This paper presents the evaluation of the signal quality based on the Error Vector Magnitude (EVM) online measurements for different radio access modulations implemented over the GNU radio platform for Software Defined Radio (SDR) developments. It is demonstrated that when a transmitted symbol is unknown by the receiver, the EVM can be calculated by measuring the closest distance between the received symbol and the ideal one of the analyzed digital constellation. The implementation of the EVM measurements through software development for multilevel digital modulations has enabled obtaining and assessing the Signal-to-Noise Ratio (SNR) and Bit Error Rate (BER) values. Results show that the SNR measurements accordingly depicts the expected behavior of the analyzed digital modulations and BER floor appears at high level modulation formats due to the way the EVM itself has been obtained.