Miniaturized RC Filters Using Phase Locked Loop

It is shown that an automatic phase‐locked loop (APLL) can be used as a bandpass filter and FM discriminator while satisfying the circuit conditions imposed by microminiaturization techniques. It has the advantage over other methods of ease of adjustment and reduction in the number of circuit components. For this reason it is to be assumed that the APLL will be added to the few practical solutions of the frequency selection problem in microminiaturization available to date. In contrast to most other applications, no assumptions can be made here about the ratio of the natural undamped system frequency to the lock range of the loop. This is taken account of in an analysis of the APLL which enables design equations and considerations to be derived. An approximation of the so‐called capture ratio, defined as the ratio of capture‐to‐lock range, is presented. It produces results that correspond very well with the equivalent measurements. The description of an APLL designed as RC channel filter and discriminator for a multichannel data receiver is presented. RC active circuits that are compatible with current microminiaturization techniques are employed exclusively. They are described in detail, and an analysis of the voltage‐controlled oscillator used is included in the Appendix.