A novel frequency synthesizer concept for wireless communications

Strict requirements for high spectral purity, high operating frequencies and fast switching speed together with the power consumption limitations make the frequency synthesizer design an extremely challenging task for wireless applications. In this paper, a combination of analog and digital synthesis techniques is discussed and analyzed. A low frequency tone generated digitally is upconverted to the final frequency range by means of I/Q mixing. Ideally, the use of I/Q processing eliminates the need for any sideband suppression filtering. In practical implementations, however, the amplitudes and phases of the I and Q branches can only be matched with finite relative accuracy, resulting in finite image tone attenuation. Apart from I/Q imbalance, another practical non-ideality is the leakage of the local oscillator signals to the I/Q mixer output ports, creating another undesired tone at the synthesizer output. A compensation structure based on digital predistortion of the low frequency signals is presented to enhance the signal quality. Furthermore, a simple and practical algorithm for updating the compensator coefficients is proposed based on minimizing the envelope variation of the synthesizer output signal. Simulation results illustrate the performance of the proposed frequency synthesizer concept.