Research Topics in Microwave and mm-Wave Microelectronics

Topics on mm-wave devices and systems

Millimetre-wave (30 – 300 GHz) transceivers have received widespread commercial adoption in imaging and automotive RADAR applications due to small size and sharp image resolution over short distances. However, due to the availability of broadcast spectrum above 30 GHz (compared to traditional GSM, LTE and 2.4 GHz ISM bands) attention is being turned to the underutilised mm-wave spectrum for future wireless communications applications, with mm-wave point-to-point links employed as a solution to last-mile fiber replacement or augmentation. In this role, the E-band spectrum at the 71-76 and 81-86 GHz ITU-regulated bands have received significant interest. The neighboring 92-95 GHz band covers a local minimum in atmospheric attenuation with properties comparable to that of the Ka-band but has received less attention due to far stricter statutory regulations.

We propose research in the following areas to address key shortcomings in the state-of-the-art.

On-chip active circuits

mm-Wave active loadpull on-chip. 

Loadpull is a critical stage in both amplifier characterization and control. A simple loadpull circuit would consist of an active load and attenuator combination, with a phase shifter. Our lab has recently prototyped both; this study will combine these developments to see if a mm-wave active load-pull circuit could be developed in a suitable microelectronic process.  

High-power broadband solid state isolators

A common requirement in transmission systems is isolation between the output port of the power amplifier and the reactive load. This is both to ensure the stability of conditionally stable output stages under variable load conditions, but also to protect the PA from catastrophic failure due to sudden short-circuit or open-circuit termination (WE1B-2,2017). With the advent of research interest in mm-wave point-to-point communications, a new need is rising for implementing these isolators in broadband, mm-wave, monolithic technology nodes such as Gallium Nitride (GaN). This project will investigate the design and implementation of broadband solid state isolators in GaN, for future application in mm-wave transmit systems. Code: TU4G-2

Broadband, outphasing, multi-way power amplifiers on-chip

To realise transceivers at mm-wave frequencies, high efficiency mm-wave amplifiers are required that perform well at back-off power levels. One such possibility is to use extended syntheses of LINC / MINC amplifiers to realise these in an N-way configuration, though the extension to mm-wave frequencies will require significant research efforts. Something else that could be done is to use wideband distributed networks for harmonics matching (rather than lumped elements or narrowband stubs). Furthermore, one might consider active Q-enhancement of the LC/stub matching sections, as low Q-factors severely affect the PAE of the switch-mode; though this would be subject to a trade-off analysis (power input for Q-enhancement vs. enhanced power output from high-Q matching). This might be further enhanced with flat outphasing Chireix networks (TU1D-4, 2017)

Broadband delay function synthesis using solid state switched slow-wave mode lines on-chip

A fundamental requirement for phased array synthesis is a reliable, broadband phase shifting element with constant insertion loss. Although numerous techniques exist to synthesise these networks in circuit synthesis, the development of high frequency phased array systems is hampered by the narrow bandwidth and high loss of the available off-the-shelf phase shift elements. This is, in part, due to the low achievable quality factors of on-chip passives at high frequencies, as well as the reduced margin between the frequency of operation and the transition frequency (ft) of the semiconductor process. One promising approach that has been demonstrated in literature is the use of microelectromechanical structure (MEMS) manufactured slats below a transmission line, that create an effective tunable slow-wave propagation mode controllable through different bias voltage sequences. This approach is, however, not feasible in high-shock / high-vibration environments, as is typically the case for RADAR / EW equipment. This study will, firstly, investigate the use of solid state switched devices (which are more resistant to shock and vibration than MEMS) to synthesise broadband controllable phase shifters, and secondly, apply this structure to the realization of more complex group delay synthesis, thereby enabling analogue pre-processing of broadband data. Code: WSI82015

Spatial combining mm-wave power amplifiers

One possibility in effecting higher output power levels at mm-wave frequencies is through spatial combining, where the combining loss is O(log(N)) (as opposed to the O(N) loss for corporate combiners). The purpose of this study is to investigate suitable distributed amplifier topologies and combining guided wave structures on-chip that can be implemented in commercial GaN (PA) or SiGe BiCMOS (PA or LNA) processes.

On-chip VCOs using negative resistance biased diodes

A critical component in realising an on-chip system is a voltage controlled oscillator (VCO). A particular type of oscillator, referred to as a negative resistance oscillator, requires an active device connected in a positive feedback configuration. An alternative would be to use a diode, designed and biased to have a negative I-V slope, such as an avalanche or resonant tunneling diode. This study will investigate the manufacturing conditions and parameters for creating a general class of negative resistance devices using either avalanche diodes, or RTDs, or both, and apply this to the synthesis of a negative resistance VCO. 

In a related development, one could consider fundamental mode on-chip oscillators with distributed couplers (TH3F-4, 2017).

