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This is a thesis report done as part of the Master of Science in Electronics Design Engineering given at Linköping University, Campus Norrköping. The thesis work is done at Ericsson AB in the spring of 2005. The thesis describes a method of removing variations in the tuning sensitivity of voltage-controlled crystal oscillators due to different manufacturing processes. These variations results in unwanted variations in the modulation bandwidth of the phase-locked loop the oscillator is used in. Through examination of the theory of phase-locked loops it is found that the bandwidth of the loop is dependent on the tuning sensitivity of the oscillator. A method of correcting the oscillator-sensitivity by amplifying or attenuating the control-voltage of the oscillator is developed. The size of the correction depends on the difference in oscillator-sensitivity compared to that of an ideal oscillator. This error is measured and the correct correction constant calculated. To facilitate the measurements and correction extra circuits are developed and inserted in the loop. The circuits are both analog and digital. The analog circuits are mounted on an extra circuit board and the digital circuits are implemented in VHDL in an external FPGA. Tests and theoretical calculations show that the method is valid and able to correct both positive and negative variations in oscillator-sensitivity of up to a factor ±2.5 times. The bandwidth of the loop can be adjusted between 2 to 15 Hz (up to ±8 dB, relative an unmodified loop). |
