I'm working on a project involving FSK-modulated signals. I'm trying to reverse-engineer these signals to determine the preamble pattern used, because the protocol is proprietary.
Shown below is a plot of the real parts of two captured transmissions that seem to follow the same pattern - except their frequencies appear to be inverted. This is further evidenced by the instantaneous frequency plots also shown. I captured these signals using an RTL-SDR dongle, which outputs complex I/Q samples. What does this mean?