Mateusz Zukocinski

Center frequency of wideband direct-coupled resonator filters

Wideband direct-coupled resonator filters with real inverters are analyzed and compared to filters based on ideal inverters (Cohns filter). Characteristic features of the wideband filters are described. A new definition of the filter center frequency based on filter frequency characteristic is proposed. Dependence of the center frequency on the filter order is presented. A new type of filter structure, new design methodology and example of 3rd order, 115% bandwidth UWB filter is presented.

Design of varactor tuned bandpass filter

The tunable bandpass filter is presented. The filter is realized in microstrip technology employing two stepped-impedance resonators tuned with varactor diodes. The filter has constant absolute bandwidth of 30 MHz in the frequency range from 380 MHz to 920 MHz. The insertion loss is lower than 5 dB and return loss higher than 17 dB in the full tuning range. The out of band attenuation is higher than 40 dB at frequencies below the passband and is higher than 50 dB at frequencies above the passband. The first spurious modes are observed at 3 GHz. Absolute bandwidth and the time delay in the full tuning range change by 33% only while center frequency changes 242%.

Real time control of RF fields using a MicroTCA.4 based LLRF system at FLASH

The Free Electron Laser in Hamburg (FLASH) is a large scale user facility, providing highly stable and brilliant laser pulses down to a wavelength of 4.1 nm. Essential for stable and reproducible photon beam is the precision control of the electron bunch parameters. The acceleration principle of the electron bunches at FLASH is based on superconducting RF technology (SRF), in which the RF fields are controlled by a digital low level RF (LLRF) system. This system has been recently upgraded to the Micro Telecommunication Computing Architecture (MicroTCA.4) to improve the performance of the field regulation. This paper presents the first measurements and operation experiences using this new electronic crate standard at a large scale research facility. RF field regulation is carried out by real time fast digital processing on several boards in a MicroTCA.4 crate, and slow automation routines running on a dual core i7 front-end CPU. Scalability and modularity of this system is one of the key parameters to meet the next steps, namely being the platform standard for the European X-ray free electron laser currently build at DESY.

Lossy inverters and their influence on coupled resonator filter characteristics

Lossy inverters have been used recently to describe circuits with lossy couplings. The subject is important for microwave filters design as well as for coupled transmission lines analysis and measurements. In this paper it is shown that lossy inverters do not satisfy the passivity condition. They should be considered as active circuits. On the base of coupled dielectric resonators examples new models of lossy coupling have been developed that preserve the passivity condition. The influence of the coupling losses on filter characteristics has been shown. Moreover, the developed models can be easily used to design coupled resonator microwave filters employing the coupling matrix method.

Resonant frequencies of helical resonators

The paper investigates the resonant frequencies of helical resonators. Full-wave 3D electromagnetic analysis is used to verify closed-form formulas known from literature. Corrections to formulas for dominant mode and new formulas for higher modes are proposed. Two-helical resonators filter was designed and measured.

3.9GHz accelerating cavity field detection hardware for the free-electron laser

The 3rd harmonic system installed in the injector at FLASH (Free-Electron Laser at Hamburg) accelerator allows to achieve ultra short electron bunches with higher peak current, which pushed the limits of high brilliance laser light generation into the so-called water window. The 3rd harmonic system operates at 3.9GHz and requires a very precise phase and amplitude control of the accelerating field below 10fs, which is done by the Low-Level RF system. One of the key component is the RF field detector that converts the detected cavity field signal to an intermediate frequency for digital sampling. The paper presents an overview of the system, particularly the low phase noise reference generator, local oscillator and multichannel down-converter. System scheme and performance are demonstrated.

FDTD Simulations of Resonators with Closely Spaced Modes

Convergence of FDTD simulations in the case of weakly or medium loaded microwave cavities is investigated. Closely spaced modes in multimode appliances are considered with special interest, due to their proliferation in microwave heating industry. Stability of electromagnetic field distributions in-between resonant frequencies is investigated as well. Full-wave 3D conformal FDTD method is used to perform the analysis.

Design method for wideband direct-coupled resonator filters with electric or magnetic couplings

The paper presents methodology for designing wideband and ultra-wideband filters with electric (capacitive) or magnetic (inductive) direct couplings based on closed-form formulas. For given filter specifications (required frequency characteristic, filter center frequency, bandwidth) the method gives filter parameters as a set of resonant frequencies, coupling factors, external quality factors and characteristic impedances of the filter resonators. The method is based on analysis of circuit models of filters with only-electric and only-magnetic direct couplings. Numerical experiments of the circuit models allowed to study properties of such filters and to build effective design method. In this paper method for designing 5th order filters is described in details. High accuracy of the presented method is also addressed.