The most accurate piston attenuators work at frequencies below 100 MHz and standard piston attenuators are usually operated at a fixed frequency such as 30 MHz or 60 MHz. In designing a comparator in which attenuators operating at any frequency up to, say, 3 GHz can be compared with a standard over a wide range of attenuation up to 100 dB, a number of problems had to be solved. Chief among these was the need to keep the noise sufficiently below the signal level at — 107 dBm and the need to design frequency changers which are linearto much better than 0 01 dB over the range of levels — 7 dBm to — 107 dBm. The paper describes how these problems were solved in the design of a comparator which would function with a c.w. source of any frequency between 0.18 GHz and 1 GHz with an uncertainty of comparison well within ±001 dB.