Circuits and Systems
Research Unit

The main expertise of Circuits and Systems (CAS) is the integrated circuit development and implementation for telecommunication and pulsed time-of-flight and time-correlated single-photon counting applications.

RFIC development include e.g. Sub-THz receivers and load modulating power amplifiers for mmWave phased arrays. Time-correlated single-photon counting enables applications such as solid-state 3-D imaging, Raman spectroscopy and time-resolved diffuse optics for the field of automotive, healthcare, medical and biopharmacy.

Circuits and Systems Research Unit

Key enablers and scientific breakthroughs

CMOS Single-photon avalanche diode (SPAD) arrays with TDCs for pulsed TOF solid-state 2-3-D imaging

CMOS technology enables to integrate both the single-photon detectors or matrix of detectors and timing electronics into the single die. Moreover, the array of enhanced gain switching based laser diodes can produce energetics short optical pulses of ~100 ps to achieve a high field-of-view and measurement distance of tens of meters in which way the pulsed time-of-flight solid-state 3-D camera can be fabricated without any moving parts. There are a lot of applications for TOF Lidar in surveying, civil engineering (e.g., construction site mapping), inspection and quality control, for example, and recently also in autonomous vehicles. In these applications, lidar systems that use mechanically steered light to obtain the 3-D range image is utilized, with drawbacks arising from relatively high costs and mechanical fragility. Moreover, it has been proven that 3-D range imaging could be used in many other applications such as robotics, small vehicle guidance (eg. UAV), virtual/augmented reality (VR/AR), consumer electronics (games) and machine control. These emerging applications benefit from the small-sized solid-state 3-D range imager.

CMOS SPAD line detector for time-gated Raman spectroscopy

Measuring the chemical environment and molecular composition of a medium is important in many applications, e.g. in the agricultural, food, oil and pharmaceuticals industries. An advanced technique for such measurements is Raman spectroscopy, which can be used to measure samples of various types, e.g. solids, liquids and gases, and allows flexible sample interfacing and preparation. CMOS technologies can be used to fabricate single-photon avalanche detectors (SPAD) or line detectors and multi-channel timing electronics into a single die. Raman spectra can be recorded by the line detector by counting the photons of the each spectral point (pixel of the line detector). Furthermore, the time gating of the detectors is possible when using CMOS technology and pulsed laser in which way the unwanted fluorescence signal can be rejected to achieve higher SNR for Raman signal. CMOS technology will pave the way to develop a small- sized Raman spectroscopy device for biomedical and biopharmacy applications.

Distortion contribution analysis tools

Optimizing the linearity of RFICs is iterative and time-consuming. The group has two in-house simulators, that show the device-by-device contributions of nonlinear distortion so that dominant contributions and possible cancellation mechanisms can be easily seen. These can be effectively used for optimizing the linearity of the circuit.

Time-resolved diffuse optics (TRDO) transceiver for medical and wearable healthcare applications

CMOS technologies have enabled to integrate all timing electronics and photodetectors (SPADs) into a single die in which case a small sized detector chip can be designed with a low cost. Secondly, the size of the laser diode driver has been a bottleneck in the miniaturized TRDO optode. However, we have demonstrated that the laser diode driver fabricated in CMOS technology can produce enough current to control the semiconductor laser diode. In that case, both the transmitter and the receiver circuits can be fabricated with a low cost integrated circuit technology and combined into a single CMOS chip to develop a small-sized transceiver chip for wearable device to measure heart rate and oxygen saturation of the muscle for example. TRDO technique have a number of in vivo applications such as optical mammography, brain mapping and muscle monitoring. In addition, TRDO technique has been used for optical characterization of pharmaceuticals, wood and food.

Integrated Millimeter-Wave Power Amplifier Architectures for 5G

Next big leap in telecommunications is taking place in the millimeter wave frequencies. Due to the increased path losses large phased arrays are proposed. The antenna sizes are decreased due to decreased wave-length, which forces the transceivers to be highly integrated. These set challenging requirements for transmitters power amplifiers (PA). For fifth telecommunication generation (5G) transmitter integrated low power PA, preceded by each antenna element in an array, is needed and the following properties are proposed in this project; PA with frequency tuning, load-pull for different power levels, and analog linearization. We have developed several integrated power amplifier (PA) structures using modern CMOS technologies at frequency range of 24-31 GHz and are compatible for 3GPP/NR FR2 bands n258 and n257. The core of the integrated PA architectures has been a 3-stack power cell that is used in single ended transformer matched PA, dual input PA with transformer-based power combining and Orthogonal Load Modulating Balanced Amplifier structure. At best these PAs are able to provide 11.4 dBm output power with -24 dB EVM by using 100MHz OFDM 64-QAM signal (Psat ~20dBm) and thus are viable option when designing front-ends for future mmWave phased arrays.

Prof. Timo Rahkonen

Unit manager
Circuits and Systems

Prof. Heli Jantunen