Prasetiadi, Ananto Eka (2017)
Tunable Substrate Integrated Waveguide Bandpass Filter and Amplitude Tuner Based on Microwave Liquid Crystal Technology.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Tunable microwave devices are needed to fulfill the requirement of the modern wireless systems. In this work, two major devices have been developed: a tunable filter and an amplitude tuner. Specifically synthesized liquid crystals (LC) have been employed as tuning elements for those devices due to their low loss property at frequency bands above 10 GHz, where printed circuit board (PCB) and low temperature co-fired ceramic (LTCC) technologies are employed for the hardware implementation.

By inserting the LC inside a filter structure, a tunable filter is obtained, so that the center frequency can be altered by using either magnetic alignment or electric biasing. To achieve a low loss filter with a high quality factor (Q-factor) and maintain the compactness, substrate integrated waveguide (SIW) structure is combined with the LC as the tuning element. Consequently, some magnetically aligned structures, which are implemented by using PCB technology, are verified first because of their simplicity. To begin with, a single resonator, which is the important constituent of the filter, is designed, fabricated and measured, resulting in a device with unloaded Q-factor of 102 to 105.6 and a tuning range of 412 MHz around a frequency of 22 GHz. In addition, 3 resonators are cascaded, forming a 3-pole Chebyshev filter. Following results are yielded: center frequency range of 21.45 to 22.06 GHz, insertion loss of 6 dB and fractional bandwidth of 2.81 to 3.54 %. Furthermore, electrically biasing systems for these PCB-based devices, have also been investigated by using glass fabrication technique. A simple demonstrator, which utilizes a tunable delay line structure, provides the following results: FoM of 32 °/dB and a response time of 5 s.

In contrary to the PCB technology, the LTCC technology enables the integration of both the microwave and biasing structures. A single resonator is fabricated first with unloaded Q-factor of 68 to 100, a tuning range of 840MHz (30.16 to 31 GHz) and a response time faster than 2 s. A 3-pole Chebyshev filter is also fabricated with the following results: operating frequency of 29.4 to 30.1 GHz, fractional bandwidth of 11.2 to 11.6% and insertion loss 2 to 4 dB.

The second device, which is the amplitude tuner, changes the amplitude of the input signal in dependence of an applied voltage. For this purpose, tunable resistors and varactors can be employed. However, with the usage of LC, a device with a lower power consumption can be obtained at frequencies above 10 GHz. The proposed amplitude tuner uses the interference concept, splitting the input signal into two branches and at the end, combining it together again at the output port. By using a tunable LC phase shifter in one of the branches, the output amplitude is controllable. Therefore, the required subcomponents are the phase shifters, power divider and combiner as well as transitions. The amplitude tuner is fabricated with the LTCC technology, yielding a device with amplitude range of 11 to 30 dB at around 30 GHz. Thus, an LC-based amplitude tuner operating at the lower millimeter wave band is achieved for the first time.

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