  We demonstrate a new technique for detecting components of arbitrarily-shaped
radio-frequency waveforms based on stroboscopic back-action evading
measurements. We combine quantum non-demolition measurements and stroboscopic
probing to detect waveform components with magnetic sensitivity beyond the
standard quantum limit. Using an ensemble of $1.5\times 10^6$ cold rubidium
atoms, we demonstrate entanglement-enhanced sensing of sinusoidal and linearly
chirped waveforms, with 1.0(2)dB and 0.8(3)dB metrologically relevant noise
reduction, respectively. We achieve volume-adjusted sensitivity of
$\delta\rm{B}\sqrt{V}\approx 11.20~\rm{fT\sqrt{cm^3/Hz}}$, comparable to the
best RF~magnetometers.
