Departure of the source impedance of a noise generator from the nominal value gives rise to an uncertainty in the measurement of noise factor. A critical examination of the problem shows that, to obtain an uncertainty on this account of less than 2%, a source v.s.w.r.of about 0.98 is required. Because existing noise generators seldom meet this requirement and because there is a need for more consistent measurements of noise than those obtained hitherto at frequencies up to about 1 200 Mc/s, a new noise generator has been developed.A description of the design, construction, calibration and use of this noise generator is given. It employs a precision source resistor of 50Ω and a pair of specially developed thermionic diodes (E2790), using a form of diode mount designed to eliminate the effects of diode capacitance so that a v.s.w.r.better than 0.98 is obtained at all frequencies in the range. It is an absolute noise generator with an accuracy in noise level of ±2% at frequencies up to about 300 Mc/s, but requires calibration at a single level for frequencies up to 1 250 Mc/s. At present, this has been achieved with an estimated uncertainty of about ±5% in noise level.