Direct measurement of noise spectra at the 1 nV/√Hz level

In order to confirm that a ultra-low noise power supply was working as designed, we needed to measure its noise spectrum at 15 V at the 1 nV/√Hz level to frequencies below 10 Hz. We accomplished this using the “cross-spectrum” technique. Since this method is not widely known, we describe it briefly, present the preamplifier we developed to make the required measurements, and show spectra from our power supply, whose noise levels approach those of battery supplies.