Phase noise and noise figure effects of active enhanced resonators

In most mm-wave oscillator topologies, the phase noise of the oscillator is determined, in large part, by the resonator Q-factor. As Q is inversely proportional to phase noise, making high-Q resonators is imperative. On-chip mm-wave resonators have, however, been shown to feature quite low Q-factors, severely impacting on phase noise performance. Active Q-enhancement may improve the phase noise of mm-wave VCOs provided that the noise injected (by active enhancement) does not exceed the 1/Q reduction. A formal theoretical study of this phenomenon has yet to be conducted. This may be extended to the study of the noise figure of active enhanced resonator filters, thereby creating "low noise active filters". A further option would be to look at ring resonators explicitly (WE2C-3, 2017). 

Something else that could be pursued is channel selection / switching (as opposed to linear enhancement) of multi-band filters (TU3F-5, 2017)

In a similar development, it would be interesting to look at band-stop, rather than band-pass filters. Here, one might consider Q-controlled variable bandwidth band stop filters (WE2C-4, TH3C-3, both 2017)

Active enhancement of transmission line filters

Most of the current theory on active Q-enhancement focuses on narrowband coupled resonator fitlers. There are, however, several use cases for wideband TEM-type coupled line filters, which (though not as acutely, but still) suffer from high losses at mm-wave frequency. This, despite the availability of wideband negative resistance circuits. This study will pursue wideband TEM-type coupled line filter synthesis of active Q-enhanced element, whilst introducing noise figure as a design goal in the fundamental synthesis.

Active filters using fundamental components

The mm-wave filter solutions on-chip higlighted above all require distributed elements. It is, however, possible to create active filters which rely solely on transistors, or a combination of transistors and lumped components (typically capacitors). These have, however, not been demonstrated experimentally at mm-wave frequencies, due to the high f0/ft ratio required for these active building blocks (NICs or gm-Cs) to operate properly; mm-wave filters are simply required to operate too close to the fundamental limitations of the active devices. The are, however, ways that this could be improved on (most notably replacing p-type components in the block with n-type components), which have not yet been explored. This project will re-investigate active filters for mm-wave applications, applying to it novel approaches for layout and parasitics extraction. This may further be extended by analytical consideration of the noise figures in filter synthesis. This is related to filter synthesis through non-Foster circuits (TU4A, 2017)

Something else that may be pursued is reconfigurability; specifically though reconfigurable transformers (TU1D-5, 2017)

Microwave BIST OBT with DC digitization

Oscillation-based testing (OBT) is a simple, yet powerful technique for built-in self-testing in microelectronics, as it detects potential faults in a vast number of circuit nodes with a zero-length input test vector and a single readout. There are, however, significant shortcomings in applying this technique at high frequencies.One such shortcoming is that the output oscillation has to be digitized to determine amplitude and frequency, which is infeasible at mirowave and mm-wave frequencies. This project will investigate different direct-to-DC detection methods, to enable detection of both amplitude and frequency with two separate DC readouts.

Microwave / mm-wave broadband OBT

Another shortcoming in current OBT BIST is that the pass / fail determination is made on a single output frequency oscillation, which may not reflect broadband degraded performance. This project will investigate oscillators with a broadband microwave output, including possible application of chaotic oscillation, comb generators, or square wave oscillators. The evaluation of total broadband spectral content is expected to detect degradation to a better extent than current OBT. 

Microwave BIST OBT of passives

Active gain devices are fairly simply connected in an unstable feedback topology for OBT. Passive devices, on the other hand, require an additional stable gain source. Even if such a source can be included, the question remains how to detect an error with output frequency. One way that this might be attempted is by interrogating the phase of the passive device (the explicit criteria in the case of an oscillator, and an implicit criteria for other passives) in either an explicit feedback microwave oscillator topology, or in a low-Q ring oscillator. This project will investigate the use of phase delay variation as a metric of OBT fault detection output, with the test of microwave phase shifters as proof of concept.

Oscillation-based tuning of microwave and mm-wave filters

Microwave and mm-wave filters (especially those using on-chip elements) are highly sensitive to constituent component value variations. For this reason, it is commonplace to tune filters post-production. This necessitates high-end test equipment (typically a VNA), making the procedure time-consuming and virtually impossible for in-field servicing. Furthermore, it may be necessary to re-tune filters in-system (for frequency agile applications, or due to environmental changes). It may, however, be possible to replace source-excited tuning with tuning based on internally generated oscillation with a low-quality amplifier and a well-known reference (high resolution lithography, bandgap references, crystal oscillators), thereby enabling oscillation-based self-tuning of filters. There is a precedent in literature for using OBT in self-healing of amplifiers, which may be drawn on for this inquiry. Note that the project may not be one in microelectronics (or even RF) design, necessarily, but rather one of machine learning.

