Real-time FPGA implementation and measured performance of I/Q modulation based frequency synthesizer

In this article, the design and implementation of mixed-signal frequency synthesis for wireless communications devices are addressed. The core of the synthesizer builds on digitally synthesized tunable low-frequency I/Q tone which is then shifted to the actual radio frequency range using I/Q modulation. An efficient real-time FPGA design of the digital synthesizer core is first provided. This is then also implemented in real time and connected to practical I/Q modulation hardware to measure the spectral characteristics of the overall synthesizer, in terms of phase noise and spurious frequencies. Specific emphasis in the implementation structure is on adaptive digital pre-distortion and calibration mechanisms that improve the limited sideband attenuation and local oscillator leakage of practical I/Q modulators. Measured results indicate that the reported design of the proposed architecture can provide more than 50dB spurious free dynamic range at the final synthesizer output.