| dc.description.abstract | When using magnetically coupled all-pass networks (MCAPNs) to realize phase shifters, the bandwidth over which the phase shift remains constant can be increased if the coupling coefficient, k, between the two inductors are designed to be negative, whereas the phase shift can be boosted if k is selected to be positive. In this work, two analog phase shifters with high phase shifts are realized using MCAPNs with positive k and ferroelectric varactors. Depending on whether the layout of the coupled inductors are symmetrical or not, the two phase shifters are respectively designated asymmetric phase shifter and symmetric phase shifter.
Among the two phase shifters, the asymmetric one is designed to operate at 10 GHz and fabricated on sapphire substrate. Measurement results show that, when the bias voltage of the ferroelectric varactors are tuned from 0 V to 6 V, the phase shift reaches its maximum at 8.5 GHz with a value of 67.3°. For all bias voltages, the insertion loss is less than 4.5 dB and the return losses are greater than 10 dB from dc to 12 GHz. The reason why the frequency where maximum phase shift occurs shifts from the originally designed 10 GHz to the measured 8.5 GHz is because the capacitance values of the varactors fabricated are larger than the designed values.
On the other hand, we develop the fabrication process for ferroelectric varactors with through substrate vias on silicon and apply it to the design of the symmetric phase shifter in this work. The symmetric phase shifter is designed at 2.45 GHz with an expected phase shift of 60°. However, measurement results show that the phase shifter does not exhibit the desired all-pass response. After checking the microphotographs of the fabricated circuit, we find that there is an unexpected piece of metal connection between the two coupled inductors after they go through gold electroplating process. After incorporating this piece of metal connection into the full-wave simultion, the simulated results match the measured results, thus verifying that the unwanted metal connection is the reason when measurement results deviate significantly from the original simulation results.
In this work, we successfully design and fabrciate the asymmetric phase shifter, demonstrating the potential for using MCAPNs with k > 0 for implemeting analog phase shifters. As for the symmetric phase shifter, we verify that its failure is due to a specific fault in the fabrication.
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