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Communication Principles - 5

AM Vestigial Sideband (VSB) – a form of amplitude modulation in which...

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Electronics Engineering (CR 061)

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LESSON CONTENT

AM Single-Sideband Reduced Carrier (SSBRC) – a form of amplitude modulation in which one sideband is totally removed and the carrier voltage is reduced to approximately 10% of its unmodulated amplitude. Sometimes called single-sideband reinserted carrier.

Pilot Carrier – it is the reinserted carrier in SSBRC for demodulation purposes.

AM Independent Sideband (ISB) – a form of amplitude modulation in which a single carrier frequency is independently modulated by two different modulating signals.

AM Vestigial Sideband (VSB) – a form of amplitude modulation in which the carrier and one complete sideband are transmitted, but only part of the second sideband is transmitted.

VSB System – is the picture portion of a commercial television broadcasting signal.

Comparison of Single-Sideband Transmission to Conventional AM It can be seen that bandwidth conservation and power efficiency are obvious advantages of single-sideband suppressed- and reduced-carrier transmission over conventional double-sideband full-carrier transmission. Single-sideband transmission requires only half as much bandwidth as double sideband, and suppressed- and reduced-carrier transmissions require considerably less total transmitted power than full-carrier AM.

Complex receivers – single-sideband systems require more complex and expensive receivers than conventional AM transmission because most single-sideband transmissions include either a reduced or a suppressed carrier; thus, envelope detection cannot be used unless the carrier is regenerated at an exalted level.
Tuning Difficulties – single-sideband require more complex and precise tuning than conventional AM receivers.

Mathematical Analysis of Suppressed-carrier AM An AM modulator is a product modulator; the output signal is the product of the modulating signal and the carrier. 𝐀𝐀𝐀 ( 2( 2( 2( 2(( 2( 2( 2( 2( 2( 2( 2( 2( 2( 2) = [1 + sin( 2 𝐀 [[[[[[[[[[[[[[[)][𝐀 𝐀 sin( 22222222222222 2 𝐀𝐀)] Where 1 + 22222222222222 sin( 2 𝐀𝐀) = constant + modulating signal 𝐀𝐀 sin( 22222222222222 2 𝐀𝐀) = unmodulated carrier

If the constant component is removed from the modulating signal, then 𝐀𝐀𝐀( ([2([2([2([2([2([2([2([2([2([2([2([2([2([2) = [ sin( 2 𝐀)][𝐀[[[[[[[[[[[[[[[ 𝐀 sin( 22222222222222 2 𝐀𝐀)] (6-1)

Multiplying yields

𝐀𝐀𝐀 ((((((((((((((() = − 𝐀𝐀 cos[2(2(2(2(2(2(2(2(2(2(2(2(2(2( 2(𝐀 + 𝐀 )))))))))))))))] + 𝐀𝐀 cos[2(2(2(2(2(2(2(2(2(2(2(2(2(2( 2(𝐀 − 𝐀𝐀 )))))))))))))))] 2 2

Where − 𝐀2𝐀 𝐀 cos[2(2(2(2(2(2(2(2(2(2(2(2(2(2( 2(𝐀 + 𝐀 )))))))))))))))] = upper side frequency component

𝐀2𝐀𝐀 cos[2(2(2(2(2(2(2(2(2(2(2(2(2(2( 2(𝐀 − 𝐀 )))))))))))))))] = lower side frequency component

Single-Sideband Generation - DSBSC Modulators – modulator circuits that inherently remove the carrier during the modulation process. - Balanced Ring Modulator o Balanced Modulator – a circuit that produces a doublesideband suppressedcarrier signal

Circuit operation – Essentially, diodes D 1 to D 4 are electronic switches that control whether the modulating signal is passed from input transformer T 1 to output transformer T 2 as is or with a 180o phase shift. With the carrier polarity as show in figure 6-5b, diode switches D 1 and D 2 are forward biased and on, while diode D 3 and D 4 are reversed biased and off. Consequently, the modulating signal is transferred across the closed switches to T 2 without phase reversal. When the polarity of the carrier reverses, as shown in figure 6-5c, diode switches D 1 and D 2 are reversed bias and off, while diode switches D 3 and D 4 are forward biased and on. Consequently, the modulating signal undergoes 180o phase reversal before reaching T 2. Carrier current flows from its source to the center taps of T 1 and T 2 , where it splits and goes in opposite directions through the upper and lower halves of the transformers. Thus, their magnetic fields cancel in the secondary windings of the transformer and the carrier is suppressed. If the diodes are not perfectly matched or if the transformers
Linear Summer – the circuit where the carrier is reinserted. - Single-Sideband Transmitter – Filter method 1 𝐀⁄ ⁄ 𝐀 = 𝐀((((((((((((((( −1 20) 2 4 ∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆ Where Q = quality factor 𝐀𝐀 = center or carrier frequency S = dB level of suppression of unwanted sideband ∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆ = frequency separation between the highest lower sideband frequency and the lowest upper sideband frequency
Crystal Filters – commonly used in single-sideband systems.
Ceramic Filters – made from lead zirconate-titanate, which exibits the piezoelectric effect. o Precautions must be taken  Impedance matching and load conditions – ceramic filters differ from coils in that their impedance cannot readily be changed  Spurious signals – in practically all cases where ceramic filters are used, spurious signals are generated. To suppress these responses, impedance matching with IF transformer is the simplest and most effective way.  Matching coils – when difficulties arise in spurious response suppression or for improvement in selectivity or impedance matching in IF stages, use of an impedance matching coil is advised  Error in wiring input and output connections – care must be taken when connecting the input and output terminals of a ceramic filter. Any error will cause waveform distortion and possibly frequency deviation of the signal  Use of two ceramic filters in cascade – for best performance, a coil should be used between two ceramic filter units. When cost is a factor and a direct connection is necessary, a suitable capacitor or resistor can be used.
Mechanical filter – is a mechanically resonant transducer. It receives electrical energy, converts it to mechanical vibrations, and then converts the vibrations back to electrical energy at its outputs.
Surface Acoustic Wave Filters – filters that use acoustic energy rather than electromechanical energy to provide excellent performance for precise bandpass filtering o Destructive Interference – reflected energy that cancels and attenuates the incident wave energy o Constructive Interference – reflected energy that aids the incident wave energy o Unidirectional Transducer – a transducer which launches the acoustic wave in only one direction Single-Sideband Transmitter: Phase Shift Method

