The File Source on the bottmo left of the flow chart is assumed to hold the
pseudo-random sequence generated by the FPGA as provided by
./flashZedBoard.py -t T --noiselen L --bitlen B with T the tap index,
L the length in chips and B the number of bits in the linear feedback
shift register. The resulting .bin file will hold one byte for each chip
equal to either 0 or 1, hence the subtraction of the mean value 0.5 before
correlating in the Fourie domain. Because the Fourier transforms might exceed
computational power for large L do not be concerned with Overflows in the
GNU Radio Companion console as the correlations might be missing a few samples
but will restart in the next data accumulation sequence.
The result is
No correlation peak if either the sequence stored in the File Source is not the one generated by the FPGA of if the FPGA has not been powered up and is not generating any signal.
Correlation peak on top, rotating phase in the middle, real and imaginary part on the bottom.
Zoom on the phase (middle graph) emphasizing the BPSK modulation despite the phase rotation due to the different local oscillators.