## 1 Introduction  
Several thousand launch activities have been performed during the last half-century, and with the rapid development of technology, satellites are playing an important role in human society. These systems are widely used for navigation, communication, and earth observation. One of the first communication experiments with laser was conducted between two Low Earth Orbit (LEO) satellites and a geostationary satellite ARTEMIS. The experiment was performed with a data rate of up to 50 Mbps. Then, other experiments followed with an increased data rate to achieve inter-satellite communication links. Today, current trends in satellites show a rapid increase in data traffic and digital processing. The throughput of next-generation satellites for digital telecom applications, as well as scientific missions, surveillance, and remote sensing, will exceed terabits per second of data that must be processed on board. For instance, the high-resolution cameras and synthetic aperture radars need high-speed communications between the instruments and the on-board data storage system [1]. The optical technology, thanks to its high bandwidth-length product, the lightweight cabling, and electromagnetic hardness, can potentially be the solution for data-rate increment in satellites. In this direction, the European Space Agency (ESA) has recently released the new SpaceFibre standard for on-board satellite communication up to 6.25 Gbps [2; 3]. The standard describes the very high-speed serial link and network technology, and it was designed specifically for use on-board spacecraft and satellites. This protocol provides a coherent quality of service mechanism able to support bandwidth reserved, scheduled, and priority-based qualities of service. SpaceFibre provides robust, long-distance communications for launcher applications and supports avionics applications with deterministic delivery constraints using virtual channels. Communication performances are strongly related to the ability to synchronize the receiver with the transmitter. This issue is typically fixed with a Clock Data Recovery (CDR), and the key block used for its synchronization is the Phase Locked Loop (PLL). The Voltage Controlled Oscillator (VCO) is the core system, inside the PLL, able to generate the required frequency of 6.25 GHz to be compliant with the SpaceFibre protocol. Although the required Total Ionizing Dose (TID) level is lower than 1 Mrad for space applications [4], the main problems are due to Single Event Effects (SEEs) that temporarily disturb the typical operation of the circuit. This work targets, as implementation technology, a commercial 65 nm CMOS from TSMC (Taiwan Semiconductor Manufacturing Company). This technology, thanks to its thin gate-oxide thickness, could be considered radiation hard up to few hundred Trad TID levels, as proved in [5], and by us in previous designs of other high-speed circuits in [6; 7; 8]. To the best of the authors' knowledge, in literature and market, there are not examples of rad-hard VCOs able to work at 6.25 GHz. The paper [9] showed the design of a PLL in the range from 0.2 GHz to 1.2 GHz, designed in 65 nm STMicroelectronics space technology. This system was irradiated up to 300 krad TID level, and its behavior was verified with different protons. In [10], a comparison between Ring Oscillator (RO) and LC-tank VCO for PLL was made for Large Hadron Collider's (LHC) applications. Both were designed for a working frequency from 2.2 GHz to 3.2 GHz, and the SEE test performed with heavy-ions showed that the LC-VCO had a larger cross-section than the RO-VCO. Varactors have been identified as the most sensitive part of LC-tank architectures, and Triple Modular Redundancy (TMR) technique has been adopted to face SEEs in the design of the phase frequency divider. The goal of this work was to compare the performances of the widely used RO and LC controlled oscillators in radiation environments and to contribute with new approaches for exploiting the characteristics that have made these systems the most implemented.