Terahertz Electronics

Xiaobang Shang, Jeff Powell, Mike Lancaster

Early SU8 work at Birmingham saw the demonstration of square micromachined coaxial cable and passive circuits at frequencies below 100 GHz. Filters, patch antennas, couplers and a Butler matrix have all been demonstrated[1]. Since then passive circuits have been demonstrated to 700 GHz[2]^-[3]. The relevance here is that coaxial cable technology will again be used, this time for the IF feed in the communication system described below. The SU8 technology is now well established and the next stage is to develop a micromachined terahertz communications.

We are aiming to build UK’s first 300GHz wireless communication link. It is essential to consider communication system requirements before designing individual components of the system. Among the requirements, the most important two are frequencies and power. The defined frequencies and power are shown in Figure 1.  Based on these system requirements, detailed component specifications are worked out. Frequency tripler, subharmonic mixer (SHB), and dielectric lens antenna, have already been designed, constructed, and will be tested shortly. The design of transmit and receive amplifiers is underway.  Realisation of the final communication system will be carried out after evaluation of each components is accomplished. Some of these components are discussed in the sections below.

The work is supported by EPSRC[1] and the mixers, triplers and non SU-8 fabrication are done at the Rutherford Appleton Laboratory. The amplifiers will be designed at Birmingham and made at the Fraunhofer Institute for Applied Solid State Physics (IAF) in Germany.

[1] W. Yi, et. al. Proc. European Microwave Conference Sept. p739 (2009)

[2] X Shang, et al. IEEE Trans. Terahertz Sci. Tech. 2, p629 (2012)

[3] X. Shang et al. IEEE Microwave Wireless Com. Lett., 23, p300 (2013)

[4] http://gow.epsrc.ac.uk/NGBOViewGrant.aspx?GrantRef=EP/M016269/1

 Hao Yang, Mike Lancaster

An example of a new 300 GHz filter based on 3 cross-coupled resonators is shown in Figure 2. 300 GHz micromachined waveguides with internal dimensions 864 µm x 432 µm were accurately made and aligned to the measurement system to produce this un-tuned bandpass filter response (Fig. 2 (d)). The filter shown in Figure 2 is a potential replacement of Frequency Selective Surface (FSS) filters used in space borne radiometers. For the same specifications, the SU-8 waveguide filter has appealing advantages such as significant reduction in size and weight over the conventional quasi-optic solution.

Xiaobang Shang, Hao Yang, Mike Lancaster, Stefan Dimov

For any communication system amplifiers and diodes are required to be integrated into the micromachined structures and these produce heat. The SU8 process as it stands is unsuitable. So modifications are being considered to the SU-8 process as well as looking at all metal micromachined techniques[1]. One method is to use laser machining in collaboration with the Department of Mechanical Engineering.

Figure 3 shows a W-band filter produced from brass using laser micromachining. The excellent measurement performance demonstrates the great potential of this manufacturing technique. The same filter design has been scaled to a higher frequency (300GHz) and made by laser machining. Figure 4 shows the responses of the first attempt. The result is not as good as the one at W-band, and efforts are being made on optimisation of laser machining process in order to achieve better results. Meanwhile, new 300GHz filter designs, which take into account of constraints in manufacturing as well as the flexibility enabled by laser machining, are also under investigation.

[1] X. Shang et. al. IEEE Trans. Microwave. Theory Techniques, 64, p. 2572 (2016)

Guo Cheng, Mike Lancaster, Jeff Powell

A frequency multiplier working at 142.5GHz based on coupled resonators is designed and fabricated in order to provide the LO signal for the sub-harmonic mixer (with RF frequency at 300GHz) in the communications system described in the precious section. The CAD model showing the fabricated metal blocks containing the waveguide are shown in Figure 5. Here the tripler is fabricated by CNC milling, but later it will be specifically re-designed for SU-8 micromachining. The simulated tripler response is shown in Figure 6.

Guo Cheng, Mike Lancaster, Jeff Powell

A similar approach to the tripler in the precious section can be made to the design of the subharmonic mixer in order to up/down convert the incoming IF/RF signal to the required RF/IF signal (15GHz for IF and 300GHz for RF, respectively). The tripler is used to provide the 142.5GHz signal at the LO port of the mixer. The CAD model with the fabricated blocks are shown in Figure 7. Again, the mixer is fabricated by CNC milling but later in our work it will be specifically re-designed for SU-8 micromachining.

Due to the use of the coupled resonators as the matching network at RF port, the mixer is inherently integrated with filtering function and provides good IM rejection for around 20 dB. The simulated mixer response is shown in Figure 8.

Guo Cheng, Yuvaraj Dhayalan, Xiaobang Shang, Mike Lancaster, Jeff Powell

As mentioned in the previous sections, both the tripler and the mixer will be specifically re-designed for SU-8 micromachining. Figure 9 shows the tripler design based on 5 layers of SU-8. The micro-machined SU-8 layers are sandwiched between the input waveguide filter and a brass plate. The simulated tripler response is shown in Figure 10.

