This paper describes an instrument capable of giving a picture of the frequency-response curves of amplifiers whose pass bands are enclosed in the bands 5–35 and 38–82 Mc/s, the accuracy of such pictures being ± 0.3 db on the low range, and ± 0.2 db on the high range. The frequency is given to ± 200 kc/s. Any band-width between about 1 Mc/s and the full extent of either band can be satisfactorily displayed.Various methods of obtaining a suitably-modulated measuring signal are discussed, together with the design problems encountered in making the instrument described.