Miniaturized RC Filters Using Phase-Locked Loop

One of the main problems in the microminiaturization of frequencyselective circuits is the considerable deterioration of Q of magnetic components when these are reduced in size to an extent comparable with other miniaturized components. 1 2 , 3 For the design of highly selective bandpass filters and frequency discriminators, completely new circuit design concepts t h a t eliminate magnetic components and overcome other limitations imposed by current microminiaturization techniques must therefore be employed. At low frequencies they invariably 823 824 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1965 lead to the synthesis of narrow-band active RC filter circuits that are compatible with the state of the microminiaturization art. To this end a number of new circuit techniques have been described in the literature, but practical solutions to the problem are still few and not wholly satisfying.4 In this paper a new approach in this direction has been taken in the field of FM multiplex signal filtering by utilizing both the filtering properties of the APLL and the possibility of designing its circuits along the lines prescribed by present-day miniaturization technology. The resulting network, characterized by its simplicity, ease of adjustment and stability, shows that the APLL is a very useful filtering device in the field of circuit miniaturization. At the same time, if so desired, it also serves as a frequency discriminator. For the purpose of introducing the APLL into the area of circuit miniaturization and to facilitate its implementation the characteristics and performance of the APLL covered in present literature are briefly reviewed.