I think I want to back off this claim of odd behavior. The more I think about general CPFSK, the more I'm convincing myself that total accumulated phase will be changing, depending on the ratio of -1's to 1's. So seeing the real part slowly change phase should not be a worry, I think?
Rich On Mon, Dec 15, 2014 at 2:21 PM, Richard Bell <richard.be...@gmail.com> wrote: > > I have an update to this behavior. It is still not fixed. > > I was using a separate multiply block after the CPFSK block to control the > amplitude. There was no good reason for this, it's just how I set it up the > first time. > > I decided to remove that multiply block and use the built in amplitude > parameter to control the output gain. This has reduced the accumulating > phase offset that creeps into the CPFSK output, but not completely. > Overtime, the real part of the output will still begin to change phase. > > This leads me to believe there is some kind of thread timing issue that is > allowing a phase offset to creep into the CPFSK output on the transmitter > side. I'm including a screenshot of my transmitter. Not much going on here. > I am also including a screenshot of the real and imaginary time series when > the program first starts and after a few minutes of run time. > > Rich > > On Mon, Dec 15, 2014 at 10:28 AM, Richard Bell <richard.be...@gmail.com> > wrote: >> >> Problem: Real and Imaginary outputs of CPFSK sometimes seem flipped in >> the transmitter. >> >> I'm using the CPFSK block to modulate for a binary FSK radio. All that I >> reference here is in the transmitter. There are 3 input paramaters for the >> block: k (modulation index), A (Amplitude) and N (Samples/Symbol). The >> relationship between input and output of the CPFSK block is shown below and >> found in the source code: >> >> out[n] = A*exp( j * k * pi * 1/N * n * input[n] ) >> >> where input[n] = +1 or -1 is the input data to the block. >> >> Now with that all setup, here is the odd behavior. Since input[n] is only >> +1 and -1, I expect the real part of the output to remain unchanged and the >> input changes, because cos(-x) = cos(x) and I expect the imaginary part to >> flip signs as the input changes because sin(-x) = -sin(x). What I see is >> sometimes the cos will be flipping signs with the sin remaining constant. >> It is as though the inphase and quadrature arms were reversed. >> >> I am looking at the real and imaginary output of the CPFSK block in the >> transmitter. Because we are in the transmitter, there are no channel >> effects or synchronization effects that come into play to cause this. >> >> Does anyone know what could cause this? >> >> v/r, >> Rich >> >
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