The design and construction of a system suitable for measuring a.m. and f.m. noise spectra from low-power, low-noise oscillators and nonlinear amplifiers is described. A radiometer technique is used to provide enhanced sensitivity, and the design factors and sources of error are considered in some detail. The system has a sensitivity equivalent to a noise temperature of 10°K in a bandwidth of 150kHz over much of its range, and is capable of measuring noise powers with a relative accuracy of 6–10% and an absolute accuracy of 15% over a sideband range of ±1 to 140MHz. The a.m.-plus-background and f.m.-plus-background components can be separated with an accuracy of some 5–10%.