Oscillator Architectures and Enhanced Frequency Synthesizer
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A voltage controlled oscillator (VCO), that generates a periodic signal whose frequency is tuned by a voltage, is a key building block in any integrated circuit systems. A sine wave oscillator can be used for a built-in self testing where high linearity is required. A bandpass filter (BPF) based oscillator is a preferred solution, and high quality factor (Q-factor) is needed to improve the linearity. However, a stringent linearity specification may require very high Q-factor, not practical to implement. To address this problem, a frequency harmonic shaping technique is proposed. It utilizes a finite impulse response filter improving the linearity by rejecting certain harmonics. A prototype SC BPF oscillator with an oscillating frequency of 10 MHz is designed and measurement results show that linearity is improved by 20 dB over a conventional oscillator. In radio frequency area, preferred oscillator structures are an LC oscillator and a ring oscillator. An LC oscillator exhibits good phase noise but an expensive cost of an inductor is disadvantageous. A ring oscillator can be built in standard CMOS process, but suffers due to a poor phase noise and is sensitive to supply noise. A RC BPF oscillator is proposed to compromise the above difficulties. A RC BPF oscillator at 2.5 GHz is designed and measured performance is better than ring oscillators when compared using a figure of merit. In particular, the frequency tuning range of the proposed oscillator is superior to the ring oscillator. VCO is normally incorporated with a frequency synthesizer (FS) for an accurate frequency control. In an integer-N FS, reference spur is one of the design concerns in communication systems since it degrades a signal to noise ratio. Reference spurs can be rejected more by either the lower loop bandwidth or the higher loop filter. But the former increases a settling time and the latter decreases phase margin. An adaptive lowpass filtering technique is proposed. The loop filter order is adaptively increased after the loop is locked. A 5.8 GHz integer-N FS is designed and measurement results show that reference spur rejection is improved by 20 dB over a conventional FS without degrading the settling time. A new pulse interleaving technique is proposed and several design modifications are suggested as a future work.
Park, Sang Wook (2009). Oscillator Architectures and Enhanced Frequency Synthesizer. Doctoral dissertation, Texas A&M University. Available electronically from