A device comprising: a substrate; a semiconductor die mounted on the substrate; a transmit antenna fabricated on the substrate and configured to transmit radio-frequency (RF) signals at least at a first center frequency; a receive antenna fabricated on the substrate and configured to receive RF signals at least at a second center frequency different than the first center frequency; and circuitry integrated with the semiconductor die and configured to provide RF signals to the transmit antenna and to receive RF signals from the receive antenna.