On-chip passive devices

Monolithic (on-chip) integration of millimetre-wave systems has been the most published development path (as opposed to machined waveguide) for future 5G E-band systems. System-on-Chip (SoC) designs have not only the advantage of reduced size and cost, but it also removes complexity from the design of the overall application device by presenting the design engineer with an integrated front-end.

Slow-wave mode harmonic suppressor filters on-chip using bar-type via structures 

Most transmitting power amplifiers work at optimal efficiently when driven into saturation. This mode of operation generates significant higher order harmonics which may not be transmitted. The requirement, therefore, exists for harmonic suppressor filters on-chip covering multiples of the lower cut-off frequency. Corrugated waveguide filters are well-known harmonic suppression devices, but have (as yet) not been attempted on-chip. The recent introduction of bar-type via structures allows for the creation of solid ridges on-chip, making corrugated waveguide possible whilst simultaneously reducing wavelength through slow wave mode propagation. This project will develop a specific class of synthesis theory for bar-type via slow-wave mode and harmonic suppressor filters. Further functionality may be added by investigating the general use of alternative guided media and absorptive mechanisms. Code: WE1A-2,3 / 2014

Novel coupled resonator filters in BEOL

A critical shortcoming in SoC design is the absence of good on-chip filters. This is, in part, due to the tolerance associated with XY-geometry placement and the uneven edges created by etching. This creates significant uncertainty in magnetic coupling between lines. A few possibilities possibilities that could be investigated are 

MIM-capacitor line coupling
Though-silicont via (TSV) coupling (Code: WSK22015)
Capacitively loaded SIW cavities, in particular deep-trench loading (Code: WSK62015)
Inductively / CSRR loaded cavities
Shielded spiral lines

This study will investigate a selection of potential resonating BEOL structures (with possible extension to Super-Q, or higher order resonance) and compare it to traditional coupled line filters on-chip.

Substrate integrated waveguide (SIW) on-chip

A significant drawback to complete on-chip systems is the high losses associated with passive circuitry surrounding the active transistors on the semiconductor wafer. This relates to, most notably, the transmission lines connecting active devices together to form circuits, especially as it relates to the achievable Q-factor of transmission line resonators. In conventional RF substrates and other planar media such as LTCC, substrate integrated waveguide (SIW) has been demonstrated a process-compatible, yet lower loss alternative to microstrip or CPW, outperforming them by significant margins at mm-wave frequencies. This medium requires reliable and low-resistance vias through the dielectric medium, achieved with a commercially available semiconductor through-silicon via (TSV) process. The most recent work on TSV-SIW reports mm-wave operation of SIW integration of silicon at mm-wave frequencies, but relies on high resistivity (5000 Ohm.cm) silicon as dielectric material as well as non-standard backside metallic deposition, precluding the integration of active components in the same monolithic process. The use of low resistivity silicon of 10 Ohm.cm and 100 Ohm.cm attained, in simulation, 4.6 dB / mm and 0.5 dB/mm, respectively, clearly showing the effect of conductivity in the silicon on insertion loss. The specific knowledge gap that exists, currently, is the attainable transmission loss by combining back-end etching (replacing silicon with free space) and TSVs into a single SIW structure, even though the effects of removing excess silicon to improve transmission characteristics of chip interconnects has been demonstrated in microstrip and CPW. If these processes can be combined in a commercial monolithic semiconductor run, on-chip transmission media with loss orders of magnitude less than the state-of-the-art may be realised. This interconnect technology may prove the medium of choice for next-generation mm-wave system-on-chip devices, if its viability in a commercial process can be demonstrated. This study will investigate the viability of such a guided medium using a commercial foundry offering, and demonstrate its effectiveness by realising passive devices such as power dividers and filters.

Combiners with low Q-factor resonators for isolation

Due to the multitude of coupling paths on-chip, and the high tolerance on resistance values, it is difficult to achieve high isolation between the output ports of Wilkinson combiners on-chip. A proposed alternative to achieving isolation through a resistor is to use a low-Q resonator (Trantanella, IMS2010), something that has already been used successfully in absorptive reflection mode filters (Gaskell & Stander, 2016). This project will attempt a wideband, high isolation Wilkinson combiner on-chip at mm-wave frequencies through isolation with low-Q distributed resonators.   

Establishing best practices for modeling back-end-of-line passives.

The BEOL of a CMOS or SiGe BiCMOS process is a complex environment for EM simulation, with various complexity-accuracy trade-offs. These include the setup of excitation and boundary conditions, modeling of passivation layers in a 2D or 3D sense, macro-modeling of via blocks, inter-layer vs. intra-layer dielectrics, surface roughness, and many more. Despite these many trade-offs, there seems to be no systematic study in literature experimentally investigating these design trade-offs. That is the goal of this study: to systematically investigate simulation environment trade-offs in simulating BEOL passives at mm-wave frequencies, and produce a list of recommendations / "best practices" for high first-iteration prototyping accuracy.

