Using optical communication is essential in modern telecommunication systems, and consequently, the Radio over Fiber (RoF) technology also plays an important role. The RoF technique involves modulating radio frequency signals onto an optical carrier for distribution over a fiber network, then transmission of the data after optical-electronic transformation. This method combines the benefits of optical networks (large capacity, bandwidth, low attenuation, insensitivity to electromagnetic interference) with the mobility of radio communication.
My thesis focuses on the design of a microwave diplexer for a Radio over Fiber receiver. The device’s function is to separate the 2.4 GHz and 2.6 GHz frequency bands, as well as the 1.3 GHz and 2.4 GHz bands.
My work includes the design and realization of a wideband Wilkinson splitter and different band-pass and band-stop filters. During the simulations I have tested multiple types of filters, and since the discrete-element circuits turned out to be effectively useless in the applied frequency range, I decided to analyse distributed element filters, including coupled transmission line filters. I also designed an impedance-matching circuit, in order to ensure the proper operation of the diplexer.
After the production of the devices with the most adequate properties, I examined whether the elements - separately and collectively - fulfil the specified criteria or not. In cases of unsatisfactory results I carried out further adjustments.
As the final step, the RoF receiver containing the diplexer unit has been tested with the help of modulated signals, while measuring the effect of the circuit on the quality of the output signal.