Microwave oscillators are nowadays an essential part of measurement equipment and telecommunication systems. They are notable for properties such as long-term stability, tuning range and phase noise. Often, the oscillator’s performance is focused on just one of these properties. For example, time-measuring systems demand a very accurate frequency. The frequency of such an oscillator is fixed and cannot change during the operation of the oscillator. In this case, the long-term stability is very good, but not so the phase noise. However, the latter can be improved by employing additional oscillators. Local oscillators are often voltage controlled, and so the frequency-tuning range is important in this case. A low phase noise is preferred, but is limited because of the lower quality factor of the resonator compared to an oscillator with a fixed frequency. In some applications, such as radar or measuring equipment, the phase noise is the most important parameter. The phase noise describes the width and the shape of the oscillator signal’s spectral line. The oscillator, for which the phase noise is the most important parameter, is the subject of this dissertation. The units for the phase noise are dBc/Hz (decibels below carrier per hertz). The ratio between the signal’s power and the noise at a specified frequency offset from the oscillator’s frequency contains the information about the phase noise. The latter depends mostly on the resonator’s quality factor. The 1/f noise also affects the phase-noise performance. These two parameters have an influence on the phase noise close to the carrier frequency. At larger frequency offsets, the thermal noise dominates.
A couple of concepts are developed for microwave oscillators with a fixed frequency and an extremely low phase noise. Different types of resonators can be used, such as a cavity resonator or an optical resonator. A phase noise of -75 dBc/Hz at a frequency offset of 10 Hz can be achieved with a cavity resonator having an unloaded quality factor of 80·103 in the X-band. An X-band oscillator with a cavity resonator that is loaded with a sapphire crystal and cooled down to 6 K achieves a phase noise of -125 dBc/Hz at a 10-Hz offset. With an optical frequency comb, a phase noise of -130 dBc/Hz at a 10-Hz offset is possible to achieve in the X-band. All these results are state of the art and are achieved with complex systems and specialized building blocks.
An oscillator that uses an optical delay line as a resonator was presented in 1995. An optical fiber enables the realization of a large delay time and thus a high quality factor. An oscillator of this type does not require a special resonator. The phase-noise level is determined with the length of the optical fiber in the delay line. An important advantage of the so-called opto-electronic oscillator is that the phase noise is independent of the oscillator’s frequency. A phase noise of -70 dBc/Hz at a 10-Hz offset can be achieved with this type of oscillator in the X-band. The photodiode’s shot noise and the laser’s relative intensity noise also have an effect on the phase noise of an opto-electronic oscillator. Multimode oscillation and temperature-dependent frequency drift are some of the problems with an opto-electronic oscillator. Solutions, such as additional optical loops and injection locking, were proposed to suppress the side modes in the spectrum of the opto-electronic oscillator. Any frequency drift can be minimized with temperature stabilization or additional measurement signals. The main problems with solutions presented in literature are, for example, to increase the number of components or the use of highly specialized building blocks. To simplify the structure of an opto-electronic oscillator and to maintain good stability, three methods were developed, which are presented in this dissertation.
To stabilize the frequency a method with a feedback control loop was developed. The achieved frequency drift was 0.05 ppm/K. The method works in such a way that the frequency is measured with a frequency discriminator and made constant with the laser’s changing wavelength. The wavelength of the light has an effect on the delay time because of the refractive index and thus also an effect on the oscillator’s frequency.
To eliminate the side modes, two methods were developed. One method increases the side-mode suppression with an additional phase modulation of the oscillator’s loop. With a frequency mixer a side mode is extracted from the oscillator’s spectrum. The phase of the oscillator’s loop is then modulated with the extracted signal. A 5-dB increase in the suppression ratio was achieved with this method. The second method includes a quality multiplier, which is a positive feedback, added to a band-pass filter in the oscillator’s loop. The filter’s bandwidth is decreased with this technique, which increases the suppression ratio by 20 dB, according to the measurements presented in this dissertation. Unfortunately, this technique increases the phase noise by 4 dB at a 1-kHz offset.
The dissertation’s core is four internationally published scientific papers. Three of them present three methods developed during the research work and are already mentioned in previous paragraphs. One paper is a state-of-the-art review, and was published as an invited paper at a conference. Besides the scientific papers, three patents (one for each of the developed methods) are also presented in this dissertation.
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