# Position Sensing with a Compact 0.1 THz Amplitude Modulated Source
## 1 Introduction  
The THz band, which is defined in the frequency range from about 100 GHz to 10 THz [1], is one of the least explored ranges of the electromagnetic (EM) spectrum, due to technological difficulties in fabricating compact and inexpensive THz sources [2]. From 1980 onwards, research in the THz range began to expand, mainly due to major advances in optics (higher frequency range) and microwave technology (lower frequency range), which reduced costs and improved access to the required elements [2]. There are two main methods used in THz systems which differ according to the source of the THz wave. The first is an optical method in which the laser beam (frequencies in the THz range (\(10^{15}\) Hz), mainly pulsed, with low power) is converted to the THz range [3]. Nonlinear crystals [4], photoconductive antennas [5] and free electron lasers (FEL--Free Electron Laser) [6] are used to achieve the THz range. The second method is the so-called electronic method, where microwaves (frequencies in the GHz range, mainly continuous waves and higher powers) are converted up to the THz range [2]. Electronic oscillators (in the MHz range) are usually used as a base and multiply the microwave input with diodes, exploiting their nonlinear property. Just as there are different THz sources, there are also different THz detectors. Further information about the development of the state-of-the-art THz sources, detectors and their use can be found in more detail in various books written on the topic [1, 2]. In the last decade, THz systems are increasingly used in industry as a safe and cost-effective replacement for X-ray systems [7] or to provide added value in industrial automation control [8]. However, in many industries, the laser beam technology is still used for distance and position monitoring. Often the heavy industries, fog or dust are produced as a by-product in the environment, which makes the surrounding area very opaque for the laser technology. Much effort is invested into compensating for the error of the laser beam when penetrating the opaque area, as it is known that it refracts light and thus introduces errors into the position measurement. In some heavier industries, for example, a steel plant, there are cases where the mixture of gasses and dust is so thick that the laser cannot penetrate the fog and the laser measurement is useless. With a THz system, those issues can be solved, as the THz waves can penetrate many materials, suchas fabrics, wood and Styrofoam, to detect hidden objects [9], and thus an opaque area comprised of fog will not pose a problem. Primarily, a frequency modulated continuous wave (FMCW) source is used as a replacement for other technologies in the industry [10]. This method is well known, as it is not complex to implement. The distance can be detected as a mixed product of the transmitted and reflected signals' frequencies, which is processed using the fast Fourier transform (FFT) algorithm [11]. The major drawback of FMCW sources is their high cost, as a wide frequency modulation bandwidth is required to achieve a good distance measurement resolution. The trend in THz radar systems is to develop less expensive and more compact sources, which are still not good enough in some industry applications, due to the low power of the source. For example, there are some semiconductor companies offering components for radar systems, such as the integrated circuit families from Texas Instruments, where the entire FMCW system including the transmitting and receiving antennas is offered on the demo board [12]. The weakness of the aforementioned system is the low bandwidth of 4 GHz, which corresponds to a distance resolution of about \(\pm\)1.875 cm, and the low transmitting power compared to other FMCW sources. Furthermore, due to the principle of operation of FMCW systems that use stronger sources, some additional components are required that occupy additional volume for proper operation.