A digital frequency synthesizer with dither-assisted pulling mitigation for simultaneous DCO and reference path coupling

摘要

Injection pulling on frequency synthesizers has become a critical design challenge for high-performance wireless transceivers, especially in highly integrated multiradio platforms, imposing stringent constraints on system-level frequency planning. For instance, the oscillator of the victim phase-locked loop (PLL) can be disturbed by the power-amplifier (PA) output or other nearby PLLs when their fundamental or harmonic frequency components are close to the victim PLL carrier frequency. Although the aggressor waveform in most cases is known a priori, the coupling path and mechanism (electric or magnetic) is complex and varying over time and operating conditions. Hence, the transfer function of the coupling path cannot be predetermined or reliably compensated with foreground calibration. Therefore, recent works [1-4] have proposed adaptive techniques that track the transfer function in the background. However, they only focus on pulling mitigation of the oscillator, and some incur high overhead, e.g., an additional ADC in the case of an analog PLL. In real operation, the interfering signal can affect not only the oscillator but also the input reference clock [1]. The compensation mechanism is different for two coupling paths, i.e. the oscillator pulling mitigation can worsen perturbation caused by reference path coupling, as noted in [5]. To alleviate those issues, this work proposes a dither-assisted pulling mitigation scheme that orthogonalizes the coupling from both DCO and reference paths and allows simultaneous rejection in the background. Moreover, the technique also aims to mitigate different aggressor waveforms while minimizing the hardware overhead. The prototype is implemented in 65nm CMOS and uses a digital PLL architecture for better technology scalability. Various types of interference are injected to the DCO and reference paths simultaneously, and the measurement shows 12 and 22.5dB spectral improvement for the PA and the oscillator mutual pulling, respectively, which validates the effectiveness of the technique.