One method of producing an SSB signal is to remove one of the sidebands via filtering, leaving only either the upper sideband (USB), the sideband with the higher frequency, or less commonly the lower sideband (LSB), the sideband with the lower frequency. Most often, the carrier is reduced or removed entirely (suppressed), being referred to in full as single sideband suppressed carrier (SSBSC). Assuming both sidebands are symmetric, which is the case for a normal AM signal, no information is lost in the process. Since the final RF amplification is now concentrated in a single sideband, the effective power output is greater than in normal AM (the carrier and redundant sideband account for well over half of the power output of an AM transmitter). Though SSB uses substantially less bandwidth and power, it cannot be demodulated by a simple envelope detector like standard AM.
The LSB spectrum is inverted compared to the baseband.
As an example, a 2 kHz audio baseband signal modulated
onto a 5 MHz carrier will produce a SSB frequency of
5.002000 MHz if USB is used or 4.998000 MHz if LSB is used.
An alternate method of generation known as a Hartley modulator, named after R. V. L. Hartley, uses phasing to suppress the unwanted sideband. To generate an SSB signal with this method, two versions of the original signal are generated, mutually 90° out of phase for any single frequency within the operating bandwidth. Each one of these signals then modulates carrier waves (of one frequency) that are also 90° out of phase with each other. By either adding or subtracting the resulting signals, a lower or upper sideband signal results. A benefit of this approach is to allow an analytical expression for SSB signals, which can be used to understand effects such as synchronous detection of SSB.
Shifting the baseband signal 90° out of phase cannot be done simply by delaying it, as it contains a large range of frequencies. In analog circuits, a wideband 90-degree phase-difference network is used. The method was popular in the days of vacuum-tube radios, but later gained a bad reputation due to poorly adjusted commercial implementations. Modulation using this method is again gaining popularity in the homebrew and DSP fields. This method, utilizing the Hilbert transform to phase shift the baseband audio, can be done at low cost with digital circuitry.
Another variation, the Weaver modulator, uses only lowpass filters and quadrature mixers, and is a favored method in digital implementations.
In Weaver's method, the band of interest is first translated to be centered at zero, conceptually by modulating a complex exponential exp(jωt) with frequency in the middle of the voiceband, but implemented by a quadrature pair of sine and cosine modulators at that frequency (e.g. 2 kHz). This complex signal or pair of real signals is then lowpass filtered to remove the undesired sideband that is not centered at zero. Then, the single-sideband complex signal centered at zero is upconverted to a real signal, by another pair of quadrature mixers, to the desired center frequency.
The front end of an SSB receiver is similar to that of an AM or FM receiver, consisting of a superheterodyne RF front end that produces a frequency-shifted version of the radio frequency (RF) signal within a standard intermediate frequency (IF) band.
To recover the original signal from the IF SSB signal, the single sideband must be frequency-shifted down to its original range of baseband frequencies, by using a product detector which mixes it with the output of a beat frequency oscillator (BFO). In other words, it is just another stage of heterodyning.
For this to work, the BFO frequency must be accurately adjusted. If the BFO is mis-adjusted, the output signal will be frequency-shifted, making speech sound strange and "Donald Duck"-like, or unintelligible. Some receivers use a carrier recovery system, which attempts to automatically lock on to the exact frequency.
A SHORT HISTORY OF SINGLE
SIDEBAND IN AMATEUR RADIO
by James W. Nash, K4HMS/V31AW
On the night of September 21, 1947, bizarre sounding voices appeared on the 75 meter phone band. These strange signals were audible in California and adjacent states. In fact, they came, not from outer space as some might have thought, but from W6YX , the club station of Stanford University, operated by O. G. Villard, Jr., W6QYT. (Villard, 1948.)
No doubt there were listeners out there who could not believe their ears. But what they were hearing was actually single sideband modulation. This event represented the beginning of the postwar SSB revolution in amateur radio. It was the most important technical development to hit the hobby since spark gave way to CW. As we have seen, its implications have gone far beyond the technical.
We all owe Villard and his co-experimenters a debt of thanks for bringing us SSB when they did. However, it’s worth knowing that single sideband had already been around for over thirty years. And, not only that, hams had been on the air with the squawky stuff as early as 1933.
During 1954, I bought my first shortwave radio, a used Hallicrafters S-53 (cost $40), and launched my short career as a pure SWL—I would receive an amateur license the next year. One night, tuning around between Radio Moscow and the BBC, or whatever, I was astonished to hear ducks talking. Yes, a twelve year-old SWL in Georgia had discovered that ducks were indeed on the radio. For there was, sure enough, Donald talking to his goofy nephews. It never occurred to me that, in theory, I could tune this stuff into English by flipping the toggle switch which said "BFO" and making some very careful adjustments.
I had, of course, discovered single sideband transmission. In the next few years, I received a Novice and then a General license. At the ramp-up to the peak of Cycle 19, I tried "phone," meaning AM, and liked it a lot. For me, the big fun was on 15 meter phone, especially since I had a 3-element Yagi for the band. After all, the Globe Scout put out only 50 watts on AM.
SSB was still in the experimental stages as far as I knew. So while I worked AM, as well as CW, I would hear the duck-talkers up there in their own part of the band. Quacking away.
Then my career on phone hit a major snag. Because then came those phone calls to my parents from the neighbors. The problem was that many of the TV’s still used 21 mc. IF’s. Well, despite the helpful efforts of the Kennehooche ARC TVI Committee, I went back to CW. Except for my long absence from the air in the later 70’s and much of the 80’s, I’ve been there for the most part since.
