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.