Chapter 3 : Single-Sideband (SSB) Communication System Chapter contents

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Chapter 3 Single-Sideband (SSB) Communication
SystemChapter contents

• 3.1 Single-Sideband System
• SSBFC, SSBSC, SSBRC
• 3.2 Transmission to Conventional AM
• Power conservation, bandwidth conservation,
  selective fading, noise reduction, complex
  receivers, tuning difficulties
• 3.3 Single-Sideband Transmitters
• Filter method, Phase-Shift Method
• 3.4 Single-Sideband Receivers
• SSB BFO Receiver, Coherent SSB BFO Receiver
• 3.5 Single-Sideband and Frequency-Division
  Multiplexing

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Chapter 3 Single-Sideband (SSB) Communication
System

• 2 main disadvantages of the conventional AM DSBFC
• Carrier power constitutes 2/3 or more of the
  total transmitted power no information in the
  carrier.
• Utilize twice as much bandwidth both the upper
  and lower sideband actually contains same
  information (redundant).
• Introduce several systems of SSB and their pros
  and contras over the conventional AM DSBFC system
• Comparison of frequency spectrum and relative
  power distribution for AM DSBFC and several
  common SSB systems

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3.1.1 AM Single-Sideband Full Carrier (SSBFC)

• The carrier is transmitted at full power but only
  one sideband is transmitted
• requires half the bandwidth of DSBFC AM
• Carrier power constitutes 80 of total
  transmitted power, while sideband power consumes
  20
• SSBFC requires less total power but utilizes a
  smaller percentage of the power to carry the
  information

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3.1.1 AM Single-Sideband Full Carrier (SSBFC)

• The output modulated signal
• as SSB only has one sideband, the peak change in
  the envelope is only half of what it is with
  DSBFC
• Therefore, the demodulated wave has only half the
  amplitude of the DSB modulated wave

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3.1.2 AM Single-Sideband Suppressed Carrier
(SSBSC)

• The carrier is totally suppressed and one
  sideband is removed
• requires half the bandwidth of DSBFC AM
• Considerably less power than DSBFC and SSBFC
  schemes
• Sideband power makes up 100 of the total
  transmitted power
• The wave is not an envelope but a sine wave at
  frequency equal to the carrier frequency
  modulating frequency (depending on which
  sideband is transmitted)

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3.1.3 AM Single-Sideband Reduced Carrier (SSBRC)

• One sideband is totally removed and the carrier
  voltage is reduced to approximately 10 of its
  unmodulated amplitude
• requires half the bandwidth of DSBFC AM
• Less transmitted power than DSBFC and SSBFC but
  more power than SSBSC
• As much as 96 of the total transmitted power is
  in the sideband
• The output modulated signal is similar to SSBFC
  but with reduced maximum and minimum envelope
  amplitudes

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3.2 Comparison of Single-Sideband Transmission
to Conventional AM
3.2.1 SSB Advantages Power Conservation

• With SSB, only one sideband is transmitted and
  the carrier is either suppressed or reduced
  significantly
• Much less total transmitted power is necessary to
  produce the same quality signal as achieved with
  DSBFC AM
• Eliminating the carrier would increase the power
  available for the sidebands by at least a factor
  of 3, providing approximately a 4.8 dB
  improvement in the signal-to-noise ratio

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3.2.2 SSB Advantages Bandwidth Conservation

• SSB requires half as much bandwidth as DSB AM,
  which is important today with the already
  overcrowded RF spectrum
• 50 reduction in bandwidth for a SSB compared to
  DSB equal to an improvement in the
  signal-to-noise ratio of 3 dB
• By combining the bandwidth improvement and the
  power advantage of removing the carrier, the
  overall improvement in the signal-to-noise ratio
  using SSBSC is approximately 7.8 dB better that
  DSBFC

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3.2.3 SSB Advantages Selective Fading

• With DSB, the carrier and two sidebands may
  propagate through the channel by different paths
  and experience different transmission impairment
  called as selective fading.
• 3 types of selective fading
• Sideband fading one sideband is significantly
  attenuated resulting in a reduced signal
  amplitude at the output of the receiver and
  causing some distortion but not detrimental to
  the signal because the 2 sidebands contain the
  same information.
• Carrier fading reduction of the carrier level
  of a 100 modulated wave will make the carrier
  voltage less than the sum voltage of the
  sidebands. Consequently, the envelope resembles
  an overmodulated envelope and causing distortion.
• Carrier or sideband phase shift as the position
  change, a change in the shape of the envelope
  will occur, causing severely distorted
  demodulated signal.
• With SSB, carrier phase shift and carrier fading
  can not occur, thus smaller distortion is
  expected.

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3.2.4 SSB Advantages Noise Reduction

• As SSB only utilizes half as much bandwidth as
  conventional AM, the thermal noise power is
  reduced to half that of a DSB system
• Considering both the bandwidth reduction and the
  immunity to the selective fading, SSB system has
  an approximately a 12 dB S/N ratio advantage over
  DSB system
• This means the DSB system must transmit 12 dB
  more powerful signal to achieve the same
  performance as the SSB system

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3.2.5 SSB Disadvantages Complex receivers

• SSB requires more complex and expensive receivers
  than DSB.
• As SSB includes either a reduced or a suppressed
  carrier, envelope detection cannot be used. SSB
  requires a carrier recovery and synchronization
  circuit, which adds to their cost, complexity and
  size.

