Optical delay line for the transmission of a 4.3 GHz signal

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Dr. Berceli Tibor
Department of Broadband Infocommunications and Electromagnetic Theory

To my diploma work I have made an optical delay line at a given 4,3 GHz frequency. This instrument is developed for Bonn Hungary Electronics Ltd. This line will be used by the firm as a reference for an altimeter used on aeroplanes. During my practice the given and accepted requirements of the firm meet the ones set in the task.

The reason why I choose this topic is that during the practice I have started to deal with this instrument and I have seen many opportunities in it which can give me further useful experiences. I spent my practice partly at the Bonn Hungary Electronics Ltd. and partly at the Optical and Microwave Telecommunication Laboratory of the university.

During the semester I have examined how to improve the transmitter circuit in terms of impedance matching and DC biasing in order to show as low RF attenuation as possible and work properly in the given temperature range (between -40 and 80 celsius degree) and at the given deviation of the supply voltage.

The appropriate matching type and current generator type were chosen in order to do simulations and produce the new circuit. For current generator a simple circuit was made which consists of two resistors, a transistor and two diodes. The impedance matching was realized with single-stub matching technique. Having done DC measurements and measurements of input impedance on the new transmitter circuit, I measured the transmission characteristic and the delay of the optical line. For measuring the delay of the line I used a fibre put at my disposal whose length is suitable for the given value of the delay. However for setting an accurate delay another fibre would be necessary whose length is even more appropriate. When the necessary measures had been done, the instrument was proved to work properly and to ensure moderate transmission at the given 100 MHz frequency range.

The bias of the newly produced transmitter circuit changes 0,75 mA in the range of the expected ±0,5 V. Its input impedance is 58,8+j16,3 Ω at 4,3 GHz which means -15,4 dB reflection coefficient. This circuit ensures less than -10 dB value of reflection coefficient between 3,8 GHz and 5 GHz. The attenuation of the transmission is 33 dB in the appointed range. The realized delay is 350 ns which can be increased to the expected value by a longer fibre.


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