Hybrid devices

Alternative mm-wave system integration media

There is a continual drive to develop millimetre-wave systems of higher quality, at lower cost, and making them mass-producible. Although complete on-chip systems are preferred, this approach is associated with high passive device losses. One alternative would be hybrid on-chip / off-chip packaging of these systems, placing passives in the packaging or mounting substrate. Although liquid crystal polymers and low-temperature co-fired ceramics have been shown effective in this role, the NRE prototyping cost for these are quite high. Experimental media, such as micro-machined glass, may prove a cheaper alternative for low production volumes. Furthermore, micro-etching creates the opportunity for air dielectric components, further enhancing achievable Q-factors. This study will investigate, in a general sense, the integration of mm-wave passives off-chip on micro-etched glass substrates and other new media. Design techniques specific to this medium will be established and demonstrated.

SIW integrated outphasing PA at mm-waves

As stated previously, outphasing topologies achieve higher efficiency at back-off power levels. This is done through fine amplitude and phase control, neither of which have been adequately addressed for implementing these devices in a distributed fashion in SIW. This project will, firstly, investigate the effective integration of switches, phase shifters and amplitude control in SIW, and then apply those in the design of an outphasing mm-wave PA in SIW.

Hybrid integrated spatial combined PAs with outphasing

Outphasing solid state power amplifier (SSPA) assemblies, such as Doherty or LINC configurations, allow for higher power-added efficiency (PAE) below the saturated output power level by selective biasing of different amplifier devices for different classes of operation (typically Class A and Class C) and selective re-combination by a prescribed phase relationship. The state-of-the-art in this field is to use two parallel devices, and although some enquiries have been made on N = 4 parallel devices, there is still no generalized theory on optimal biasing and combining of N arbitrary devices. Furthermore, the N = 4 devices use corporate dividing and combining structures, which are far less efficient than planar radial combiners and dividers. This study will pursue a general synthesis theory of N-way outphasing SSPA assemblies, and apply this theory in the realization of a mm-wave outphasing SSPA using planar radial spatial combiners and dividers.

Hybrid integrated cross-coupled VCO

To realise transceivers in this frequency range, VCOs are required that have low phase noise, are stable and have, ideally, voltage controllable characteristics. Off-chip substrate integrated waveguide (SIW) has been demonstrated to provide higher Q-factors than conventional planar resonators or LC tanks on-chip or in off-chip media, but have not yet been demonstrated in this role. Furthermore, it has not yet been demonstrated that these VCOs can be realised with cross-coupled topologies. Code: TU2B2015, TU2B32015

Active enhanced SIW coupled resonator filter

Although off-chip SIW resonators have fairly good Q-factors, the values are still far below what is achievable with ceramic or air-filled metallic waveguide resonators. One possibility is the active Q-enhancement of SIW cavities by an external negative resistance circuit. This project will investigate suitable SIW cavity perturbations and negative resistance circuits to effect such a device, with extension to frequency agility and noise figure analysis.

Arbitrary 3D phased array synthesis

The conventional approach to phased array synthesis is position the elements in a regular m x n matrix and apply individual control of relative amplitude and phase.  Fixing the synthesised array's element positioning to a rectangular grid reduces the complexity of the array synthesis problem, but also removes two degrees of freedom (three, in the case where arbitrary Z-axis placement is allowed) in the synthesis of the array.  This study will investigate a generalized method for optimized array synthesis for the case where the element placement is no longer restricted to a fixed half-wavelength rectangular grid, but as an arbitrary point in 3D space.
 
Radially distributed antenna array synthesis
 
A feasible upper limit on mm-wave array size is imposed by the use of corporate divider networks to feed the network. The power lost in the distribution network feeding the array elements scales with an order N (where N is the number of elements) making large planar arrays unfeasible at high frequencies. This study will investigate the use of spatial distribution (where the loss scales only with order log(N)) to feed the array of radiating elements, thereby creating viable planar arrays for high element counts. Furthermore, the project will investigate the realizable geometries that can be implemented using radial power distribution, and develop feasible array synthesis methods for these geometries. This will extend to integration of active phase and amplitude control elements in the radial propagating path.
 
mm-Wave conformal arrays
 
Another shortcoming in state-of-the-art mm-wave planar arrays is the limited scan angle. Planar arrays typically feature beam steer angles of ±50° from boresight, but may be extended by using planar arrays that conform to a convex surface. Current mm-wave conformal arrays are, however, limited to simple beam switching schemes. If suitable synthesis methods for continuous steering across multiple facets can be found, then wide scan angle sensors can be realised.
 
