Digitally Controlled Oscillator Topologies for mm-Wave Pulsed Coherent Radar

Abstract

The advancement of future generations of wireless communication and radar sensing warrants the need for mm-wave digitally controlled oscillators (DCOs) with high-frequency trade-offs in consideration. The purpose of this project is to investigate DCO topologies inspired from scientific literature. The DCO is an electronic component that takes a digital code as input to tune the output operation frequency. Acconeer's application, a pulsed coherent radar system, sets unconventional performance requirements on the DCO compared to continuous wave systems. A specific type of oscillator core, namely the cross-coupled differential-pair harmonic oscillator, has been investigated. The performance of this oscillator is subject to special trade-offs in the DCO implementation. A strategy of multiple stages was used to guide the work. First, possible solutions were crudely investigated, then promising solutions selected, and finally pre- and post-layout simulation results of said solutions were provided. Five solution alternatives were identified and benchmarked with respect to a digital equivalent of the cross-coupled differential-pair LC-VCO. These solutions present different opportunities to relax trade-offs in the design of the DCO. Benchmarking the post-layout results of the solutions with literature revealed competitive performance, considering requirements set by the radar application, which is the context of this work. The analysis of this investigation provides a foundation and suggestive guidelines of future action for Acconeer. This work also captures the state-of-the-art of DCOs for pulsed coherent radar, priming further research within the field.

Popular Abstract

In the hunt for faster wireless communication and more advanced radar technology, the design of digital oscillators is of utmost importance.

Consider a regular day in your life, such as today. It is likely that you made or will make a phone call. Even more likely is that you will use the mobile phone that is in your pocket right now to search the web or simply scroll through social media. Even though not every single person has these habits, a large portion of today's society relies on being connected wirelessly. Wireless communication can be defined as the act of transferring information seemingly instantly, at the speed of light, from one device to another. As the technological advancement of the human race progresses, the demand for faster data transfer rates increases, which is achieved by increasing the bandwidth and modulation complexity of the transmitted electromagnetic waves. Larger bandwidths are available at higher frequencies. The frequencies in use in today's technology are typically in the gigahertz (GHz, one billion oscillations per second) region. The wavelength of a wave with a frequency of a few tens of GHz is in the order of a few millimeters, hence the name millimeter wave (mm-wave) technology. Additionally, research is currently underway to support the sub-terahertz (THz, one trillion oscillations per second) region.

The properties of electromagnetic waves can also be used for sensing applications, and the most relevant for this work, radar. Radar is a technique where electromagnetic waves are sent out, reflected on objects, and then received. The properties of the reflected light, as well as the time it took for the light to return, can be used to determine properties about the surroundings, such as position, shape, and speed of objects. The capabilities of radar increases as support for higher frequencies is developed. In the next generation of wireless communication, 6G, there's talk about integrating wireless communication with sensing to achieve ultra high data transfer rates.

When creating devices for wireless communication and radar technologies, a lot of different components must be designed. A commonly known component is the antenna, which is used to transmit and receive wireless signals. Another less commonly known component, which is also at the focus of this work, is the oscillator. An oscillator is a component that generates a periodic electric signal, and is important for the functions of the wireless communication devices. The fundamental operation of the electrical oscillator is analogous to that of a classic pendulum. As the position of the pendulum is changing from left to right, the energy is constantly converted between kinetic energy and potential energy. The same is true for an electrical oscillator, but the position is instead a voltage value, and the energy alternates between being stored in the magnetic field of a coil and the electric field of a capacitance. However, in practice, a pendulum will ultimately stop due to some of the energy being lost to air resistance and friction between the anchor and rope. The same is true for the electrical oscillator, but the energy is instead lost to unforeseen resistance in the metal wires and components of the circuitry. The design of an electrical oscillator is fundamentally about working against the resistive parts that makes the oscillator stop. You add other components that add energy to the circuit, which keeps the oscillation stable. This is like adding a mechanism that pushes the pendulum a tiny amount on each swing to keep it from stopping.

Ultimately, this work aims to shed light on these design considerations and trade-offs under certain conditions that may turn out as useful information for this research field as well as for the host company Acconeer.

• author: Junskog, Klas
• supervisor: Lars Ohlsson Fhager
• organization: Department of Electrical and Information Technology
• course: EITM01 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: oscillators, mm-wave, radar
• report number: LU/LTH-EIT 2023-918
• language: English
• id: 9123412
• date added to LUP: 2023-06-13 10:48:08
• date last changed: 2023-06-13 10:48:08