3.2.6 SSB Disadvantages Tuning difficulties

• SSB requires more complex and precise tuning than
  the DSB receiver.

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3.3 SSB Transmission

• transmitters used for SSB suppressed and reduced
  carrier transmission are identical except that
  the re-inserted carrier transmitters have an
  additional circuits that adds a low amplitude
  carrier to the single sideband waveform after
  suppressed-carrier modulation has been performed
  and one of the sideband has been removed.
• the re-inserted carrier is called a pilot
  carrier.
• the circuit where the pilot carrier is
  re-inserted is called a linear summer.
• 3 transmitter configurations are commonly used
  for single sideband generation
• Filter method
• Phase shift method
• Third method

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3.3.1 Filter Method

• Block diagram for a SSB transmitter using
  balanced modulators to suppressed the unwanted
  carrier and filters to suppress the unwanted
  sideband.
• The low frequency IF is converted to the final
  operating frequency band through a series of
  frequency translation
• 3-stages of frequency up-conversion
• modulating signal is an audio spectrum that
  extends from 0 kHz 5 kHz

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3.3.1 Filter Method

• modulating signal mixes with a low frequency (LF)
  100 kHz carrier in the balanced modulator 1 to
  produced a DSB frequency spectrum centered at the
  suppressed 100 kHz carrier.
• bandpass filter 1 (BPF 1) that is tuned to a 5
  kHz bandwidth centered at 102.5 kHz used to
  eliminate the lower sideband and pass only the
  upper sideband.
• the pilot carrier or reduced amplitude carrier is
  added to the single-sided waveform in the carrier
  re-insertion stage (summer).
• the summer is a simple adder circuit that
  combines the 100 kHz pilot carrier with the 100
  kHz 105 kHz upper sideband frequency spectrum.

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3.3.1 Filter Method

• Output of the summer is the SSBRC waveform. the
  SSBRC waveform is mixed in the balanced modulator
  2 with a 2 MHz medium frequency (MF) carrier.
• output is a DSB suppressed carrier signal in
  which the upper and lower sidebands each contain
  the original SSBRC frequency spectrum.
• upper and lower sidebands are separated by a 200
  kHz frequency band that is void of information.

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3.3.1 Filter Method

• the lower sideband then is filtered (cut) through
  the BPF 2 (5 kHz bandwidth centered at 2.1025
  MHz.
• the output from BPF 2 is once again a single
  sideband reduced carrier waveform with a reduced
  2.1 MHz carrier and a 5 kHz wide upper sideband.
• then the SSBRC waveform from BPF 2 is mixed in
  the balanced modulator 3 with the 20 MHz high
  frequency carrier (HF), producing a double
  sideband suppressed carrier signal in which the
  upper and lower sidebands each contain the
  original SSBRC frequency spectrum.
• upper and lower sidebands are separated by a 4.2
  MHz frequency band that is void of information.

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3.3.1 Filter Method

• the lower sideband then is filtered (cut) through
  the BPF 3 (5 kHz bandwidth centered at 22.1025
  MHz.
• the output from BPF 3 is once again a single
  sideband reduced carrier waveform with a reduced
  22.1 MHz RF carrier and a 5 kHz wide upper
  sideband.
• Conclusion
• the original modulating signal frequency spectrum
  was up-converted in 3 modulation steps to a final
  carrier frequency of 22.1 MHz and a single upper
  sideband that extended from the carrier (22.1
  MHz) to 22.105 MHz.
• after each up-conversion (frequency translation),
  the desired sideband is separated from the double
  sideband spectrum with a bandpass filter (BPF).

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3.3.1 Filter Method

• Why not using single heterodyning process (1
  balanced modulator, 1 bandpass filter single HF
  carrier) ?
• Block diagram of a single conversion SSBSC
  transmitter
• the output of the balance modulator is a DSB
  spectrum centered around a suppressed carrier
  frequency of 22.1 MHz.

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3.3.1 Filter Method

• to separate the 5 kHz upper sideband from the
  composite spectrum, a bandpass filter with
  extremely high Q is required.
• for fixed modulating bandwidth, the filter Q
  increase rapidly with the centre frequency.
• the difficulty with this method the filter with
  high Q is difficult to construct and not economic.

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3.3.1 Filter Method

• the solution to this direct filtering is to use a
  3-stages up-conversion as explained previously.
• the advantages of the 3-stages up-conversion as
  compared to single-conversion transmitter on the
  selection of BPF.
• to construct a 5 kHz wide, steep-skirted BPF at
  100 kHz (BPF 1) is relatively simple as only a
  moderate Q is required.
• the sideband at BPF 2 are 200 kHz apart, thus a
  low Q-filter with gradual roll-off
  characteristics can be used with no danger of
  passing any portion of unwanted sideband.
• if multiple channel are used and the HF carrier
  is tunable, a broadband filter can be used for
  BPF 3 with no danger of any portion of the
  undesired sideband leaking through the filter.