Advanced mm-wave on-chip nearfield scanning techniques and custom probe design
 
The radiation patterns of mm-wave antennas can be ascertained either by nearfield scanning, or by farfield measurement. Nearfield scanning of on-chip antennas would allow for antenna measurement integration into existing wafer probe measurement setups. The problem, here, is that state-of-the-art mm-wave nearfield probes have apertures that match (or exceed) that of the individual radiators, making it impossible to characterize individual antennas. This project will develop suitable small-aperture nearfield scanning probes, and develop suitable error correction techniques to process measurement results. In addition, the option of M + N (as opposed to M x N) scanning of a small aperture device will be investigated, using a large aspect ratio probe that effectively captures all of the radiated energy in a cut-plane simultaneously.
 
N-order stacked patch antenna synthesis
 
Stacking a parasitic patch on top of a microstrip patch antenna inreases the effective input reflection bandwidth of the antenna by creating an apparent second resonance. However, the synthesis techniques for this configuration may be aided significantly by rigorous application of lossy / absorptive filter synthesis techniques, whereby an explicitly synthesised wideband N-order multi-stacked patch can be designed from first principles. This has never been attempted, and may provide an invaluable tool for wideband microstrip antennas in multi-layer PCBs.
 

Slow-wave mode nanowire transmission structures

In a continuing effort to miniaturize microwave and mm-wave devices, slow-wave mode transmission lines seem to offer significant advantages. One way to effect this is to partially grow metallic nanowires through a porous medium, using that is substrate for a conventional microstrip line. There has, however, been very little work done on exploring different materials, processes, and transmission line geometries using this principle. Furthermore, there have been no published examples of typical RF passives (filters, antennas, couplers, etc) in this medium. All of these represent research inquiries, which this study will address.
 
A related inquiry would be to look at mixed mode propagation medium with EBG PGGWG and SIW vias (TH4B-1, 2017)
 

Topics for radio astronomy

Alternative manufacturing technologies for front-end EM components in single pixel dish receivers

Cryo-cooled low noise amplifiers integrated in RF CMOS and BiCMOS

Substrate Integrated Waveguide front-end for fast survey instruments

Monolithic integration of high dynamic range low-noise amplification and digitization circuits under cryogenic conditions

Synthesis and monolithic integration of in-system analogue data pre-processing networks

 

Final year project topics

These topics are best suited to final year projects, but have the possibility of extension to postgraduate level if sufficient novel research questions are found.

Broadband mm-wave field sensor (suitable for extension to Master's level)

A distinct requirement exists for a broadband mm-wave dual-polarization field sensor to characterize the quiet zone of a mm-wave anechoic chamber. Furthermore, the device is required to have extremely low RCS, as to minimally perturb the sensed field. This project will investigate suitable topologies for BEOL EM field harvesting topologies, as well as the required active detection circuitry.

Phased array dual-band GPS receiver (suitable for extension to Master's level)

Dual-band military GPS receivers (L1 and L2) are prime targets for electronic warfare (EW) attacks, either through saturation of the receiver or "spoofing". To overcome this, there is a need for phased array receivers that can steer multiple individual nulls towards interfering sources, or point multiple main beams to specific satellites. This project will develop a simple dual-band phased array receiver to steer multiple beams and nulls independently. 

mm-Wave waveguide filter manufactured by plating a machined dielectric slab (suitable for extension to PhD level)

The front-end transmitting amplifiers of RADAR and EW systems systems typically generate unwanted harmonics (2f0, 3f0, 4f0 etc, where f0 is the centre frequency of transmission) that, according to MIL-STD-461, have to be suppressed. A well-known machined waveguide topology known to perform this function is the waffle-iron filter, but this is a bulky, expensive device unsuitable for small, mobile, low-cost transmitters. A new manufacturing methodology has been proposed, whereby a dielectric block is machined and plated with a thin metallization coating. This will decrease the size of the filter, make it lighter, and reduce the manufacturing cost. The student will have to design and simulate a harmonic suppressor filter to suppress unwanted harmonics from a transmitter, and have it manufactured by studying and selecting appropriate plating processes.

Harmonic suppressor filter in multi-layer substrate integrated waveguide (suitable for extension to PhD level)

The front-end transmitting amplifiers of RADAR and EW systems systems typically generate unwanted harmonics (2f0, 3f0, 4f0 etc, where f0 is the centre frequency of transmission) that, according to MIL-STD-461, have to be suppressed. Substrate integrated waveguide (SIW) has been demonstrated to be a low-loss medium, with filters in this medium designed according to the same principles as rectangular waveguide filters. A well-known machined waveguide topology known to perform this function is the waffle-iron filter, but this is a bulky, expensive device unsuitable for small, mobile, low-cost transmitters. It has been proposed, but not proven, that a multi-layer microwave substrates could serve as a medium for implementing waffle-iron filters in SIW. The student will investigate the suitability of conventional harmonic suppression techniques currently used in conventional waveguide filters (corrugation, waffle iron boss patterns, etc) for application in SIW, and produce a filter based on this topology

Compact folded Gysel combiner for SSPA applications in SIW (suitable for extension to Master's level)

Most current RADAR and EW transmitters make use of travelling wave tube (TWT) amplifiers. These devices, though efficient, have intricate start-up procedures and finite shelf life. There is currently a major drive to move away from TWT amplifiers towards parallel assemblies of multiple solid state power amplifiers (SSPAs). For the combination of only a few amplifiers, Gysel combiners may be used, but their implementation in substrate integrated waveguide have not been demonstrated yet. This topology is especially useful in cases where one of the active devices fails, maintaining low input reflection at the input port whilst diverting mismatched power to one of the multiple external loaded ports. The student will deliver a matched divider in SIW, integrated with commercial-off-the-shelf amplifiers, and a recombiner of similar topology.

