A magnetic feedback modulator for improved FM carrier recording

Magnetic multivibrators are often used as voltage-controlled oscillators in FM carrier recording systems. These oscillators generally consist of two switching transistors connected to a center tapped coil on a square loop magnetic core. The transistors operate alternately in saturation, driving the core between its positive and negative saturation limits. Frequency then depends upon saturation flux and applied voltage. In the Magnetic Feedback Modulator, stability and linearity improvement of an order of magnitude is possible with a new method of applying negative feedback to a square-loop core magnetic multivibrator. In addition, circuit complexity is reduced considerably over previous methods using conventional frequency detectors to derive feedback voltage. Based on Faraday's Law, a voltage which is a function of frequency and flux change is derived from the magnetic circuit. This voltage is then used as negative feedback to the dc differential amplifier to drive the magnetic multivibrator. Since the magnetic circuit is included in the negative feedback loop, frequency errors occurring in the modulator cause corresponding changes in the magnetic feedback voltage. The errors are then reduced in magnitude by the negative feedback.