Hello again, I asked some questions about a a month ago. First, thanks for all the answers! I've been keeping busy with some other stuff but now I want to return to this subject once more. Unfortunately I still don't seem to be able to grasp much. To reiterate, this is not my field at all! If anyone does take time to help me, please feel free to treat me as much like an idiot as you feel is necessary for me to have a chance at understanding this stuff :)>>>Take a system h that operates on a signal x to generate another signal>>>y, such that y(t) = h(x(t)). If x(t) = x1(t) + x2(t) thensuperposition>> >> >>>holds if y(t) = h(x1(t) + x2(t)) = h(x1(t)) + h(x2(t)). So it holds in>>>the BPSK system that I was babbling about above, but it doesn't >>>necessarily* hold in a FSK system because the value of one bit might >>>color the phase of all succeeding bits. >> >> >> See, I know what superposition means and I know PSK is linear and FSK >> is supposed to be non-linear, and I can follow your argument abouth(t),>> x(t) etc. and when I apply it to the PSK equation I can see that the >> superposition holds. The problem is that when I apply it to FSK, thesame>> thing happens and it appears superposition holds in this case too! >> >> I must not understand how to apply it correctly, so is there anychance>> you could walk me through the process? >> >In the case of PSK (B-, Q- or M-, you choose) the contribution of the >n'th symbol is always the same thing for the same signal, and it's >always additive. In the general case for FSK the n'th symbol causes the>phase of the signal to rotate through a certain amount, starting from >wherever it left off at the end of the (n-1)'th bit-time. Thus >superposition (in general) fails to hold.I'm afraid I still don't understand you. Can you help me through it again, with tiny, tiny baby steps this time? :) I'm not mathematically gifted but I think it still might be easier for me to follow equations. Let's consider a general BFSK system where the modulated signal is generated by switching between two LOs, ie not phase-continuous. Using some "pseudo"-LaTeX notation, I represent such an FSK signal by: s(t) = Re[ sum_{m} e^{j*2*pi*a_{m} * f * (t - mT)} * e^{j*2*pi*f_{c}*t} ] where a_{m} are the symbols, f is the frequency deviation, f_{c} is the carrier frequency, and T is the symbol period. Say that we want to transmit the sequence a={1, -1}. Using the criterion for linearity given above, the superposition h(x1(t) + x2(t)) = h(x1(t)) + h(x2(t)) should hold. Plugging in the appropriate values from the BFSK equation gives: x1(t) = e^{j*2*pi*f*t} x2(t) = e^{-j*2*pi*f*(t-T)} h(u(t) = Re[ u(t) * e^{j*2*pi*f_{c}*t} ] which results in: y(t) = h(x1(t) + x2(t)) = = Re[ (x1(t) + x2(t)) * e^{j*2*pi*f_{c}*t} ] = = Re[ x1(t) * e^{j*2*pi*f*t} ] + Re[ x2(t) * e^{j*2*pi*f*t} ] = = h(x1(t)) + h(x2(t)) => superposition holds => linear modulation?! Where my error(s) at? :( This message was sent using the Comp.DSP web interface on www.DSPRelated.com