Short-Range Frequency Synthesizer Design Using PLL Architecture forMulti-Protocol 2.4 GHz Transceivers

Abstract

Introduction/Objective: This work presents a fully integrated Integer-N frequency synthesizer targeting multi-standard short-range wireless protocols operating in the 2.4 GHz ISM band, including Bluetooth LE (BLE 5.4), ZigBee, Thread, and Wi-Fi coexistence. The design addresses the need for fast lock time, low residual FM, excellent spectral purity, and low-cost implementation in resource-constrained IoT applications. Methods: The synthesizer is implemented in 0.35 µm CMOS and centers on a resistorless discretetime loop filter (DT-LPF), the core innovation of this work, which eliminates on-chip resistors entirely by replacing them with a delayed charge injection mechanism using switched-capacitor networks. This approach improves stability of the loop, eliminates thermal noise associated with passive resistors, and by avoiding on-chip resistors entirely, reduces sensitivity to resistor mismatch and process variation, while enabling a more compact loop filter topology in standard CMOS processes. Complementing this, the design employs a gm/Id-optimized differential LC-VCO, a dead-zone-free tri-state PFD with 1 ns reset-path delay, and a current-scaled CML multi-modulus 2/3 divider that reduces highfrequency power dissipation. All blocks are co-optimized for 1 MHz reference frequency operation, and the system is fully designed and simulated using industry-standard EDA tools. Results: The PLL achieves a 230 MHz tuning range (2.28–2.51 GHz) with 1 MHz channel resolution, fully covering the 79-channel Bluetooth band, ZigBee/Thread channels (2.405–2.480 GHz), and the entire 2.4 GHz ISM band used by Wi-Fi. It locks in approximately 100 µs, satisfying the <150 µs fast-hopping requirement for BLE 5.4 and enabling low-latency operation in ZigBee/Thread networks. The proposed architecture exhibits –117.5 dBc/Hz phase noise at 1 MHz offset, exceeding ZigBee’s –110 dBc/Hz specification and supporting robust Wi-Fi coexistence, along with 5 kHz RMS residual FM (well below the 10 kHz Bluetooth budget), reference spurs below –67 dBc (within ZigBee’s –60 dBc and Bluetooth’s –55 dBc limits), and 32.6 mW total power consumption from a 3.3 V supply. The Figure of Merit (FoM) is –170.1 dB. Discussion: The DT-LPF enhances integrability, reduces thermal noise, and assures PLL lock stability, while the current-scaled CML divider cuts prescaler power by >30%. Despite using a lowcost 0.35 µm process, the architecture rivals advanced-node PLLs in FoM and protocol compliance. The 1 MHz reference frequency and explicit residual FM validation ensure real-world suitability for Bluetooth and ZigBee, metrics often omitted in recent works. The design demonstrates that architectural innovation can compensate for technological constraints in cost-sensitive IoT. Conclusion: The proposed synthesizer meets all essential performance metrics for ISM-band wireless protocols and demonstrates architectural efficiency and scalability. Its design offers a competitive and low-cost solution for power-sensitive short-range communication systems, particularly in cost-constrained IoT applications where advanced CMOS nodes are economically prohibitive.