  We demonstrate an atomic radio-frequency (RF) receiver and spectrum analyzer
based on thermal Rydberg atoms coupled to a planar microwave waveguide. We use
an off-resonant RF heterodyne technique to achieve continuous operation for
carrier frequencies ranging from DC to 20 GHz. The system achieves an intrinsic
sensitivity of up to -120(2) dBm/Hz, DC coupling, 4 MHz instantaneous
bandwidth, and over 80 dB of linear dynamic range. By connecting through a
low-noise preamplifier, we demonstrate high-performance spectrum analysis with
peak sensitivity of better than -145 dBm/Hz. Attaching a standard rabbit-ears
antenna, the spectrum analyzer detects weak ambient signals including FM radio,
AM radio, Wi-Fi, and Bluetooth. We also demonstrate waveguide-readout of the
thermal Rydberg ensemble by non-destructively probing waveguide-atom
interactions. The system opens the door for small, room-temperature,
ensemble-based Rydberg sensors that surpass the sensitivity, bandwidth, and
precision limitations of standard RF sensors, receivers, and analyzers.
