An insertion loss and phase measuring set has been developed for making small‐signal measurements on transistors and general two‐port networks with maximum inaccuracy of 0.1 db and 0.5 degree over a frequency range from 5 to 250 mc. In order to realize accuracy substantially independent of test frequency, the measurement information is heterodyned to a fixed intermediate frequency, where detection is performed with the aid of adjustable loss and phase‐shift standards. Use of a rapid sampling technique to compare the unknown with a high‐frequency standard eliminates errors from circuit drifts and also reduces the magnitude of the “instrument‐zero line” to a small value. Besides discussing the over‐all operation of the new test set, the paper presents the design approaches used in solving problems related to purity of terminations as seen from the unknown, automatic control of beat oscillator frequency, conversion, signal‐to‐noise, and design of loss and phase standards. Particular attention is given to the features of the set which especially adapt it to the measurement of transistor parameters.