In chapter 2, basic principles of SSB were shared once. In Amplitude Modulation (AM), the net result of modulation is two side bands, both modulated with the modulating intelligence and a carrier to take both these away out of the transmitter. The lower side band will be lower than the carrier frequency by carrier frequency minus audio frequency and the upper side band will be higher than the carrier wave frequency by carrier frequency plus modulation frequency. The creation of Side bands is not like one each on both sides to represent all sounds but two side bands each for every sound in the modulating Audio Frequency. Putting everything in another words, the number of sidebands in a common complex audio signal could be countless because any sound is a mix of many simple sounds. If you consider the modulating audio frequency as intelligence, you will also easily see intelligence being transferred to sidebands first and the amplitude of the carrier changes proportionally only next.
In a transmitter, two third of the power is consumed by the carrier. Since carrier is necessary for demodulation, it should necessarily be there with the sidebands. Some percentage of energy can be saved through transmitting carrier and one side band only, because both the sidebands contain modulated intelligence. Anther possible observation was that the carrier need not necessarily have travelled a long distance to be fit to join the signal for demodulation - a signal at the carrier frequency can easily be oscillated within the Receiver itself. If there is provision in the receiver to generate the carrier signal even if both the sidebands are transmitted energy is saved by one third and the quality is not spoiled by even a tiny percent. Modifications and alterations went on until it was found that a single side band only is required for effective transmission of intelligence. Self oscillated carrier in the receiver added with the received side band is enough for the demodulator to function. Even though power is heavily saved in SSB transmission, the audio quality is highly challenged making SSB transmissions unacceptable to Broadcast Commercial transmission companies.
SSB transmission technique was a blessing in disguise for the Radio Amateurs. The saved energy and space. An SSB transmission using the same power flew to 10 times more distance. In Dx, AM transmissions are generally considered odd now because in any allotted Ham Band spectrums three times more contact groups are possible with SSB. Without everyone compromising to limited spaces, it is natural that all bands become crowded and unusable.
SSB transmission technique was a blessing in disguise for the Radio Amateurs. The saved energy and space. An SSB transmission using the same power flew to 10 times more distance. In Dx, AM transmissions are generally considered odd now because in any allotted Ham Band spectrums three times more contact groups are possible with SSB. Without everyone compromising to limited spaces, it is natural that all bands become crowded and unusable.
In SSB, instead of the local oscillator in an AM transmitter, it is the exciter that develops the necessary signal frequencies. If modulation is in the final stage in AM transmissions, it is in the early stages in SBBs. Another difference is that if in AM carrier is continuous in SSB carried is generated only with sound. That is no sound means no carrier presence. In the various SSB generation methods, only in the phasing method that carrier frequency itself is used for modulation. Here, the audio and the carrier signals are separated by a 90 degree shift in phase, before bing fed into the balanced modulator. Balanced modulator refers to the stage in which the audio and the carrier gets mixed.Whatever be the the of balanced modulator that is used, modulation should happen there along with suppression of the carrier wave. That is, at the out put of the balanced modulator there should only be the two sidebands. The suppression that the carrier gets in the balanced modulation could vary from 40db to 80 db. The less the carrier suppression the more the carrier leak. In a general purpose ordinary SSB transmitter, a 40 to 50 db suppression only is required. A simple diode ring balanced modulator is actually enough to serve that purpose.
Briefing everything, a self generated carrier signal frequency mixes with the audio in the balanced modulator. After modulation, the carrier is suppressed and two sidebands appear at the out put of the balanced modulator. Further, one of the side bands is filtered out there and the remaining signal is fed into the linear amplifier stage. See fig. C-17/1A.
Two important features that encourage home brewers to assemble SSB transmitters are that balanced modulator and filter designs can be changed or modified according to the oscillator frequency and the in house feed back problems are practically nil because both the fundamental frequency and the transmission frequencies are different. Where the modulated signal is multiplied it may cause uncontrollable generation of distortion. Heterodyning is practically the only solution here. The purpose of the VFO here is also the same. Heterodyning is the production of a lower frequency from the combination of two almost equal high frequencies. Perhaps, the most expensive and crucial portion of an exciter could be the filter. That is why circuits designs that separate USB and LSB are using a single filter is common among Amateur home brewers. See fig. C-17/1B.
Take a BEL 9 MHz Crystal Filter for example. Together with this 9 MHz Crystal Filter two oscillators of 8.9985 MHz and 9.0015 MHz also are given. Suppose it is the 8.9985 MHz Crystal that we trigger, when it is mixed with a 3 KHz audio in the balanced modulator, a USB of 8.9985 MHz + 3 KHz and an LSB of 8.9985 MHz - 3 KHz are produced. Because the carrier is suppressed in the balanced modulator only sidebands come out of this stage. If the width of the 9 MHz Crystal Filter is 3 KHz, shouldn't it be from 8.9985 MHz to 9.0015 MHz? That is, only the USB that is generated when mixed with 8.9985 MHz come out through the Crystal Filter. The band width range of the LSB signal that is produced when the audio is mixed with 9.0015 MHz also will be the same. In short, if it is 9 MHz LSB signal that is expected from the filter, use 9.0015 MHz Crystal Oscillator and if it is USB that is expected, use the 8.9985 MHz Crystal Oscillator. Care should be taken in keeping Crystal Filter and the Oscillators matched, along with keeping the audio frequency width limited to the crystal filter width.
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