  A method to perform spectrum analysis on low power signals between 0.1 and 10
THz is proposed. It utilizes a nanoscale antiferromagnetic tunnel junction
(ATJ) that produces an oscillating tunneling anisotropic magnetoresistance,
whose frequency is dependent on the magnitude of an evanescent spin current. It
is first shown that the ATJ oscillation frequency can be tuned linearly with
time. Then, it is shown that the ATJ output is highly dependent on matching
conditions that are highly dependent on the dimensions of the dielectric
tunneling barrier. Spectrum analysis can be performed by using an appropriately
designed ATJ, whose frequency is driven to increase linearly with time, a low
pass filter, and a matched filter. This method of THz spectrum analysis, if
realized in experiment, will allow miniaturized electronics to rapidly analyze
low power signals with a simple algorithm. It is also found by simulation and
analytical theory that for an ATJ with a 0.09 $\mu$m$^2$ footprint, spectrum
analysis can be performed over a 0.25 THz bandwidth in just 25 ns on signals
that are at the Johnson-Nyquist thermal noise floor.
