Mixing two signals creates power at dc?
If I understand correctly, you are mixing a 1500 Hz sinusoid with an MSK signal, which uses 1200 Hz to transmit 0 (or 1) and 1800 Hz to transmit 1 (or 0). The spectrum of the MSK signal, though, is quite large and includes energy at 1500 Hz. So, when mixing, you can expect to find a DC value.