Duc Dinh, Mike Lancaster

Collaborative work supported by the National Physical Laboratory UK is undertaking research in to extending the frequency range of power standards in the UK. Initial work has been done on the design of the WR-3 microwave power sensor, as shown in Figure 11. The proposed sensor is going to be realized by six SU-8 layers. The heat-based sensing element will be 10 nm thin film platinum developed on 50 µm SU-8 substrate. The other SU-8 layers are fully metal coated, except for one bare SU-8 layer is used to create electrical isolation. The designed sensor works at 281 GHz and the simulate response in Figure 11 demonstrates the design can apply at terahertz frequencies.

Tianhao He, Yang Gao, Mike Lancaster, Jeff Powell

For several years the group has been working on passive devices based on coupled resonator structures. These circuits are based on filter design principles but we have now extend the applicability to multiplexer, antennas and other components. The newest part of the work is to extend this to amplifiers and multipliers ready for inclusion into the communications system described above. Figure 12 shows the design and results of a 9 GHz waveguide to microstrip amplifier based on the principles. Here there is a 3 resonator waveguide filter, removing the matching network entirely form the microstrip circuit which outputs the amplified signal via microstrip to an SMA coaxial connector. Designs are now been undertaken at 300 GHz.

Rashad Mahmud, Mike Lancaster, Fred Huang

Another novel use of the coupled resonator technique and the coupling matrix is in the design of antennas and antenna arrays. Here the idea is to use the antenna elements as the resonators in a filter in addition to their radiating function. This enables a filter to be included in the antenna array itself without (or fewer) external resonators. For the communications system we require high gain (~32dbi) and wide bandwidth (~15%) and preferably planar so it fits in with the waveguide micromachining process. Figure 13 shows a new type of waveguide antenna array based on the principle modelled and tested at 10 GHz. In this case it is a 4x4 array. The results of simulation and measurement are also shown and an exceptional filtering characteristic.

300 GHz Network Analyser Based  Radar System [7]

Kostas Konstantinidis, Alexandros Feresidis, Costas Constantinou, Edward Hoare, Marina Gashinova, Mike Lancaster, Peter Gardner

We have developed a novel ultra-broadband high gain antenna, based on a modified extended hemispherical lens directly fed by a waveguide. A novel matching technique based on an air pocket etched off the lens dielectric is employed. A new taper shaped lens extension is proposed for the first time for improved sidelobe level and gain performance. The antenna is compatible with our waveguide-based THz automotive radar and THz communication systems.

The antenna has an operating 3dB gain bandwidth of 30% (80GHz) achieving a maximum of 30dB measured gain. The measured S11 is well below -14dB across the entire WR-3 band. The achieved beamwidths (around 3 degrees) show that the antenna can be incorporated to obtain a high-resolution imaging system. Moreover the antenna weights just 5 g and is composed of low cost and easy to machine material (Rexolite).

Xiaobang Shang, Mike Lancaster

There is an increasing interest in the application of 3D printing to manufacture microwave and millimetre-wave components with high geometrical complexity. The merits of components made by 3D printing are reduced fabrication time, reduced component weight (if made from plastics and plated with metal), elimination of the need of assembly, and increased design flexibility.

In collaboration with Swissto12 Ltd[1], a W-band 5^th order Chebyshev filter has been demonstrated, as shown in Figure 14. The filter was printed from photosensitive resin and was subsequently coated with 10 µm thick copper all around. Measurement responses show visible frequency shift and this is attributed to fabrication inaccuracies. The modified filter model taking account of these inaccuracies was simulated in CST and the results agree well with measurements. The deviation in dimensions is due to enlargement in post-curing step of the printing process, and could be corrected by remaking the filter with modified dimensions. This filter represents one of the highest frequency 3D printed waveguide filters demonstrated to date. The manufacturing process will improve over time and we expect 3D printed filters moving into the sub-terahertz region soon.

[1] Swissto12. Lausanne, Switzerland, www. Swissto12.com

Fatemeh Norouzian, Rui Di, Edward Hoare, Marina Gashinova, Costas Constantinou, Peter Gardner, Mike Cherniakov

The signal reduction due to contamination of the radome of antennas at low-THz frequencies (150 and 300 GHz) have been investigated for outdoor applications. Lack of knowledge on the signal degradation of the contaminated radome at the low-THz frequencies was the motivation for this research, as these data are very important to assess the feasibility of the future of the low-THz sensors for outdoor applications. A new measurement methodology is proposed to characterise the effect of different obscurant on electromagnetic wave propagation. The low-THz experimental equipment and the measurement setup are shown in Figure 15 and 16. A sample of contaminant is placed between the antennas and the reference target in the path of the signal. The signal reduction that occurs in the presence of the sample is due to a combination of various phenomena such as: reflection, refraction, absorption and scattering. It is labelled as “Transmissivity”, and obtained by measuring the ratio of the signal reflected from a reference target measured when the sample is present and when there is no sample.

