Abstract: A fractional-N frequency synthesizer with low quantization noise includes a phase frequency detector, a charge pump, a loop filter, a voltage controlled oscillator a divider and a Delta-Sigma modulator. The Delta-Sigma modulator computes “N” to the loop filter in the output current of the charge pump. Quantization step size in the form of current is narrowed, and that reduces the absolute energy of quantization noise reduced, resulting in wider loop bandwidth which has advantages for the fractional-N frequency synthesizer over the lock time, energy saving, and alleviates voltage controlled oscillator design constraints. Even more, frequency synthesizer achieves high frequency transmission in direct modulation transmitter.

Abstract: A voltage-controlled oscillator with high loop gain includes a first transistor, a second transistor, an inductor, a third transistor, a fourth transistor and a gain circuit. The gain circuit provides a high loop gain in the voltage-controlled oscillator, which operates at wider tuning range but with low power consumption due to capacitance value provided by transistors thereof.