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This makes practical realization of exact phase shifts difficult, requiring high precision components. Even then maintaining constant input/output from the modulators with changing temperature and voltages is difficult. However the technique does allow low power and low cost (no filters) generation of a SSBSC signal with no need to lower the signals frequency to aid filter cut-off characteristics. As well as this the system requires less time to install and switching to lower or higher sideband is simply a matter of switching inputs rather than connecting another filter.
Weaver Method The third method is really a hybrid of the second method and was arguably developed by D. K. Weaver in the 1950’s. It is covered extensively in his book A Third Method of Generation and Detection of Single-Sideband Signals, D. Weaver, Proceedings of the IRE, 1956. It is very similar to the phase method except that the broadband 90? phase difference is implemented by a quadrature mixing process by two quadrature oscillators.
A quadrature oscillator creates a cosine and sine wave with the same frequency at the same time.
The difference in frequency between the two oscillators will be the resulting offset. The information signal is fed into the first two balanced modulators and multiplied with the signal from the first quadrature oscillator, one signal in-phase (sine) and one signal in-quadrature (cosine) which constitutes the 90? shift. The resulting modulated signals are then passed to their own low pass filters. These two filters can be ordinary low-pass filters and their phase response is not an issue but they must be identical filters.
Each of the emerging two signals is then fed into a second pair of balanced modulators.
The signal initially mixed with the in-quadrature signal is mixed in the modulator with another in-quadrature signal from the second quadrature oscillator. The signal initially mixed with the in-phase signal is mixed in the modulator with another in-phase signal again from the second quadrature oscillator. The resulting two modulated signals are mixed and their sum becomes the SSBSC signal (diagram 3). The Weaver method is the most complicated way of generating the SSBSC signal and requires the use of filters. These filters must closely track each other in amplitude and phase for adequate rejection of the unwanted sideband.
However these filters can be just standard in frequency so avoiding the more expensive high performance models. The Weaver method does address distortion problems affecting the phase cancellation method, which are then carried on the remaining sideband. As always efficiency comes with complexity as its price. Conclusion An important aspect of SSBSC generation is the measurement of unwanted output energy compared to wanted output energy. In other words by how much the unwanted sideband is suppressed. A ratio of 40dB output power was once acceptable but recently suppression of 60dB or more has been targeted and achieved.
The phase method and the Weaver method became unpopular as high performance filters began to enter mass production and fell in price. The filter method became the practical solution for SSBSC signal generation and is the very system your company have installed. However with the advent of digital signal processing (DSP) image rejection performance of the levels mentioned above have begun to be realized with the phase cancellation and Weaver methods. This is possible due to DSP allowing precise number control in what is essentially a mathematical process.
This new found accuracy has resulted in the second two methods being the current choice for low cost, low power SSBSC signal generation with the Weaver method the more refined of the two. Although difficult to develop and install we recommend the Weaver method as the best alternative for your company.
Bibliography Web sites: http://search. ieice. org/2000/pdf/e83-b_6_1246. pdf http://www. arrl. org/tis/info/pdf/020708qex013. pdf http://www. elec. mq. edu. au/~cl/files_pdf/elec321/lect_ssb. pdf http://www. utdallas. edu/~kamran/courses/EE4250/lab5. pdf http://home. online. no/~lilandh/hfkomm2/app_b. pdf http://members.ozemail. com. au/~timhoop/otheraps. htm http://spigot. anu. edu. au/~haley/Teaching/ENGN3214/Lectures/Lecture23/lecture23. pdf http://www. vhfcomm. co. uk/hfssbcw. htm http://www. ssb. it/SSBOnline. nsf/V2010/4A8E2AB8FEB74C5D00256B0300530574 Text books: Electronic communiction fundamentals, T.
Adamson, Bergwell Productions, 1996 A Third Method of Generation and Detection of Single-Sideband Signals, D. Weaver, Proceedings of the IRE, 1956. SSB Technology Fights its Way into the Land-mobile Market, B. Manz, Microwaves and RF, Aug. , 1983. Robert J. Zavrel Jr. , EDN, May 16, J. Howells Page 1 5/8/.
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