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Single sideband systems

AM Single-Sideband Full Carrier

AM single-sideband full carrier (SSBFC) is a

form of amplitude modulation in which the

carrier is transmitted at full power but only

one of the sidebands is transmitted.

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AM Single-Sideband Suppressed Carrier

• AM single-sideband suppressed carrier (SSBSC) is a form of amplitude modulation in which the carrier is totally suppressed and one of the sidebands removed.

• The frequency spectrum and relative power distribution for SSBSCwith upper sideband transmission are shown in Figure 6-lc.

• It can be seen that the sideband power makes up 100% of the total transmitted power.

• Figure 6-3 shows a SSBSC waveform for a single-frequency modulating signal. As you can see, the waveform is not an envelope; it is simply a sine wave at a single frequency equal to the carrier frequency plus the modulating-signal frequency or the carrier frequency minus the modulating-signal frequency, depending on which sideband is transmitted.

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AM Single-Sideband Reduced Carrier

• AM single-sideband reduced carrier (SSBRC) is 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.

• To produce a reduced carrier component, the carrier is totally suppressed during modulation and then reinserted at a reduced amplitude. Therefore, SSBRC is sometimes called single-sideband reinserted carrier.

• The frequency spectrum.and relative power distribution for SSBRC are shown in Figure 6-1d. The figure shows that the sideband power constitutes almost 100%

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AM Independent Sideband

• AM independent sideband (ISB) is a form of amplitude modulation in which a single carrier frequency is independently modulated by two different modulating signals.

• The single-sideband output signals from the two modulators are combined to forma double-sideband signal in which the two sidebands are totally independent of each other except that they are symmetrical about a common carrier frequency.

• One sideband is positioned above the carrier in the frequency spectrum and one below. For demodulation purposes, the carrier is generally reinserted at a reduced level as with SSBRC transmission. Figure 6-le shows the frequency spectrum and power distribution for ISB.

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AM Vestigial Sideband

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

• The carrier is transmitted at full power. In VSB, the lower modulating-signal frequencies are transmitted double sideband, and the higher modulating-signal frequencies are transmitted single sideband.

• The frequency spectrum and relative power distribution for VSB are shown in Figure 6-If.

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MATHEMATICAL ANALYSIS OF SUPPRESSED-CARRIER AM

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SINGLE-SIDEBAND GENERATION

• A circuit that produces a double-sideband suppressed-carrier signal is a balanced modulator.

• The balanced modulator has rapidly become one of the most useful and widely used circuits in electronic communications.

• In addition to suppressed-carrier AM systems, balanced modulators are widely used in frequency and phase modulation systems as well as in digital modulation systems, such as phase shift keying and quadratureamplitude modulation.

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Balanced Ring Modulator

• Essentially, diodes D, to D4 are electronic switches that control whether the modulating signal is passed from input transformer T1 to output transformer T2 as is or with a 180°phase shift.

• With the carrier polarity as shown in Figure 6-5b, diode switches D1 and D2 are forward biased and on, while diode switches D3 and D4 are reverse biased and off. Consequently, the modulating signal is transferred across the closed switches to T2 without a phase reversal.

• When the polarity of the carrier reverses, as shown in Figure 6-5c, diode switches D1 and D2 are reverse biased and off, while diode switches D3 and D4 are forward biased and on. Consequently, the modulating signal undergoes a 180°phase reversal before reaching T2.

• Carrier current flows from its source to the center taps of T1 and T2, 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 are not exactly center tapped, the circuit is out of balance, and the carrier is not totally suppressed. It is virtually impossible to achieve perfect balance; thus, a small carrier component is always present in the output signal. 12

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Single-Sideband Transmitter: Filter Method

• Figure 6-12 shows a block diagram for a SSB transmitter that uses balanced modulators to suppress the unwanted carrier and filters to suppress the unwanted sideband.

• The figure shows a transmitter that uses three stages of frequency up-conversion. The modulating signal is an audio spectrum that extends from 0 kHz to 5 kHz.

• The modulating signal mixes with a low-frequency (LF) 100-kHz carrier in balanced modulator 1 to produce a double-sideband frequency spectrum centered around the suppressed 100-kHz IF carrier.

• Bandpass filter 1 (BPF 1) is tuned to a 5-kHz bandwidth centered around 102.5 kHz, which is the center of the upper sideband frequency spectrum.

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• The low-frequency IF is converted to the final operating frequency band through a series of frequency translations.

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

• The output is a double-sideband suppressed-carrier signal in which the upper and lower sidebands again each contain the original SSBRC frequency spectrum.

