Sous la tutelle de :

 

FMCW Frequency-Modulation Continuous-Wave RADAR

This project aims to propose a THz reflectometry bench to obtain a 3D imaging of a material on one side, to detect any defects and to characterize it (index measurement for example). Radiation in the microwave field would not allow such a fine spatial resolution and the optics offer too little transparency. For reasons of cost as well as resolution, the field of study in which we will focus is between 75GHz and 110GHz, so in the area of millimeter waves.

To realize this system, there are two approaches, either pulsed operation as for the TDS (Time domain spectroscopy), or continuous with the modulation of frequency like the radar FMCW (Frequency Modulated Continuous Wave). For the latter case, the signal frequency changes over time, usually with a scan over a fixed bandwidth. The difference in frequency between the transmitted and received signals (reflected by the target) is determined by the mixing of the two signals. Called beat, this signal can be used to determine the distance and / or speed of a target, and offers a simple and often used method. The application is conventionally the detection of objects and weapons in airports. The advantages of FMCW radars are multiple: excellent performance for short range applications, excellent resolution distance, low cost and simple implementation with the use of small components, low power consumption. With these different systems, 3D imaging is performed only by penetration of THz signal at tissue level. It allows to visualize objects hidden under clothes. Our system allows us to perform a real 3D imaging of a material and to detect its possible defects.

 

 

FMCW Radar Set-up

 

 We are interested in characterizing materials that may have multiple defects, so multiple beats. In order to reduce the number of beats, we chose the signal reflected at the front of our target as a reference signal. We first studied a transparent and low diffusing material like Teflon. The source signal is collimated by a parabolic mirror and sent to the target. A splitter plate is used to recover the signal reflected by the target on a detection diode which measures the beat between the signal reflected at the front face and the back side. In order to correct the signals and eliminate the noise, the signal is processed using the Matlab tool. A first correction is to take into account the variation of the power of the emission source as a function of the frequency.

One problem put forward is the resolution of the system as well as the signal to noise ratio. In order to improve the characteristics of this system, signal processing algorithms must be studied and implemented and more efficient components must be used. The first results showed that millimeter defects can be detected. Parasites have been observed due to variations in the power of the source as a function of frequency. After correction, we observe an improvement of the signal-to-noise ratio of 20 dB for low frequencies.

 

Antennas

TeHO group has a great expertise in the field of design, production and characterization of various antenna :

* Printed Antennas

868 MHz

5.76 GHz

 

 

 

* Large anechoic room & Radiation pattern measurements

 

* Slotted Antennas

 

Integrated devices for THz detection and communication

Thanks to its great expertise in the field of millimeter waves, TeHO group is therefore transferring its know-how in the field of THz systems.

In the frame of an ANR work with the L2C and III-V lab, the platform is involved in the study of the detection of these THz radiations by a heterojunction bipolar transistor, from InGaAs and InP technologies. We thus characterize this transistor mounted as a common emitter, polarized only through its base (VBE = 0.65 V), and study the detected signal consisting of a DC voltage measured on the collector. The sensitivity of the detector thus measured is 0.35 V / W with a cut-off frequency close to 10 GHz and a relatively high output impedance compared to the standard for microwave telecommunication lines (50 Ohms). Therefore, this measured sensitivity is not sufficient to establish an ultra-fast communication system (at least 10 Gbps throughput) with the currently available THz source operating at a power of 10 μW. Indeed, for such a sensitivity and a signal-to-noise ratio (only thermal noise) taken equal to 10, the calculated maximum communication bandwidth is 0.1 MHz. Thereafter, impedance and sensitivity measurements show that increasing the bias voltage decreases impedance and sensitivity. Therefore, a compromise must be made to improve the matching of the impedance of the transistor to that of the external circuit while preserving the Thz sensitivity. Also, it is planned to use smaller focal length teflon lenses in order to reduce the size of the propagated beam to the diffraction limit, thus coupling more energy on the detector and thus gain a considerable factor on the sensitivity. We also studied the same transistor coupled on its base with a logarithmic spiral antenna to increase the sensitivity.


Determining the polarization window of the component for sufficient sensitivity and impedance matching of the component allowed us to perform heterodyne communication at a rate of 1.5 Gpbs, allowing the transmission of a 300 GHz carrier HD video with enough signal (> 350 ~ mVpp) at the output of two amplifier stages (gain of 50 dB) to allow display on a screen. This experiment was possible thanks to the acquisition of a 50 mW CW source for the local oscillator.


The relevant experimental set-up is already described in the section "THz Generation & Detection" of this webpage, in the item "THz Communication & Detection".

 

Furthermore, TeHO group supports the technical platform HERMES  - for further information visit the following webpage : Hyperfrequences Expertise Recherche MESures