The main cause of potentially degraded performance of sensors is presence of water on a radome. The loss due to the presence of the water film is strongly dependent on the frequency, permittivity and formation of the water. Therefore, characterisation of transmission through uniform layer^[1]and randomly distributed droplets (as it happened in real-life scenario) of pure and contaminated water with salt and road dirt^[2]is required as a part of longer-term automotive low-THz radar research program. All our measurement results are compared with a theoretical model: uniform layer have been modeled by the Fresnel theory^[3] of reflection and transmission for multilayer structures and randomly distributed droplets model is based on Kirchhoff diffraction theory^[4]. Both model represent good agreement with the measurement results.

Measuring attenuation of low-THz wave through different weather condition is another aspect of this research to assess feasibility of outdoor application for low-THz sensors. Our measurement setup is shown in Figure 21.

[1] F. Norouzian, et al. IEEE Radar Conference, 2016

[2] F. Norouzian, et al. to be submitted in Dec 2016 in IEEE Journal

[3] W. H. Hayt, et. al. Engineering Electromagnetics, sixth edition, McGraw-Hill Companies, 2001

[4] M. Born, et. Al. Principles of Optics, seventh edition, Cambridge University Press, 1999.

Rui Du, Fatemeh Norouzian, Emidio Marchetti, Marina Gashinova, Mikhail Cherniakov

To quantify the attenuation at Low–THz frequency range (150 - 300 GHz) and evaluate the performance of Low-THz radar in outdoor environment, laboratory experiments have been conducted to characterise attenuation of a contaminated radome by sand. Transmissivity of sand has been measured as a function of thickness and moisture at 150GHz and 300GHz.  The results indicate that the attenuation is not only proportional to the thickness and the moisture of the sand as well as the frequency, but also to the size of sand granules, so that coarser particles lead to greater attenuation.

Five kinds of particle-size-calibrated sand were tested as well as natural sand, Figure 22. Transmissivity through different thickness of dry sand and 10. Vol% moisture at 150 GHz and 300 GHz are measured and plotted in Figure 23 and 24.

Rui Du, Emidio Marchetti, Fatemeh Norouzian, Marina Gashinova, Mikhail Cherniakov

Micro Doppler (u-D) effect/signature is usually used to identify the target, because the movement of non-rigid target will introduce modulation into the returned signal from the target illuminated by radar, which will generate sidebands besides target’s Doppler frequency. This signature contains information about the movements of different segments from target which is very helpful to the identification of targets. Human body is a typical non-rigid model consists with different segments (like upper/lower arm and upper/lower leg) and connected with joints (such as elbow and knee). Movements of human being will also produce u-D signature because of the different motions of limbs and body, which is used in some applications to distinguish human being from other targets, recognise different human motions, identify unarmed/armed pedestrians and human’s gender. u-D signature is also investigated at low-Terahertz frequency region. Two sceneries were tested: towards to radar and cross to radar.

Emidio Marchetti, Ben Willetts, Rui Dui, Edward Hoare, Marina Gashinova, Mikhail Cherniakov

Low-THz radar sensing is one of the most promising technologies that has the potential to provide safe navigation for autonomous cars due to the expected high resolution imaging capability that THz radar can deliver.  Most active pedestrian safety systems are based on the fusion of video and radar sensors and the higher the similarity between each of the obtained images, the higher the expected efficiency from such hybrid systems.  Low-THz sensors have the potential to improve these systems due to its ability to measure optical-like images. During the development process, the designer of an automotive sensor system needs to assess, quantitatively, the detection performance for pedestrian targets and also to quantify the maximum detection range. This requires the knowledge of electromagnetic reflection characteristics of the object and the Radar Cross Section (RCS) can be used as a measure of such reflectivities. The primary objective of our work is to obtain and analyses pedestrian RCS at 150 GHz and 300 GHz and compare these measurements with results previously reported in literature.

Therefore, a measurements campaign, aimed to characterize the pedestrian reflectivity using a child mannequin in the low-THz band, has been carried out. An analysis of the various RCS signatures collected shows that the observed global average RCS in the three frequency bands, 24, 150 and 300 GHz are quite close and in good agreement with the results reported in literature. This is an important finding as it suggests that sensors operating at low-THz frequencies have the same detection performances of common automotive sensors operating at lower frequencies. A first qualitative analysis of the target range/azimuth profile shows the possibility of extracting some peculiar features of the target; in fact the range spread at different angles is a characteristic signature of the target and can be used for identification purpose. Moreover, the average RCS measured for different aspect angles of the mannequin does not show a significant differences in the three different frequencies measured. A set of measurements carried out to assess the influence of clothing on pedestrian reflectivity shows that the variability of reflectivity for different clothes increases with the rise of the frequency, however the spread is only in the range of few dBsm.

Further work will be done with measurements of other typical road-related actors and objects, along with the development of techniques to enhance pedestrian identification.