2. The History of Single Sideband Modulation
Single sideband in general practice, and as referred to in this article, 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.
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.
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