Low Power Microwave Signal Detection With a Spin-Torque Nano-Oscillator in the Active Self-Oscillating Regime
A spin-torque nano-oscillator (STNO) driven by a ramped bias current can perform spectrum analysis quickly over a wide frequency bandwidth. The STNO spectrum analyzer operates by injection locking to external microwave signals and produces an output dc voltage <inline-formula> <tex-math notation="LaTeX">$V_{\mathrm{ dc}}$ </tex-math></inline-formula> that temporally encodes the input spectrum. We found, via numerical analysis with a macrospin approximation, that an STNO is able to scan a 10 GHz bandwidth in less than 100 ns (scanning rate <inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula> exceeds 100 MHz/ns). In contrast to conventional quadratic microwave detectors, the output voltage of the STNO analyzer is proportional to the amplitude of the input microwave signal <inline-formula> <tex-math notation="LaTeX">$I_{\mathrm{ rf}}$ </tex-math></inline-formula> with sensitivity <inline-formula> <tex-math notation="LaTeX">$S = dV_{\mathrm{ dc}}/dI_{\mathrm{ rf}} \approx 750~\rm mV/mA$ </tex-math></inline-formula>. The minimum detectable signal of the analyzer depends on the scanning rate <inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula> and, at low <inline-formula> <tex-math notation="LaTeX">$R \approx 1~\rm MHz/ns$ </tex-math></inline-formula>, is about 1 pW.