Equivalent variable centre-frequency amplifiers

This paper attempts to demonstrate the equivalence of two variable centre-frequency amplifiers. The first consists of an i.f. amplifier preceded by a mixer and a voltage-controlled oscillator; the second consists of a tuned amplifier whose centre frequency is altered by the use of voltage-controlled capacitors across the resonant circuits. The first is the type normally employed in frequency feedback detectors, whilst the second is the basis of `dynamic selector¿ or `dynamic filter¿ systems. A method of analysing the response of a dynamic filter to a frequency-modulated signal is developed using this equivalence. The application of the dynamic filter to f.m. detection is considered and it is shown that one possible circuit arrangement is equivalent to a feedback f.m. detector.