Chapter 3 : Single-Sideband SSB Communication System Chapter contents

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Chapter 3 : Single Sideband Transmission . BENG 2413 Communication Principles Faculty of Electrical Engineering . 2. Chapter 3 : Single-Sideband (SSB) Communication System. 2 main disadvantages of the conventional AM DSBFCCarrier power constitutes 2/3 or more of the total transmitted power ? no inf
Chapter 3 : Single-Sideband SSB Communication System Chapter...

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1. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 1 Chapter 3 : Single-Sideband (SSB) Communication System Chapter 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

2. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 2 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

3. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 3 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

4. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 4 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

5. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 5 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)

6. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 6 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

7. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 7 3.2 : Comparison of Single-Sideband Transmission to Conventional AM 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

8. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 8 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

9. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 9 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.

10. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 10 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

11. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 11 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.

12. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 12 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

13. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 13 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

14. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 14 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.

15. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 15 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.

16. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 16 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.

17. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 17 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).

18. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 18 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.

19. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 19 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.

20. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 20 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.

21. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 21 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 90? 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).

22. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 22 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 90? (cos wmt) and fed to balanced modulator 2. carrier signal (sin wmt) is also fed directly to balanced modulator 1 and shifted 90? (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)

23. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 23 3.3.2 : Phase Shift Method the final output from the linear summer : (3) which is the lower sideband of the AM wave.

24. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 24 3.4 SSB Receivers 3.4.1 : 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.

25. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 25 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.

26. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 26 3.4.1 : SSB BFO Receiver Ex 6-2 :

27. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 27 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.

28. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 28 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 :

29. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 29 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 :

30. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 30 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.

31. Chapter 3 : Single Sideband Transmission BENG 2413 Communication Principles Faculty of Electrical Engineering 31 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


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