Hello, I am writing a filter ( for pulse shaping ) followed by a modulation ( BPSK ) scheme for a satellite transmission. I have a result, but I would like to hear your opnion about it. I am using a root raised cosine filter for the input stream, and applying quadrature modulation after it. For instance, suppose I want to transmit this symbol stream : 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1 After I apply the root raised cosine filter, I get this http://ns.lsc.ic.unicamp.br/~samuel.goto/modulation1.png I thing this is right, 'cause it has the same input stream form but shaped ... Does this look ok for you ? It looks nice for me =) Well, ok. I guess this is right. But things get a bit stranger when I do the quadrature modulation. As far as I know, the nice thing about quadrature modulation is that it shows when symbols change values with phase changes ... so, for this simple input, with only one symbol change ( from 1 to -1 ), I get a nice phase change in the carrier signal ( first graph, in red ). http://ns.lsc.ic.unicamp.br/~samuel.goto/modulation2.png The second graph shows the carrier signal modulated by the filtered sinal, and it also shows the phase change. My problem is : isn't pulse shaping ( with root raised cosine filter ) done to eliminate high frequencies ? In this second graph ( wich is the filtered signal modulated ), seems to have higher frequencies than the first one ( wich isn't filtered ). I was expecting a smoother wave for the filtered signal. What is the problem ? Thanks in advance, samuel goto
Strange result of a filter + modulation
Started by ●January 17, 2006