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

In this work, we have developed a PLL circuit implementing a high performance spin torque oscillator (STO) into it . The STO consists of a magnetic tunnel junction (MTJ) having an in-plane magnetized reference layer and perpendicularly magnetized free layer [6][7] . This STO shows a very high power and Q factor simultaneously, thanks to a large magnetoresistance (MR) ratio of the MTJ stack and absence of edge effects in the perpendicular free layer magnetization, respectively . Fig . 1 shows a block diagram of the PLL built in this work . A STO showing a Q factor of about 800 under free running oscillation and red shift for a small change of bias voltage is nominally biased through a pick-off tee to generate a 5.12 GHz microwave signal . This signal is amplified by a broadband low noise amplifier (LNA), and its frequency is down converted to 80 MHz by a 1/64 down counter . This signal and 80 MHz reference clock are fed to a phase frequency detector (PFD), which outputs a voltage signal proportional to the phase difference between the two signals (phase error signal, PES) . The PES is low pass filtered and fed back to the bias control circuit to fine tune the STO frequency . That way, the phase of the STO output should be locked to that of the 80 MHz reference clock .