Design of Microwave Mixer for Radio over Fiber receiver

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Cseh Tamás
Department of Broadband Infocommunications and Electromagnetic Theory

With the improvement of wireless communication the need for high capacity wireless services significantly arose. The number of users is multiplied whereas the available frequency spectrum is limited. As a solution to this problem, the use of higher carrier frequencies and smaller cell size is needed. The usage of systems with smaller cell sizes means that large numbers of Base Stations are needed, which leads to higher costs of the system, ehile high carrier frequencies rise the cost of the Base Stations and Mobile Units.

Radio over Fiber system is a solution for the problems caused by the smaller cell sizes. Radio over Fiber systems achieve the simplification of Base Stations, because signal processing functions, which need high-cost devices are executed at a centralised head-end, which share the information with multiple base stations, which are simplified to single Radio Antenna Units this way.

With the higher carrier frequencies, microwave mixers are needed to perform the RF-IF frequency conversion in the centralised head-end. There are numerous types of microwave mixers, nowadays mixers are regularly produced with conversion losses around 4 dB at frequencies around 50 GHz.

As a part of my thesis, I designed a balanced mixer. The passive components of the mixer such as microwave hybrids and filters were designed and simulated in Microwave Office. Two mixer circuits, a low pass filter and a band pass filter were implemented. I measured the transmission and the isolation of the mixers. Furthermore I analysed the degradation of the quality of modulated signal transmission over a Radio over Fiber system.


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