Design Of High Precise Oscillator With Average Voltage Feedback

Since the invention of crystal oscillator, it has been widely used in sophisticated instruments and systems due to its high frequency stability and tiny jitter. Then the transistor had been created, and its processing technology continuously updated, more and more systems had been shifted from separate device to be integrated as a chip.Integrating the oscillator into the systems can greatly reduce cost and power consumption, otherwise the oscillator will be off chip.Relaxation Oscillator has the advantage of compatible with standard CMOS processing. It had broadly adopted in SOC, biomedical sensors, micro-computer and high speed interface. It form square waves through iterated charging and discharging.When the voltage reached to turn point, the comparator will shift the charging or discharging state. But,the delay of the comparator is susceptible to temperature and power supply, which makes the precision of frequency limited. This is prominent especially in the design of high frequency oscillator.In this article we applied a voltage averaging feedback to stabilize the delay of comparator. Voltage averaging feedback is realized through inserting an active filter between reference voltage and comparator. The filter automatically adjusts its output to equalize DC value of its two inputs, which results in a stable desired frequency. If comparator delay is longer, the filter output increase to keep the same frequency, and if the delay is shorter, the output decreases in a similar fashion. Thanks for the feedback the comparator used in this paper are quite simple rather a complex one in ordinary relaxation oscillator. The ultimate frequency will be determined by the passive resistor and capacitor with a rather stable comparator delay.The designed oscillator in this thesis is aim to SMPS applications. The comparator delay and frequency performance have great improvement compared to the original oscillator used in SMPS. The ultimate frequency is at 10 MHz order with 50% duty cycle. The frequency can be reduced to 2.5MHz through frequency division, and the duty cycle can also be reduced through digital circuit to meet the SMPS application environment.The temperature coefficient of oscillator is mainly determined by resistor. The frequency will vary 0.3% with temperature changed from 30℃ to 120 ℃, after the compensation for resistor temperature coefficient. There are 2% varies in frequency with the power supply changed from 2.7V to 3.9V.This work adopted 0.13 um CMOS processing, output frequency are tuneable from6 MHz to 11 MHz by trimming. The layout for the design had implemented, which had been taped out.