Single Sideband (SSB) | Double Side Band Modulation

What is Single SideBand ?

In amplitude modulation, two-thirds of the transmitted power is in the carrier, which itself conveys no information. The real information is contained within the side bands. One way to improve the efficiency of amplitude modulation is to suppress the carrier and eliminate one side band. The result is a single-sideband (SSB) signal. SSB is a form of AM that offers unique benefits in some types of electronic communication.

Double Side Band DSB Signals

The first step in generating an Single Sideband (SSB) signal is to suppress the carrier, leaving the upper and lower side bands. This type of signal is referred to as a double-side band suppressed carrier (DSSC or DSB) signal. The benefit, of course, is that no power is wasted on the carrier. Double-side band suppressed carrier modulation is simply a special case of AM with no carrier. A typical DSB signal is shown in Fig. 3-15. This signal, the algebraic sum of the two sinusoidal sidebands, is the signal produced when a carrier is modulated by a single-tone sine wave information signal. The carrier is suppressed, and the time-domain

DSB signal is a sine wave at the carrier frequency, varying in amplitude as shown. Note that the envelope of this waveform is not the same as that of the modulating signal, as it is in a pure AM signal with carrier. A unique characteristic of the DSB signal is the phase transitions that occur at the lower-amplitude portions of the wave. In Fig. 3-15, note that there are two adjacent positive-going half-cycles at the null points in the wave. That is one way to tell from an oscilloscope display whether the signal shown is a true DSB signal. A frequency-domain display of a DSB signal is given in Fig. 3-16. As shown, the spectrum space occupied by a DSB signal is the same as that for a conventional AM signal. Double-sideband suppressed carrier signals are generated by a circuit called a balanced modulator. The purpose of the balanced modulator is to produce the sum and difference frequencies but to cancel or balance out the carrier. Balanced modulators are covered in detail in Chap. 4. Despite the fact that elimination of the carrier in DSB AM saves considerable power, DSB is not widely used because the signal is diffi cult to demodulate (recover) at the receiver. One important application for DSB, however, is the transmission of the color information in a TV signal.

Single Sideband (SSB) Signals

In DSB transmission, since the side bands are the sum and difference of the carrier and modulating signals, the information is contained in both side bands. As it turns out, there is no reason to transmit both side bands in order to convey the information. One sideband can be suppressed; the remaining side band is called a single sideband suppressed carrier (SSSC or SSB) signal. SSB signals offer four major benefits.

  1. The primary benefit of an SSB signal is that the spectrum space it occupies is only one-half that of AM and DSB signals. This greatly conserves spectrum space and allows more signals to be transmitted in the same frequency range.
  2. All the power previously devoted to the carrier and the other side band can be channeled into the single side band, producing a stronger signal that should carry farther and be more reliably received at greater distances. Alternatively, SSB transmitters can be made smaller and lighter than an equivalent AM or DSB transmitter because less circuitry and power are used.
  3. Because Single Sideband (SSB) signals occupy a narrower bandwidth, the amount of noise in the signal is reduced.
  4. There is less selective fading of an SSB signal over long distances. An AM signal is really multiple signals, at least a carrier and two side bands. These are on different frequencies, so they are affected in slightly different ways by the ionosphere and upper atmosphere, which have a great influence on radio signals of less than about 50 MHz. The carrier and side bands may arrive at the receiver at slightly different times, causing a phase shift that can, in turn, cause them to add in such a way as to cancel one another rather than add up to the original AM signal. Such cancellation, or selective fading, is not a problem with Single Sideband (SSB)since only one side band is being transmitted.

An Single Sideband (SSB) signal has some unusual characteristics. First, when no information or modulating signal is present, no RF signal is transmitted. In a standard AM transmitter, the carrier is still transmitted even though it may not be modulated. This is the condition that might occur during a voice pause on an AM broadcast. But since there is no carrier transmitted in an SSB system, no signals are present if the information signal is zero. Side bands are generated only during the modulation process, e.g., when someone speaks into a microphone. This explains why SSB is so much more efficient than AM. Fig. 3-17 shows the frequency- and time-domain displays of an SSB signal produced when a steady 2-kHz sine wave tone modulates a 14.3-MHz carrier. Amplitude modulation would produce side bands of 14.298 and 14.302 MHz. In Single Sideband (SSB), only one side band is used. Fig. 3-17(a) shows that only the upper side band is generated. The RF signal is simply a constant-power 14.302-MHz sine wave. A time-domain display of this SSB signal is shown in Fig. 3-17(b). Of course, most information signals transmitted by SSB are not pure sine waves. A more common modulation signal is voice, with its varying frequency and amplitude content. The voice signal creates a complex RF Single Sideband (SSB) signal that varies in frequency and amplitude over the narrow spectrum defined by the voice signal bandwidth. The waveform at the output of the SSB modulator has the same shape as the base band waveform, but it is shifted in frequency

Disadvantages of DSB and Single Sideband (SSB)

The main disadvantage of DSB and SSB signals is that they are harder to recover, or demodulate, at the receiver. Demodulation depends upon the carrier being present. If the carrier is not present, then it must be regenerated at the receiver and reinserted into the signal. To faithfully recover the intelligence signal, the reinserted carrier must have the same phase and frequency as those of the original carrier. This is a difficult requirement. When Single Sideband (SSB) is used for voice transmission, the reinserted carrier can be made variable in frequency so that it can be adjusted manually while listening to recover an intelligible signal. This is not possible with some kinds of data signals. To solve this problem, a low-level carrier signal is sometimes transmitted along with the two sidebands in DSB or a single sideband in SSB. Because the carrier has a low power level, the essential benefits of SSB are retained, but a weak carrier is received so that it can be amplified and reinserted to recover the original information. Such a low-level carrier is referred to as a pilot carrier. This technique is used in FM stereo transmissions as well as in the transmission of the color information in a TV picture.

