. The maximum voltage rating of the traps places a limitation on the amount of peak power which may be fed into the antenna. As an example, if the peak power rating of a given antenna is 1000 watts, an AM transmitter used with this antenna must be limited to 250 watts carrier output power. This is true because the PEP of an AM signal, at 100-percent modulation, is four times the carrier power. In comparison, an SSB transmitter rated at 1000 watts PEP output, all of which is sideband power, may be used with this same antenna.
An interesting sidelight related to this comparison involves television and broadcast interference. For a given amount of total sideband power output, the PEP output of a conventional AM transmitter must be eight times that of an equivalent SSB transmitter. Therefore, even though such an AM transmitter is no more effective in transmitting the desired intelligence, it is more likely to cause fundamental overload interference than is the SSB transmitter.
Advantage of SSB with Selective Fading
The signal-to-noise comparison between SSB and AM, as discussed in a previous paragraph, is based upon ideal propagating conditions. Over many transmission paths, however, signals are subject to a phenomenon known as selective fading. This type of fading is caused by multipath propagation and is characterized by selective attenuation of the individual components which make up the transmitted signal. An AM signal is subject to severe distortion under these conditions principally because of its dependence upon a received carrier.
A few of the multiple paths over which a transmitted signal could be propagated . On the lower h-f bands, the signal often reaches the receiver by means of both sky waves and ground waves. Multiple sky-wave paths predominate on the upper h-f bands. As indicated in this illustration, the propagating medium is not a single, uniform reflecting surface. It is subject to continual variations in density, stratification, and refractive index. The effect of these propagating conditions is to cause varying instantaneous phase relationships among identical signal components arriving at the receiver over different paths. At a given moment, some of the arriving signal components are out of phase with those propagated over another path. The net result is that nulls are created in the spectrum of the received signal, and these nulls move across the signal spectrum as the propagating medium changes. The effect is somewhat like tuning a narrow rejection notch across the receiver passband. Some loss among the sideband components can be tolerated, since the practical effect is to upset only the amplitude and frequency response of the received signal. With a conventional AM signal, attenuation of the carrier by these selective nulls causes a serious problem. In this situation the carrier voltage at the receiver can be appreciably less than the sum of the two sideband voltages. Consequently the RF envelope does not retain its transmitted shape, and distortions severe upon demodulation. Frequently the fading is sufficient to render the signal unintelligible a large percentage of the time even though average signal strength may be quite high. Distortion caused by selective fading can be largely avoided by the use of the exalted carrier technique, but this method requires the local carrier to be phase locked to the transmitted carrier. The means to accomplish this, such as automatic-frequency control of the receiver BFO, requires a substantial amount of added circuitry. A SSB signal is not degraded significantly by selective fading. Since the received signal is not dependent upon a received carrier, no degradation results from the loss of carrier power. Transmission of only one sideband removes the necessity for precise carrier re-insertion at the receiver. Selective fading within the one sideband of a SSB signal alters only the amplitude and frequency response of the signal. Rarely does it cause enough distortion to make the signal unintelligible.
Comparison of SSB with FM
Good FM reception requires received signals that are strong enough to overcome the limiter threshold in the FM receiver. When this condition prevails, the s/n ratio for a given signal strength is better in the FM receiver than in the SSB receiver. However, when the signal strength falls below the FM limiter threshold, the s/n ratio in the FM receiver deteriorates rapidly, and the SSB receiver provides the best s/n ratio. The practical results are these: (1) With strong signals, FM can provide better s/n ratios than SSB. The advantage to be gained from this situation is dubious, since strong signals also provide good intelligibility in the SSB system, and a further increase in s/n ratio does not materially increase the readability. (2) With weak signals, the SSB system will provide an intelligible signal where the FM system fails. (3) The SSB system provides considerable savings in spectrum compared to the FM system.
Single Sideband for VHF Propagation in the VHF range is marked by a great variety in the means by which signals get from one spot on the earth to another. In many cases, more than one mode of propagation is involved on a given contact thus causing fading and resultant distortion. Such degradation of signals can be substantially reduced through the use of SSB. Various atmospheric and man-made noise sources which must be considered in the h-f range are, for the most part, small factors at VHF. As a result, propagation modes can be used at VHF which are either non existent in the hf range or are obscured by other factors. Many of the propagation modes usable at VHF, such as the various forms of scatter, require effective weak-signal techniques. Single sideband is well suited for this type of communication. The use of scatter is a relatively new technique with a challenge for amateurs who enjoy being close to the state of the art. Although there are several mechanisms by which scatter occurs, the general effect is that signals are scattered by irregularities in the troposphere or ionosphere in a manner similar to the scattering of light from automobile headlights on a foggy night. In this way VHF signals are returned to the earth at distances considerably beyond the normal radio horizon. Since the transmitted signal is widely scattered, received signal levels are typically quite weak, however, signals propagated in this way also exhibit a high degree of reliability. To achieve maximum reliability, transmitter powers of several kilowatts are used for commercial and military circuits, but when the reliability requirements are relaxed to a level still quite acceptable by amateur standards, scatter contacts are possible within amateur power limitations. In either case, low-noise receivers and high-gain antennas are necessities. The possibilities for amateur phone communications via scatter are somewhat marginal, however, this serves only to make the problem more interesting and successful results more rewarding. Single sideband holds the greatest promise for success, since an SSB transmitter operating at the maximum legal power input produces considerably more sideband power output than a comparable AM transmitter. Auroral reflection, using CW, is regularly used for VHF DX work when aurora conditions occur. Voice contacts by this means, especially on the 144 MHz band, frequently are not too successful due to the doppler effect prevalent in this mode of propagation. Although it has yet to be fully exploited, SSB has demonstrated an ability to get through where AM fails. Much is still to be learned about certain aspects of VHF propagation. This fact alone makes VHF operation both interesting and challenging for many amateurs. By means such as SSB, the possibilities for increasing the normal working distances for phone signals are by no means exhausted. For those inclined to a more casual type of operation, SSB can provide a high percentage of enjoyable contacts in addition to making efficient use of available facilities.
The use of SSB for phone operation is particularly beneficial to the amateur. Because amateur operation is generally on random frequencies within a band, rather than on specific channels, suppression of the high-energy carrier eliminates the din of heterodynes common to AM operation. Since a great concentration of stations exists within many of the amateur phone bands, the spectrum economy of SSB is an important factor. It allows a greater number of satisfactory contacts to take place at the same time in comparison to other modes. The power economy of SSB permits a considerable reduction in the size of power supply equipment and reduces relative cost, particularly higher power levels. In situations where maximum power is required, such as in VHF scatter circuits or difficult HF paths, SSB provides the most sideband power output presently available within amateur power limitations . The benefits of SSB are greatest and most easily observed under poor propagating conditions. As a given transmission path deteriorates due to a combination of noise, severe selective fading, and narrow-band interference, the superiority of SSB over AM becomes increasingly evident. Studies have been made which give SSB a theoretical performance edge of several dB when conditions are marginal. Since the variables are sometimes difficult to relate, one of the most convincing methods of comparison from an amateur standpoint is to listen on the amateur DX phone bands. Signals from SSB stations are the first to become readable as the bands open and are the last to fade as the bands go out.
DJ4BR, Ing. Peter Weber