Wireless portable scalar network analyser (suitable for extension to Master's level)

Body area networks (BAN) is to be an integral part of the future Internet of Things (IoT) technology movement, which requires seamless communication between multiple personal items and wearable electronics. This requires high-bandwidth channels to be characterised and existing models validated by measurement of propagation characteristics between different parts of the body. Current measurements require that the “phantom” (an appropriate representation of a human body) be wired to a scalar or vector network analyser for propagation measurements between antennas placed on different body parts. This precluded the modelling of propagation under movement or in real-life environments. The student will deliver a battery-operated, wireless scalar network analyser and suitable wearable antennas for body area network propagation characteristics.

Test fixture for on-chip antenna measurements 

Our research group has a mm-wave farfield anechoic measurement facility, which currently operates on a WR-12 waveguide interface. We would, however, like to measure on-chip antennas on this facility, which would require a specialized low-vibration measurement setup to position, and maintain in place, a microelectronic GSG RF wafer probe. This project will investigate the effect of the mm-wave probe on the antenna's measured radiation pattern, and design a low-vibration measurement fixture to minimize the probe's interference on the measurement. 

Automated tester for audio amplifiers

Testing of electronic circuits is a major cost driver in production in both integrated and regularly assembled circuits. To reduce this cost, tests are usually automated by connecting a produced circuit to an automated tester that produces a pre-programmed set of inputs, records the outputs, and analyses the performance of the device-under-test (DUT). This automation reduces the time occupied by the test and increases the repeatability of the test procedure.  The challenge in this project will be to design and build an automatic tester for an audio amplifier. The system would need to bias the amplifier and apply suitable source and load impedances, after which certain amplifier metrics (gain, frequency range, compression, distortion, input and output impedance) need to be evaluated automatically. 

mm-Wave single-chip proximity sensor (suitable for extension to Master's level)

mm-Wave sensors are mostly immune to suspended particulate matter (dust & smoke) unlike optical sensors, and immune to ambient noise and air pressure changes unlike ultrasonic sensors. This makes them ideal for military or mining applications. However, to achieve sharp resolution and limit interference with communication bands, these sensors need to be implemented in the mm-wave (30 – 300 GHz band). This project will involve the microelectronic implementation of a single-chip mm-wave proximity sensor (including antennas) with simple binary detection output. The scope would depend on the level of study. 

A digital predistortion testbed for microwave power amplifiers

Power amplifiers (PAs) are notoriously nonlinear, and distort the broadcast signal significantly to achieve the required output power levels. This distortion significantly reduces the error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR) in multi-carrier systems such as OFDM used in LTE and WiFi. To counteract this distortion, input signals are often subjected to digital predistortion (DPD) where the inverse of the amplifier’s distortion is applied to the signal before amplification. The combined effect of DPD and amplifier distortion is a high output power signal that closely resembles that of the input signal. This project involves the design and implementation of DPD software on a PC, or suitable computing platform, for S-band (2 – 4 GHz) PAs, and integrate seamlessly with existing RF test equipment. 

An ELF/SLF wireless communications link using rotating permanent magnet antennas

Although radiofrequency (RF) wireless communication is fairly ubiquitous, it is highly susceptible to attenuation when propagating though seawater and soil. To communicate in these environments, low frequency (ELF, SLF, or VLF) carriers below 1 kHz are required. Given the considerable wavelength of these antennas, conventional distributed antennas are no longer feasible, requiring compact alternatives. One such alternative is the rotating permanent magnet, which has been used with some success in generating low bit rate communications underwater or underground. This project involves the design, simulation, and fabrication of a rotating permanent magnet antenna for ELF or SLF operation. Furthermore, the system needs to be able to transmit low data rate digital data using an appropriate communications protocol and modulation scheme. Finally, a suitable low frequency detector for this antenna’s radiation pattern needs to be designed and built, which would include a readout circuit for the data message. Communication through soil and saltwater would have to be demonstrated. 

A nearfield-farfield transceiver for mm-wave upconversion of LTE smartphones (suitable for extension to Master's level)

LTE cellphone networks are prohibited in radio-quiet environments, such as radio astronomy sites. Wireless connectivity may be established above the receivers' band of interest, but this would require custom mobile devices. An alternative solution would be to “capture” the LTE signal generated by a smartphone through an appropriate nearfield probe, upconvert it to a higher frequency band, and then rebroadcast it to distant infrastructure. This project will develop such a system; the scope would depend on the level of study. 

