My IQ sampler is measuring a QPSK carrier at a set frequency/bandwidth (40MSamp/s), but shifts the signal spectrum to baseband. My signal is 1.5Msym/s, but sampling at 1.5Msym/s gives me a constellation of a circle due to the phase difference between the carrier and the IQ sampler clock.
I still need to synchronize to the carrier, but I now have a 0Hz carrier? Do I try a costas loop with a 0Hz center VCO? So far I haven't had much luck. Any ideas? Thanks everyone.