Ultra-fast Spectrum Analysis at GHz frequencies using Spin-Torque Nano-Oscillators

Abstract

In modern radar and communication systems it is important to determine the frequency composition of complex external signals on the µs time scale and faster [1]. Common swept-tuned spectrum analyzers (SA) are limited to sweep times of 10-100 ms determined by the characteristic times of macroscopic VCOs and YIG-tuned reference oscillators. In contrast, recent progress in spintronics has led to the development of spin-torque nano-oscillators (STNO) generating in the GHz frequency range, and naturally having time constants of the order of several nanoseconds determined by the intrinsic properties of magnetization dynamics at nano-scale [2]. In [3] it was proposed theoretically and in [4] demonstrated experimentally for a vortex-state configuration that STNOs can be used as a central element of an ultra-fast spectrum analyzer. A fastest sweep rate of 0.67us was achieved for spectral analysis in the range of 25MHz around center frequency of 300MHz. Here we demonstrate that an ultra-fast sweep-tuned spectrum analysis can be performed at a central frequency of 3 GHz using uniform-state STNOs. The obtained SA performances sufficiently exceed the results obtained with a vortex-state STNO with a reduced sweep time of T =0.1µs, sweep range of 400 MHz (limited by the STNO power and not by tuning range capabilities) keeping the RBW (resolution bandwidth) close to the theoretical limit (14 MHz). This GHz-frequency technique of spectrum analysis was implemented as shown in Fig.1. The STNO was used as a local oscillator whose frequency was sweep-tuned by injecting a saw-tooth sweep signal. The STNO output (a) is, then, mixed with the input signal (b), digitized (c), and processed with a matched filter, resulting in a narrow output peak (d) whose temporal position corresponds to the input frequency while the width determines the RBW. Financial support is acknowledged from the EC program ERC MAGICAL 669204, from the NSF of the USA (Grants # EFMA-1641989 and “ECCS-1708982), and by the US AFOSR (MURI grant # FA9550-19-1-0307). The EC program GREAT is acknowledged for provision of perpendicular MTJ devices operating at GHz frequencies.