A sensor for remote monitoring of power factor and other performance metrics on high voltage lines (suitable for extension to Master's level)

Maintaining high power factors (in-phase voltage and current waveforms) is critical to the efficient use of high voltage transmission lines. Remote monitoring of the PF on lines would enable engineers to apply PF correction when and where necessary without physically attaching measurement equipment to the network. As the electric field (E-field) around a transmission line is proportional to the voltage on the line, and the magnetic field (H-field) proportional to the current, the power factor could be detected by phase comparison of the remote sensed E-fields and H-fields. This project will involve the design, simulation, and construction of such a system. It would involve designing and EM simulating appropriate E-field and H-field sensors with readout circuitry, as well as the integration of an embedded platform to calculate and display the power factor. 

A wireless battery charger operating over varying distances (suitable for extension to PhD level)

Wireless power transfer has been commercialized for contactless charging of consumer devices. These systems typically use resonant coils, effecting high energy coupling efficiency at specific frequencies. An often overlooked problem in this system is that the frequency of peak coupling depends significantly on the distance between the resonant elements; something that can quite easily be addressed through appropriate application of microwave filtering theory. This project will involve the design, simulation, and construction of a wireless battery charger that operates efficiently over multiple offset distances, either through appropriate broadband coupling or through real-time control of coupling frequency. For extension to postgraduate levels, the study would have to extend on current microwave filtering theory to establish coupling structures (and synthesis methods) specifically suited for varying coupling distances.

A tunable microwave group delay network (suitable for extension to PhD level)

It has recently been shown that various computational processes (such as Fourier transforms) can easily be implemented over broad bandwidths using suitably synthesized multi-order analog group delay networks, at a fraction of the cost required to implement these functions with DSP processors over the same bandwidth. Furthermore, these networks can be synthesized in a computationally efficient manner in real time. However, to demonstrate the versatility of this algorithm, a suitable hardware test platform is required. TThe design task of this project is to implement a microwave testbed for a multi-order tunable group delay network, and implement the existing group delay synthesis network in an appropriate embedded computing platform to control the network. For extension to postgraduate levels, the study would have to extend on current group delay synthesis methods by (a) including variable magnitude responses as well, or (b) improving on current synthesis methods which have been shown to take a large number of iterations top optimize. 

A C-band FMCW RADAR (suitable for extension to Master's level)

The challenge of this project is to design, build, and test an FMCW RADAR at an unlicensed frequency in C-band (4 – 8 GHz). The student is required to design and construct the RADAR antennas, the RF front-end (though use of off-the-shelf components would be recommended), as well as implementing the RADAR equation in the digital back-end (on an appropriate computing platform) to generate range-time plots. The signal processing would involve (amongst others) appropriate filtering, windowing, as well as range extraction. For extension to Master's degree level, the study would have to be ported to the existing 77 GHz automotove RADAR band, and novel integration methods devised.

A microwave reconfigurable switched delay line filter (suitable for extension to PhD level)

An alternative approach to creating frequency agile filters (i.e. not using tuned resonators) is to use delay line filtering, where pass-bands and stop-bands are created through constructive and destructive interference on recombined parallel transmission lines. By varying the electrical length of these lines using appropriate microwave switches, the center frequency may be controlled post-production. This project proposes the design of a microwave switched delay line filter on RF PCB. The centre frequency must be digitally controllable through an appropriate embedded computing platform. PhD extension for this topic would necessitate an improvement on existing delay line / switched delay line filtering methods (eg. introduction of variable delay line control, such is possible with distributed phase shifters). 

A radiometric Sun seeker

To stimulate interest in STEM (Science, Technology, Engineering and Mathematics) career paths, and in radio astronomy in particular, it is imperative that radiometric observation of stellar bodies be demonstrated in a conceptually simple way to learners. This project will develop a microwave passive radiometer, with control and data management platform, to demonstrate how radio emissions can be used to observe celestial bodies. This will involve the selection of an appropriate dish feed, design of the low noise receiver with digitization, data storage and uplink implementation, and suitable integration with a mechanical assembly of a two-axis motorized commercial reflector.  The radiometer would need to find the Sun through a sky search, and track its movement accurately across the sky.

An SIW mm-wave diplexer for E-band communications (extendable to Master's or PhD level)

ITU regulations have designated two frequency blocks in the E-band for mm-wave point-to-point communications, namely 71-76 and 81-86 GHz. Current system-on-chip receivers only operate over one of the two bands, requiring a diplexer to connect the two separate integrated circuits to a single front-end antenna. The state-of-the-art in manufacturing E-band components implements machined waveguide, which carries high non-recurring engineering cost. Substrate integrated waveguide has, however, shown to be a suitable low-cost alternative in most applications.The student is required to design, simulate, and manufacture a 71-76/81-86 GHz SIW diplexer in a suitable media, making provision for SoC and antenna integration. Extension to Master's degree would require an extension of scope, while PhD research would have to introduce some novelty (frequency agility, switching, novel manufacturing processes) not currently exhibited by state-of-the-art diplexers in this frequency range.

