CCII/CFA active-RC oscillators based on cascaded first-order allpass building blocks

In this paper, the general theory of oscillators based on two cascaded first-order allpass networks is developed. As design pointers to the relative merits of single- and dual-element frequency control, frequency stability and harmonic rejection factors are evaluated as functions of the ratio of the time constants in the two allpass sections. Two new oscillators are then described. One of these uses three CCII+ and its frequency may be adjusted by adjusting the values of one or two capacitors. The second uses one CCII+ and one CFA but requires tracking dual-element frequency control by adjusting two equal resistors or capacitors. These oscillators incorporate the Analog Devices AD 844 amplifier, which may be operated either as a CFA or CCII+ with a unity-gain output buffer. Experimental results are given and the relative merits ofthe oscillators are discussed.