3rd qrtr p comm sc part 2

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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 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).

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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 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 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

With SSB, only one sideband is transmitted and the carrier is either suppressed or reduced significantly

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

3.2.1 SSB Advantages : Power Conservation

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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

3.2.2 SSB Advantages : Bandwidth Conservation

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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.

Carrier or sideband phase shift : as the position change, a change in the shape of the envelope will occur, causing severely distorted demodulated signal.

3.2.3 SSB Advantages : Selective Fading

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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

3.2.4 SSB Advantages : Noise Reduction

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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.

3.2.5 SSB Disadvantages : Complex receivers

3.2.6 SSB Disadvantages : Tuning difficulties SSB receivers require more complex and precise tuning than the DSB

receivers.

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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.

B

fQ

c

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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)

tt

tt

mcmc

cm

cos2

1)cos(

2

1

))(sin(sin

tt

tt

mcmc

cm

cos2

1)cos(

2

1

))(cos(cos

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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.

tttt mcmcmcmc cos2

1cos

2

1cos

2

1cos

2

1

tmc cos

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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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3.4.2 : 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.

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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