A dual-band SIW integrated slotted waveguide array antenna for E-band communications (extendable to Master's or PhD level)

ITU regulations have designated two frequency blocks in the E-band for mm-wave point-to-point communications, namely 71-76 and 81-86 GHz. The state-of-the-art in point-to-point antenna arrays in this frequency range is to use slotted waveguide arrays mechanically assembled from machined copper shim and alumimium. This process carries high non-recurring engineering cost. Substrate integrated waveguide has, however, shown to be a suitable low-cost alternative in most applications. The student is required to design, simulate, manufacture, and measure a slotted waveguide static array antenna in SIW, operating at both the 71-76 and 81-86 GHz bands. Extension to Master's degree study would involve investigating different array topologies, while a PhD study would have to extend significantly on the state-of-the-art (eg. co-integration with phase shifting to achieve beam-steered arrays)

A broadband mm-wave SIW integrated end-fire antenna for CubeSat deployment (extendable to Master's degree level)

The microwave and mm-wave microelectronics research group is currently developing a mm-wave Sun observation instrument for CubeSat deployment. This instrument will observe solar radiation in the 56-92 GHz spectrum, with a front-end divided into four distinct receiver bands (56-65, 65-74, 74-83 and 83-92 GHz). Since the exterior of a CubeSat is populated with solar panels, it may be infeasible to integrate broadside radiating antennas. However, gaps between the solar panels may allow for end-fire SIW-integrated antennas to receive, though their performance in this role has not been evaluated. The student will design, simulate, manufacture, and measure an SIW-integrated end-fire antenna array, capable of covering the four bands as outlined above. Extension to Master's degree level would involve investigation into different radiator shapes and / or materials. 

mm-Wave through-hole via transitional evanescent / combline filters (extendable to Master's or PhD level)

Although substrate integrated waveguide (SIW) has been shown effective in replacing conventional machined waveguide devices in various applications, the filters produced in this way still occupy significantly more surface area than planar resonators of the same frequency. An alternative approach is to use a grounded via as a quarter-wave line, and use these as combline filter elements. This has been demonstrated in simulation at Ka-band, but has it been manufactured. This project will implement a transitional evanescent / combline filter in in a suitable mm-wave band, using a conventional RF soft substrate. Extension to Master's or PhD level would involve migration to a suitable on-chip medium (such as through-silicon vias) and integration with other MMIC circuitry for enhancement or agility control.

Switches and isolators in SIW (extendable to Master's or PhD level)

It is notoriously difficult to integrate systems in printed circuit board (PCB) technology above 40 GHz, as most conventional transmission media start radiating energy. Substrate integrated waveguide (SIW) has, however, been shown to operate extremely well at mm-wave frequencies in PCB, driving the exploration of different system components to match this interconnect medium. Many passives (antennas, filters, combiners / dividers) have been demonstrated successfully, and there has been some success in integrating amplifiers in SIW too. What is missing, however, is diode-based nonlinear circuits, specifically switches and isolators. This project will involve the design, simulation, implementation and experimental verification of isolators and switches in SIW. Extension to Master's degree or PhD research would necessitate accurate analytical modeling and / or the development of novel switching / isolating circuit topologies to which SIW is uniquely suited.

A planar filter with dispersive coupling
See WE1C-1, 2017

Short-term experiments

These are descriptions of experiments for any degree-seeking study, but may rather be conducted as part of a lab internship (those suitable for final year projects marked as *)

- Combine existing SiGe negative resistance with existing off-chip micromachined cavities to create high-Q resonators or VCOs.- Design IIR filters in FPGAs or ROACH boards for pulsar dedispersion
- Design an experiment to measure the presence of multiple propagating modes on rectangular waveguide.
- Theoretical designs of two radiometers: one TPR, one DDR, at 183 GHz.
- Re-design and prototype a subharmonic stub filter.
- Design and prototype a substrate through-hole via coupled filter at mm-wave frequencies.[*]
- Design filters by machining and plating a dielectric slab.[*]
- Design an experiment for single-port characterization of RF metals and dielectrics for cryo experiments.
- Design an active Q-enhanced filter on soft substrate with a negative resistance chip.
- Extract RLC models for GSG taper transitions
- Plan a planar probed measurement setup for RFID antennas
- Nearfield scanning probe for on-chip antenna measurements.[*]
- Refurbishing mm-wave anechoic chamber for mm-wave on-chip antenna measurements
- 1.5x multiplier for harmonic blocks.[*]

Published by Tinus Stander

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