A visual selectivity meter with a uniform decibel scale

Visual examination of the selectivity or frequency response of a filter by means of a frequency-modulated oscillator and cathode-ray tube is well known, but few attempts have been made to produce a selectivity curve to a decibel scale. A very successful method, which depends on A.G.C.J action (virtually negative feedback), has been developed to give logarithmic delineation of frequency response, and the apparatus is described with particular reference to decibel scale production. The carrier or mid-frequency range of the instrument is capable of variation from 50 to 1 500 kc./s., and four frequency-modulation ranges, giving band widths of 100, 50, 20 and 10 kc./s. are possible. The decibel frequency response scale is ¿linear¿ over a range of 60 db. Check tests at three different carrier frequencies, 1 500, 500 and 100 kc./s., are described and show the performance of the apparatus to be satisfactory and its degree of accuracy to be good.