Signal Power Considerations

In conventional AM, the transmitted power is distributed among the carrier and two side bands. For example, given a carrier power of 400 W with 100 percent modulation, each side band will contain 100 W of power and the total power transmitted will be 600 W. The effective transmission power is the combined power in the side bands, or 200 W. An SSB transmitter sends no carrier, so the carrier power is zero. A given SSB transmitter will have the same communication effectiveness as a conventional AM unit running much more power. For example, a 10-W SSB transmitter offers the performance capabilities of an AM transmitter running a total of 40 W, since they both show 10 W of power in one side band. The power advantage of SSB over AM is 4:1. In SSB, the transmitter output is expressed in terms of peak envelope power (PEP), the maximum power produced on voice amplitude peaks. PEP is computed by the equation P = V2 /R. For example, assume that a voice signal produces a 360-V, peak-to-peak signal across a 50-V load. The rms voltage is 0.707 times the peak value, and the peak value is one-half the peak-to-peak voltage. In this example, the rms voltage is 0.707(360/2) = 127.26 V. The peak envelope power is then

The PEP input power is simply the dc input power of the transmitter’s final amplifier stage at the instant of the voice envelope peak. It is the final amplifier stage dc supply voltage multiplied by the maximum amplifier current that occurs at the peak, or

For example, a 450-V supply with a peak current of 0.8 A produces a PEP of 450(0.8) = 360 W. Note that voice amplitude peaks are produced only when very loud sounds are generated during certain speech patterns or when some word or sound is emphasized. During normal speech levels, the input and output power levels are much less than the PEP level. The average power is typically only one-fourth to one-third of the PEP value with typical human speech:

With a PEP of 240 W, the average power is only 60 to 80 W. Typical single side band (SSB )transmitters are designed to handle only the average power level on a continuous basis, not the PEP. The transmitted sideband will, of course, change in frequency and amplitude as a complex voice signal is applied. This sideband will occupy the same bandwidth as one sideband in a fully modulated AM signal with carrier.Incidentally, it does not matter whether the upper or lower sideband is used, since the information is contained in either. A fi lter is typically used to remove the unwanted sideband.

Example 3-7 An SSB transmitter produces a peak-to-peak voltage of 178 V across a 75-V antenna load. What is the PEP?

Example 3-8 An SSB transmitter has a 24-V dc power supply. On voice peaks the current achieves a maximum of 9.3 A.

a. What is the PEP?

Classification of Radio Emissions

Fig. 3-18 shows the codes used to designate the many types of signals that can be transmitted by radio and wire. The basic code is made up of a capital letter and a number, and lowercase subscript letters are used for more specific definitions. For example, a basic AM voice signal such as that heard on the AM broadcast band or on a CB or aircraft radio has the code A3. All the variations of AM using voice or video intelligence have the A3 designation, but subscript letters are used todistinguish them. Examples of codes designating signals described in this chapter are as follows:

  • DSB two sidebands, full carrier = A3
  • DSB two sidebands, suppressed carrier = A3b
  • SSB single sideband, suppressed carrier = A3j
  • SSB single sideband, 10 percent pilot carrier = A3a
  • Vestigial sideband TV = A3c
  • OOK and ASK = A1

Note that there are special designations for fax and pulse transmissions, and that the number 9 covers any special modulation or techniques not covered elsewhere. When a number precedes the letter code, the number refers to bandwidth in kilohertz. For example, the designation 10A3 refers to a 10-kHz bandwidth voice AM signal. The designation 20A3h refers to an AM SSB signal with full carrier and message frequency to 20 kHz. Another system used to describe a signal is given in Fig. 3-19. It is similar to the method just described, but with some variations. This is the definition used by the standards organization International Telecommunications Union (ITU). Some examples are

A3F amplitude-modulated analog TV
J3E SSB voice
F2D FSK data
G7E phase-modulated voice, multiple signals

Radio emission code designations.

Letter        A             Amplitude modulation
                 F              Frequency modulation
                 P              Phase modulation
Number    0              Carrier ON only, no message (radio beacon)
                 1              Carrier ON/OFF, no message (Morse code, radar)
                 2              Carrier ON, keyed tone ON/OFF (code)
                 3              Telephony, message as voice or music
                 4              Fax, nonmoving graphics (slow-scan TV)
                 5              Vestigial sideband (commercial TV)
                 6              Four-frequency diplex telegraphy
                 7              Multiple sidebands each with different message
                 8              General (all others)
                 None        Double sideband, full carrier
                 a               Single sideband, reduced carrier
                 b               Double sideband, no carrier
                 c               Vestigial sideband
                 d              Carrier pulses only, pulse amplitude modulation
                 e               Carrier pulses only, pulse width modulation (PWM)
                 f                Carrier pulses only, pulse position modulation (PPM)
                 g               Quantized pulses, digital video
                 h               Single sideband, full carrier
                 j                Single sideband, no carrier

ITU emissions designations.

Type of Modulation
N Unmodulated carrier
A Amplitude modulation
J Single sideband
F Frequency modulation
G Phase modulation
P Series of pulses, no modulation
Type of Modulating Signals
0 None
1 Digital, single channel, no modulation
2 Digital, single channel, with modulation
3 Analog, single channel
7 Digital, two or more channels
8 Analog, two or more channels
9 Analog plus digital
Type of Intelligence Signal
N None
A Telegraphy, human
B Telegraphy, machine
C Fax
D Data, telemetry, control signals
E Telephony (human voice)
F Video, TV
W Some combination of any of the above

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Raheem Kolachi

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