Diplomski radovi rade se na Zavodu za radiokomunikacije pod vodstvom profesora Dubravka Babića, a izvršavat će se u suradnji s tvrtkom Eridan Communications, Inc. iz Santa Clare u Kaliforniji. Teme diplomskih radova usuglašene su s tehničkim potrebama Eridana. Ako vas zanimaju navedene teme javite se profesoru Babiću radi daljnjih dogovora.
1. A multi-octave low-pass tunable MEMS RF filter
Micro-electro-mechanical components are presently researched for building tunable filters at the input or output of mobile-phone amplifiers and receivers. The task is to (a) characterize the performance of an off-the-shelf MEMS filter digitally controlled via MIPI serial interface, and (b) design and build a high-order low-pass filter with cutoff frequency tuning between 1 and 2.5 GHz.
The project involves analytic design and ADS simulation, designing and building the serial interface using off-the-shelf MIPI serial interface, writing the firmware to control the filter via USB or similar standard, designing a test printed circuit board, s-parameter characterization of the filters with extraction of filter parameters (parasitic inductance and capacitances) and finally designing the low-pass tunable filter.
2. Polar RF transmitter with harmonic management
Polar RF transmitters generate amplitude and phase modulated square-waves at their output. This output needs to be filtered to remove spurious emissions, which can be done with standard low-pass filters. However, standard lossless filters exhibit high reflection to higher harmonics which has profound effect on the operation of the output stage of the polar transmitter and hence certain degree of harmonic management at the output of polar transmitters is required. The project involves designing and characterizing various low-pass filters with cutoff frequency 100 MHz and studying their effect on the output of a polar transmitter.
The project involves (a) characterization of the capacitors and inductors to be used in the filter, (b) analytic design of required filter and ADS simulation, (c) designing and building a printed circuit board with the filter and the output stage with a switching field-effect transistor, (d) characterization of the transmitter output for energy efficiency, linearity, and harmonic content, and (e) extracting the filter parameters from measured data and developing a complete model for the filter.
3. UHF Class-F amplifier using GaN high-electron mobility transistor
Obtaining high efficiency for modern LTE-type signals is many RF electronics manufacturer's challenge today. Envelope tracking and Doherty amplifiers are in production today, but still can be used only in narrow bands. Polar transmitters offer more than an octave in bandwidth and are of high interest in environments where frequency hopping and carrier aggregation is needed. The question is whether it is possible to create high-efficiency class F amplifiers and over what band can they be tuned for applications in base stations (BS) and mobile phones.
The project involves an analytic and numerical study of the practicality of employing class F architecture with GaN field-effect transistors, selecting components, designing the filters at the output from the transmitter using either lumped or distributed components, building printed circuit boards and testing the finished amplifier, and finally, extracting the amplifier and filter parameters to develop a model. This is the first version of the amplifier, later versions will be designed for higher frequencies and ultimately will be made tunable.
4. Switching driver circuit for GaN high-electron mobility transistor
Presently, AlGaN/GaN field-effect transistors are actively investigated for use in switchmode RF power amplifiers operating in classes D, E, and F, and polar modulators as they provide high energy efficiency. The key challenge in obtaining high efficiency is to be able to open and close the output power transistor sufficiently fast. The rate of transistor state change critically depends on the ability of the driver citcuit (preamplifier) to deliver sufficiently large current impulses to charge and discharge the gate-capacitance of the output power transistor. Relative to energy conversion applications (solar inverters and DC/DC converters) where switching occurs at low frequencies (~100 kHz), at GHz switching frequencies, where the switching has to occur within several tens of picoseconds, this a major circuit and device design challenge: commercial manufacturers of such circuits keep theirs solutions secret.
The task is to design a circuit that will drive an AlGaN/GaN high-electron mobilty transistor with 1 W output power. The project comprises analytic design, selection of components or circuits that will drive a commercial AlGaN/GaN FET, perform ADS simulations, add a model for the device, design, build, and populate a printed circuit board, and finally characterize the completed circuit at 1 GHz.
5. Reflectionless Gauss low-pass filter in the S-band
There is a number of ways to build a reflectionless filter - a filter that maintains input impeance equal to Z0 over a frequency band that exceeds the filter cutoff frequency. In theory this is always possible with lumped components and works well at frequencies below 100 MHz. However, the parasitic capacitances of the inductors and parasitic inductances of the capacitors make this a difficult if not impossible task to achive in the GHz range. Rather than lumped components, one then uses distributed absorbing filters. Fourth-order Bessel-Thomson filter is an example of a low-pass absorbing filter and it is commonly used as a standard receiver filter in optical communications owing to its minimal group delay disperson for a given band pass.
The task is to design an absorbing low-pass filter using a microstrip or coplanar waveguide line that has reflection |s11|2 < 1/10 between frequency zero and double the cutoff frequency of the filter. The filter should be designed analytically and optimized using ADS, printed citcuit board should be designed, built, and populated. Finally, the filter is to be characterized for |s11|2 and |s21|2.
6. Metoda i sustav za karakterizaciju sporih promjena u izlaznoj snazi RF pojačala
Radiofrekvencijskog pojačala u bilo kojem mobilnom sustavu vrlo često mijenjaju izlaznu snagu zbog kretanja primopredajnika u odnosu na baznu stanicu. Učestalost promijena može biti svakih deset milisekundi, a intenzitet se može promijeniti i do tisuću puta. Iako je digitalna kontrola ovakvih promijena jednostavna, za izlazni tranzistor u pojačalu snage ovo može biti nemoguće pratiti precizno ako se tranzistorske karakteristike mijenjaju sporo uslijed karakteristika materijala ili problema u proizvodnom postupku (dominantni femomen je tzv. "drain lag"). Utjecaj ovakvih fenomena se najčešće ne vrednuje na samom tranzistoru već kad je pojačalo ili bar izlazni stupanj pojačala već izrađen. Postoji nekoliko testova koji su razvijeni u industriji da bi se ovakvi fenomeni otkrili i neke tehnologije su iz ovog razloga eliminirane iz primjene.
Zadatak ovog rada je implementirati mjerni sustav koji će modulirati izlaznu snagu nekolicine komercijalno dobavljivih tranzistora i vrednovati ove spore promjene na frekvencijama koje se koriste za 4G/LTE sustave. Gotovi test sustav će se onda koristiti za evaluaciju mikrovalnih pojačala.
7. Evaluation of the impedance loading the dynamic power supply in a polar amplifier
In a polar modulator (PM), the amplitude modulation (DC to 100 MHz baseband) is delivered by a dynamic power supply (DPS) coupled to the output-stage via the power supply port. One of the key issues in the design of a DPS is planning for the impedance that the DPS will see when it drives the PM output stage operating in the GHz range. This problem can be analyzed numerically using state-variable analysis and based on this a number of analytic approximations have been developed. The question remains whether these models correctly describe reality and whether they can used in establishing the stability and performance the DPS/PM combination.
The task is to design several printed-circuit boards with test circuits and evaluate low-frequency impedance of a polar modulator output stage operating in the GHz range. The measurements and models need to be matched with simulations. The goal is to establish a method for determining the equivalent impedance of a switching polar modulator output stage seen from the power supply port.