frequencyModulation
On the Internet I found FSK generator connection diagram. And I'm trying to connect up it in Qucs.
I' sure, 555 timer is working properly. But at the output of the whole schema I get unclear result.
P.S. Output.Vt does not depend on the input bit stream.
Look, please, what could be wrong?
FSK is difficult to visualize as you increase in order. The reason for this is when you are using FSK, you have orthogonal frequencies that essentially add an extra dimension to your plane. You can visualize up to 3d (3 frequencies) as shown below (pardon the hand paint drawing), but once you get greater then that FSK just can't be represented this way. However, just because we aren't able to plot it on a graph, the matrix math still applies equally the same, just with added dimensions.
I believe you mean a discontinuous phase when switching between two symbols. If the phase jumps at the border between two symbols, your modulated signal is discontinuous. This means that it contains many (theoretically all) frequencies, so it has a large bandwidth. On the other hand, if you always have a continuous phase, there are no discontinuities in the modulated signal, which requires much less bandwidth. Note that the smoother a time-domain signal is, the smaller is its high-frequency content. FSK with a continuous phase is called (what else?) continuous phase FSK (CPFSK). It is a special case of continuous phase modulation (CPM). Also check this link.
Best Answer
If you look at the left side of the original circuit you will see that the input data rate is expected to be 150 Hz, and the output frequency then switches between 1070 Hz and 1270 Hz.
Your input signal is 100 MHz, much too fast for this circuit. To perform FSK with an input frequency of 100 MHz you will need an oscillator with a nominal frequency of 1 GHz or so...and a 555 timer will never achieve that.