Generation of highly stable 5 GHz microwave from a spin torque oscillator by phase locked loop referenced to a 80 MHz clock

Abstract

Various modern electronics systems require multiple high frequency (HF) signal sources, often in microwave frequency range, with very low phase noise. Spin torque oscillators (STOs) are a promising candidate for such HF signal sources that can be directly integrated into semiconductor chips with an extremely small footprint, which should lead to significant benefit in terms of packing density and fabrication cost. However, performances of STOs, such as the output power and quality factor (Q factor) under free running oscillation, have not been good enough for most practical applications yet. Regarding phase noise, a technique called injection locking has successfully stabilized oscillation frequency (f STO ), in which a microwave power from a reference source with identical or integer multiple of the oscillation frequency (f Ref = n f STO , where n is an integer) is directly injected into the STO. As a result, an extremely narrow line width, i.e. low phase noise, has been achieved [1]-[4]. Unfortunately, this technique is not a commercially viable option because of the necessity to have another HF reference source. In contrast, phase locked loop (PLL) is widely used in real electronics systems to stabilize the frequency. In PLL, a HF signal is phase locked to a low frequency reference clock with low phase noise generated by a crystal oscillator, which is typically in 10–100 MHz frequency range (f STO >>f Ref ). However, it has been difficult to implement an STO into a PLL circuit because of the large phase noise of STOs, especially when f Ref is much lower than f STO [5].