This paper proposes a system design, working on the basis of the Internet of Things (IoT) LoRa protocol, for giving an entire and reliable design guidance for ADPLL circuits and system specifications. The design model consists of a phase-domain model, which gives a verification on phase noise and specification, and a time-domain behavior model, which gives an complete observation during the ADPLL working procedure. Besides the conventional design procedure, which often only uses a phase-domain model, the proposed design model not only offers the system behavior-level operating state, but also obtains the noise and performance results of the loop. In addition, the design model built is completely time-domain-based, so the implementation of the actual digital circuit can be guided by the data flow inside the model. The built system model also covers 400M-510MHz, including the ISM frequency point used by LoRa.