As for sideband, I forgot it was even there for a long time. In 1988, however, inspired by news stories of the newly approaching peak of Cycle 22, I acquired a TS-680S. With the help of my two youngest children I erected a G5RV antenna. After a long absence I got back on the air. I soon discovered this newfangled transceiver had SSB on it—soon I tried it out and was overwhelmed with its superiority to AM. How could I have missed this for so long? And how long had it really been around? I remembered hearing sideband 30 years before, but I had thought it was something brand new. I was wrong.
Bell Labs and Others
Well, it’s like this. The story starts in 1915, when mankind discovered sidebands. Now possessing this superior understanding of the AM signal, radio scientists began to understand the implications of their discovery. Soon afterwards, our old friends at Bell Labs, who have discovered practically everything, developed a method for removing one of the sidebands of an AM signal but retaining all the essential modulation components. As an expert of that day supposedly said, "both sidebands are saying the same thing" (Goodman, 1948).
Or, as the Messenger said in one of his mysterious transmissions many years ago, "some day you will gain many dB, for both sidebands say same thing."
Bell soon discovered that it was possible to increase the capacity of overseas telephone circuits using SSB modulation, and such circuits were in telephonic use by 1918. The first single sideband radio transmissions occurred by 1923. From the beginning equipment stability problems slowed development of SSB. In 1927, a trans-Atlantic SSB commercial radio circuit was in place, but at low frequencies. In fact, it was in the mid-1930’s before commercial shortwave SSB was practical—the frequency stablity problems were solved by using new generation AFC equipped receivers and a pilot carrier from the transmitter.
Hams at Work
By the time the 1930’s came along, licensed amateurs were hard at work on the problem. Apparently the all-time ham radio SSB pioneer was Robert M. Moore, W6DEI. He published three articles on the subject in R9 magazine in 1933 and 1934. At that point, this experimental mode was referred to as "single sideband suppressed carrier" transmission, or SSSC. The articles reveal that what Moore was talking about was real single sideband. In his articles he acknowledges the prior work of Bell Telephone Laboratories in developing the techniques he was using. (Moore, 1933, 1934.)
Another pioneer in this endeavor was J. Evans Williams, W2BFD, who in 1948 wrote a letter to QST pointing out that Moore was indeed the original 1930’s ham radio SSB pioneer. Williams himself build a 500 watt SSSC transmitter based on Moore’s designs, and reported in his letter that a half-dozen or so sideband stations had been on 20 meters during the 1930’s.
As Williams also pointed out, the idea simply did not yet catch fire. One reason was that the bands were not yet so crowded. Another was that many hams had only recently become involved in AM in the first place—those were the days when one started in CW—and had recently built or purchased new AM equipment. Finally, FM was coming on the scene and many of its advocates claimed that it alone was the mode of the future. So SSSC had to wait a while.
SSB Takes Over
What finally made SSB feasible for amateurs was a workable solution to the problem of transmitter stability. This came after World War II, with the work of Villard and others. The techniques which were ultimately most workable involved a balanced modulator which generated the sidebands, but no carrier, followed by a sideband filter which eliminated one of them.
The phone bands were really jammed in the 1950’s, much worse so than today. This was largely because of the width of AM signals. The ham community soon grasped the fact that SSB was more than a way to get 8 dB. It was a long term solution to the problem of bands jammed every night with heterodynes howling like banshees. I’m not sure how many of us today realize how vital SSB has been to the history of the avocation. After sideband became well-established the process of working DX on phone with moderate power and antennas became vastly easier. By the early 1970’s, there was more DX on sideband than on CW, and the gap has probably continued to grow ever since.
Of course, in the 1950’s we non-SSB heathens still referred to it as "duck talk." Yet it was soon established that any receiver with adequate frequency stability and a variable BFO could receive this squawky stuff so that it sounded more or less human. Also, a clever ham without an expensive receiver could get some help from a frequency meter or VFO (most of us had separate ones in those days), for use in carrier reinsertion.
It was the early sixties before commercial rigs with sideband were widely available. I’m not sure which manufacturer put out the first commercial ham SSB rig. (If you know, let me know.) But by the late 1960’s SSB had come to dominate the phone bands. Today the terms "phone" and "sideband" are synonymous.
As for me, a ham who had worked AM in the 1950’s, my first QSO on SSB came in 1988. I was amazed at the ease with which one could work phone DX with 100 watts and modest antennas. The clarity and capability of reading signals in QRM was remarkably improved over the AM of my boyhood. And although today my preference still remains strongly for CW, that is a personal preference. Without single sideband, I’m not sure amateur radio would be nearly as strong an institution as it is today. Who knows what will come next?
The Collins KWM-1: One of the first SSB rigs. >>
Goodman, Byron, W1DX. "What is Single-Sideband Telephony?" QST, January 1948, pp. 13-15, 126 ff. If you don’t know, Goodman was the first editor of "How’s DX" back in the 1930’s.
Moore, Robert, W6DEI. "Single Sideband Transmission," R9, September, December, and January, 1933-1934 (three installments).
Villard, O.G., W6QYT. "Single-Sideband Operating Tests, " QST, January 1948, pp. 16-18, 128, 130.
Williams, J. Evans, W2BFD. "SSSC," letter to the editor, QST, February 1948, p. 64.
About the writer:
*Jim Nash, K4HMS,email@example.com,
first licensed in 1955, has published articles in QST and the DX Magazine. He is a practicing attorney in Houston.