Phase-locked loop (PLL) systems are widely employed in integrated circuits for space analog devices and communications systems that operate in radiation environments, where significant perturbations, especially in terms of phase noise, can be generated due to radiation particles. Among all the blocks that form a PLL system, previous research suggests the voltage-controlled oscillator (VCO) is one of the most critical components in terms of radiation tolerance and electric performance. Ring oscillators (ROs) and LC-tank VCOs have been commonly employed in high-performance PLLs. Nevertheless, both structures have drawbacks including a limited tuning range, high sensitivity to phase noise, limited radiation tolerance, and large design areas. In order to fulfill these high-performance requirements, a current-model logic (CML) based RO-VCO is presented as a possible solution capable of reducing the limitations of the commonly used structures and exploiting their advantages. The proposed hybrid VCO model includes passive components in its design which are the key parameters that define oscillation frequency of this structure. This tunable oscillator has been designed and tested in 65nm Bulk and 28 nm Fully depleted silicon-on-insulator (FDSOI) CMOS technologies
The 65nm testchip was designed to compare the behavior of the proposed CML VCO with a current-starved RO and a radiation hardened by design (RHBD) LC-tank VCO in terms of tuning range, phase noise, Single event effect (SEE) sensitivity and design area. Simulations were carried out by applying a double exponential current pulse into different sensitive nodes of the three VCOs. In addition, SEE tests were conducted using pulsed laser experiments. Simulation and test results show that a CML VCO can effectively overcome the limitations presented by a RO-VCO and LC-tank VCO, achieving a wide range of tuning, and low sensitivity to noise and SEEs without the need for a large cross-section.
Further studies of the proposed CML VCO were done on 28nm FDSOI in order to reduce the leakage current and increase the switching speed. the same current-starved VCO and CML VCO were implemented on this testchip, and simulations were performed by injecting a double exponential current pulse energy into the previously defined sensitive nodes. The results show SEE sensitivity improvement without narrowing the tuning range or affecting the phase noise response
