RF test signals are a requirement for the implementation of effective BIST techniques in transceivers. In this work a method to encode a binary signal with the desired RF frequency is presented. The approach employs high-pass sigma delta modulators, in contrast to conventional low-pass or band-pass approaches, allowing signal generation close to the Nyquist limit of FS/2 (FS=Sampling Frequency). As a digital signal is used, only a 1-bit DAC is needed, reducing test costs. Practical results using a 3Gbps transceiver illustrate the performance achievable by the method.