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3.3.2 Phase Shift Method

• with phase-shift method, the undesired sideband
  is cancelled in the output of the modulator.
• Block diagram of a SSB transmitter using
  phase-shift method
• use 2 separate DSB modulators (balanced modulator
  1 2).
• modulating signal and carrier are applied
  directly to one of the modulators, then both are
  shifted 90º and applied to the second modulator.
• the outputs from the two balanced modulators are
  DSBSC signals with the proper phase (when they
  are combined in a linear summer, the upper
  sideband is cancelled).

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3.3.2 Phase Shift Method

• Mathematical analysis of the phase-shift
  transmitter
• modulating signal (sin wmt) is fed directly to
  balanced modulator 1 and shifted 90º (cos wmt)
  and fed to balanced modulator 2.
• carrier signal (sin wmt) is also fed directly to
  balanced modulator 1 and shifted 90º (cos wmt)
  and fed to balanced modulator 2
• the outputs of the balanced modulators are
  expressed as
• Output of balanced modulator 1
• (1)
• Output of balanced modulator 2
• (2)

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3.3.2 Phase Shift Method

• the final output from the linear summer
• (3)
• which is the lower sideband of the AM wave.

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3.4 SSB Receivers3.4.1 SSB BFO Receiver

• Block diagram for a simple noncoherent SSB BFO
  receiver
• in a receiver, the input signal (suppressed or
  reduced carrier and one sideband) is amplified
  and then mixed with the RF local oscillator
  frequency to produce intermediate frequency.
• the output from the RF mixer is then goes through
  further amplification and band reduction prior to
  second mixer.
• the output from the IF amplifier stage is then
  mixed (heterodyned) with beat frequency
  oscillator (BFO) frequency.

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3.4.1 SSB BFO Receiver

• BFO frequency is equal to the IF carrier
  frequency. Thus the difference between the IF and
  the BFO frequency is the information signal.
• i.e. the output from the IF mixer is the sum and
  difference frequencies between the IF and the
  beat frequency. The difference frequency band is
  the original input information.
• the receiver is classified as noncoherent because
  the RF oscillator and the BFO signals are not
  synchronized to each other and to the oscillators
  in the transmitter.
• Consequently, any difference between the
  transmitter and receiver local oscillator
  frequencies produces a frequency offset error in
  the demodulated information signal.
• the RF mixer and IF mixer are product detectors.
  A product detector and balanced (product)
  modulator are essentially the same circuit.

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3.4.1 SSB BFO Receiver

• Ex 6-2

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3.4.2 Coherent SSB BFO Receiver

• Block diagram for a coherent SSB BFO receiver
• this type of receiver is identical to the
  previous noncoherent type, except that the LO and
  BFO frequencies are synchronized to the carrier
  oscillators in the transmitter.
• the carrier recovery circuit is a narrowband PLL
  that tracks the pilot carrier in the SSBRC
  signal.
• the recovered carrier is then used to generate
  coherent local oscillator frequencies (RF LO
  frequency BFO frequency) in the synthesizer.

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3.4.2 Coherent SSB BFO Receiver

• any minor changes in the carrier frequency in the
  transmitter are compensated in the receiver, and
  the problem of frequency offset error is
  eliminated.
• Ex 6-3

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3.5 SSB and Frequency-Division Multiplexing

• the most common application of SSB (especially
  SSBSC) is frequency-division multiplexing (FDM)
  due to the bandwidth and power efficiencies of
  SSB system.
• Frequency-division multiplexing is an analog
  method of combining two or more analog sources
  that originally occupied the same frequency band
  in such a manner that the channels do not
  interfere with each other
• Example of simple FDM system where four 5 kHz
  channels are frequency-division multiplexed into
  a single 20 kHz channel

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3.5 SSB and Frequency-Division Multiplexing

• channel 1 signals modulate a 100 kHz carrier in a
  balanced modulator. The output is a DSBSC with a
  bandwidth of 10 kHz.
• the DSBSC wave is then passed through BPF
  producing a SSBSC signal occupying the frequency
  band between 100 kHz and 105 kHz.
• channel 2 signals modulate a 105 kHz carrier
  producing a DSBSC wave that is converted to SSBSC
  by passing it through a BPF.
• the output from the BPF occupies the frequency
  band between 105 kHz and 110 kHz.
• similar process is used to convert channel 3 and
  channel 4 signals to the frequency bands 110 kHz
  to 115 kHz and 11f kHz to 120 kHz, respectively.

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3.5 SSB and Frequency-Division Multiplexing

• the combined frequency spectrum produced by
  combining the outputs from 4 filters is shown
  below.
• the total combined bandwidth is equal to 20 kHz
  and each channel occupies a different 5 kHz
  portion of the total 20 kHz bandwidth.
• FDM is used extensively to combine many
  relatively narrowband channels into a single ,
  composite wideband channel without the channel
  interfering with each other.
• Ex public telephone systems