• The sidebands are separated by a 4.2-MHz frequency band that is void of information. BPF 3 is centered on 22.1025 MHz with a 5-kHz bandwidth.

• Therefore, the output of BPF 3 is once again a single-sideband waveform with a reduced 22.1-MHz RF carrier and a 5-kHz-wide upper sideband. The output waveform is amplified in the linear power amplifier and then transmitted.

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• In the transmitter just described, the original modulating-signal frequency spectrum was up-converted in three modulation steps to a final carrier frequency of 22.1 MHz and a single upper sideband that extended from the carrier to 22.105 MHz.

• After each up-conversion (frequency translation), the desired sideband is separated from the double-sideband spectrum with a BPF. The same final output spectrum can be produced with a single heterodyning process: one balanced modulator, one bandpass filter, and a single HF carrier supply.

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Crystal filters

• The crystal lattice filter is commonly used in singlesideband systems. The lattice comprises two sets of matched crystal pairs (X1 and X2, X3 and X4) connected between tuned input and output transformers T1 and T2.

• Crystals X1 and X2 are series connected, whereas X3 and X4 are connected in parallel. Each pair of crystals is matched in frequency within 10 Hz to 20 Hz.

• X1 and X2 are cut to operate at the filter lower cutoff frequency, and X3 and X4 are cut to operate at the upper cutoff frequency. The input and output transformers are tuned to the center of the desired passband, which tends to spread the difference between the series and parallel resonant frequencies.

• C1 and C2 are used to correct for any overspreading of frequencydifference under matched crystal conditions.

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Single-sidebands receivers:Single-Sideband BFO Receiver

• The output from the IF amplifier stage is heterodyned (beat) with the output from a beat frequency oscillator (BFO). The BFO frequency is equal to the IF carrier frequency; thus, the difference between the IF and the BFO frequencies is the information signal.

• Demodulation is accomplished through several stages of mixing and filtering. The receiver is noncoherent because the RF local oscillator and BFO signals are not synchronized to each other or to the oscillators in the, transmitter.

• Consequently, any difference between the transmit and receive local oscillator frequencies produces a frequency offset error in the demodulated information signal.

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• The RF mixer and second detector shown in Figure 6-19 are product detectors. As with the balanced modulators in the transmitter, their outputs are the product of their inputs.

• With both the modulator and detector, the single-frequency carrier is the switching signal. In a receiver, the input signal, which is a suppressed or reduced RF carrier and one side-band, is mixed with the RF local oscillator frequency to produce an intermediate frequency.

• The output from the second product detector is the sum and difference frequencies between the IF and the beat frequency. The difference frequency band is the original input information.

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Coherent Single-Sideband BFO Receiver

• Figure 6-20 shows a block diagram for a coherent single-sideband BFO receiver.

• The carrier recovery circuit is a narrowband PLL that tracks the pilot carrier in the composite SSBRC receiver signal and uses the recovered carrier to regenerate coherent local oscillator frequencies in the synthesizer.

• The synthesizer circuit produces a coherent RF local oscillator and BFO frequency. The carrier recovery circuit tracks the received pilot carrier.

• Therefore, minor changes in the carrier frequency in the transmitter are compensated for in the receiver, and the frequency offset error is eliminated. If the coherent receiver shown in Figure 6-20 had been used in Example 6-2, the RF local oscillator would not have been allowed to drift independently.

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Single-Sideband Envelope Detection Receiver

• Figure 6-21 shows the block diagram for a single-sideband receiver that uses synchronous carriers and envelope detection to demodulate the received signals.

• The reduced carrier pilot is detected, separated from the demodulated spectrum, and regenerated in the carrier recovery circuit. The regenerated pilot is divided and used as the stable frequency source for a frequency synthesizer, which supplies the receiver with frequency coherent local oscillators.

• The receive RF is mixed down to IF in the first detector. A regenerated IF carrier is added to the IF spectrum in the last linear summer, which produces a SSB full-carrier envelope.

• The envelope is demodulated in a conventional peak diode detector to produce the original information signal spectrum. This type of receiver is often called an exalted carrier receiver

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SINGLE-SIDEBAND MEASUREMENTS

• Single-sideband transmitters are rated in peak envelope power (PEP) and peak envelope voltage (PEV) rather than simply rms power and voltage.

• Figure 6-26a shows the waveform produced in a SSBSC modulator with a two-tone modulating signal. The waveform is the vector sum of the two equal-amplitude side frequencies and is similar to a conventional AM waveform except that the repetition rate is equal to the difference between the two modulating-signal frequencies.

• Figure 6-26b shows the envelope for a two-tone test signal when a low-amplitude pilot carrier is added.

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