Graduate Thesis Or Dissertation
 

Characterization and reduction of local oscillator phase noise effects in communications systems

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  • Recent developments in digital communications at microwave frequencies have revealed that local oscillator phase noise is often a factor in the bit error rate (BER) analysis. Digital signals transported across microwave radio links acquire waveform jitter from local oscillator phase noise. As jitter increases so does BER. The main goals of the investigations described in this dissertation are to demonstrate the feasibility of determining rms jitter from measured phase noise and to develop mathematical models to describe how local oscillator phase noise is added to an information signal passing through a radio link. The first goal of estimating jitter from phase noise data has many applications. An obvious use is to specify the phase noise performance of a local oscillator for a given jitter specification which in turn may be specified for a desired BER level. A less obvious application is the ability to estimate the jitter of a microwave or millimeter wave signal based on measured phase noise. At these high frequencies it is often impractical or impossible to measure jitter directly due to performance limitations of time domain equipment such as the digital sampling oscilloscopes (DSO) which are typically limited to about 22 GHz. Conversely spectrum analysis techniques are well developed that allow accurate phase noise measurements to be performed well beyond 100 GHz. Experiments which validate the known relation between an oscillator's single sideband phase noise and associated mean square jitter [8, 28] are presented. Test equipment was developed to allow the addition of phase noise in a controlled manner to a clean reference signal which for practical purposes has no inherent jitter. By performing the experiments at the relatively low frequency of 33.333 MHz both the phase noise and jitter could be measured easily. Comparing the rms jitter predicted from phase noise data to direct measurement with a Digital Sampling Oscilloscope determined that the relation gave typically less than 14% error with a worst case disagreement of 24%. The experiment had an estimated uncertainty of ± 17%. This level of agreement is acceptable for many BER applications, which often specify jitter to an order of magnitude. The second goal of the research was to develop a model which describes how the phase noise of transmitter and receiver local oscillators add to an information signal carried over a communications link. It is shown that this added phase noise can in principle be eliminated in a double sideband communications system when the relative phase difference between the two local oscillators is synchronized to Nπ, where N is any integer. Experiments were performed which validated the predicted results. It was found that using real components allowed a 24 dB reduction added phase noise when compared to the case when no synchronization was used. A practical circuit is proposed to implement the technique in a practical manner for real radio systems. A final area of research presented phase noise measurements for a Gunn diode microwave integrated circuit (MIC) voltage controlled oscillator (VCO) in the 18 GHz region. The single sideband phase noise ratio of -96 dBc/Hz at 100 kHz offset frequency was significantly better than current published data for MESFET, HBT, and PHEMT VCOs at similar frequencies. These results are important in the area of digital radios, since improved phase noise allows higher data rates and reduces adjacent channel power.
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