qam calculater snr
To make the proper calculation, you need to know your system bandwidth also.
You can proceed like this:
signal: -30 dBm->att 10 dB ->-40 dBm
noise: -75 dBm. The attenuator reduces input noise as well but adds it's own noise (that has to be integrated into system bandwidth).
For example, if you have B=20 MHz bandwidth, the output noise of the attenuator (assuming it is matched to the load and chatacteristic impedance is 50 Ohm) will be -101dBm(-174+10LOG(B)) that is negligible respect to -85 dBm.
So your SNR is still 45 dB.
Mazz