Independent Sideband

b. A 0% drift would cause a decrease in the RF local oscillator frequency of ∆∆∆∆∆∆∆∆∆∆∆∆∆∆∆ = (0)(20 ) = 200 Thus, the RF local oscillator frequency would drift down to 19 Hz and the output from the RF mixer is 𝐀𝐀𝐀 = (30 30 ) − 19 = 10. 10 𝐀𝐀𝐀 The demodulated information signal spectrum is the difference between the intermediate frequency band and the BFO, or 𝐀𝐀 = (10 10 ) − 10 = 200 5200 The 0% drift in the RF local oscillator frequency caused a corresponding 200 Hz error in the demodulated information signal spectrum.

Coherent Single-Sideband BFO Receiver - This receiver is identical to the BFO receiver shown in figure 6-19 except that the LO and BFO frequencies are synchronized to the carrier oscillators in the transmitter.

Single-Sideband Envelope Detection Receiver – it uses synchronous carriers and envelope detection to demodulate the received signals

Multichannel Pilot Carrier Single-Sideband Receiver – uses a PLL carrier recovery circuit and a frequency synthesizer to produce coherent local and beat frequency oscillator frequencies

Amplitude-Companding Single-Sideband (ACSSB) - Systems that provide narrowband voice communications for land-mobile services with nearly the quality achieved with FM systems and do it using less than one-third the bandwidth

Single-Sideband Suppressed Carrier and Frequency-Division Multiplexing Multiplexing – the process of combining transmissions from more than one source and transmitting them over a common facility, such as metallic or optical fiber cable or a radio-frequency channel. Single-Sideband Suppressed-Carrier Transmission – a transmission that can be used to combine hundreds or even thousands of narrowband channels into single, composite wideband channel without the channels interfering with each other.

Single-sideband transmitters are rated in peak envelope power (PEP) and peak envelope voltage (PEV) rather than simply rms power and voltage.
The voltage developed across the load is:

𝐀𝐀𝐀𝐀𝐀𝐀

Where E 1 and E 2 are the rms voltages of the two test tones. Therefore,

𝐀𝐀𝐀

𝐀

And because E 1 = E 2 , 𝐀𝐀𝐀 = 2 2 𝐀

However, the average power dissipated in the load is equal to the sum of the powers of the two tones: 𝐀 12 𝐀 22 2222222222222222 𝐀 2

𝐀𝐀𝐀𝐀 = + = = 222222222222222 222222222222222 222222222222222 𝐀

Which simplifies to 𝐀𝐀𝐀𝐀 = 𝐀𝐀𝐀 2

Two equal-amplitude test tones are used for the test signal for the following reasons:

One tone produces a continuous single-frequency output that does not produce intermodulation.
A single-frequency output signal is not analogous to a normal information signal.
More than two tones make analysis impractical
Two tones of equal amplitude place a more demanding requirement on the transmitter than is likely to occur during normal operation.

Example: For a two-tone test signal of 1 kHz and 3 kHz and a carrier frequency of 100 kHz, determine for a single-sideband suppressed-carrier transmission a. Output Frequency spectrum if only the upper sideband is transmitted. b. For E 1 = E 2 = 5 V and a load resistance of 50 ohms, the PEP and average output power.

Solution a. The output frequency spectrum contains the two upper side frequencies: 𝐀𝐀𝐀𝐀1 = 100 + 1 = 101. 𝐀𝐀𝐀𝐀2 = 100 + 3 = 103

b. 𝐀𝐀𝐀 = 2 (0×5) 2 = 0 50 𝐀𝐀𝐀 0 𝐀𝐀𝐀𝐀 = = = 0. b. For E 1 = E 2 = 20 Vp and a load resistance RL = 100 ohm, determine PEP and average power.

For the SSB receiver shown in figure 6-22, RF input frequency fRF = 36 MHz, RF local oscillator frequency fIO = 26 MHzm and a 3-kHz modulating-signal frequency, determine the following : BFO output frequency and detected information frequency.

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