Integrated circuit for a controllable frequency oscillator

Claim 1

A single chip MOS integrated circuit for a controllable frequency oscillator comprising: a semiconductor substrate of one type of conductivity; two first terminals for connection to a resonator; two second terminals for connection to an electrical power supply source; and a circuit formed on said semiconductor substrate and connected to said first and second terminals for establishing and sustaining the oscillations of a resonator connected to said two first terminals, said circuit comprising at least one active element connected to said first and second terminals, at least one control terminal, and means connected to said active element and said control terminal for adjusting the frequency of the oscillation of the resonator in dependence on a control signal applied to said control terminal, said adjusting means comprising: at least one adjustable capacitor having a capacitance which varies continuously in dependence on a voltage applied thereto, said capacitor being connected to said active element for determining the frequency of the oscillations of the resonator in dependence on said capacitance, said adjustable capacitor comprising a first and a second semiconductor region of opposite conductivity types arranged relative to each other for forming a junction therebetween, said second semiconductor region being a diffusion portion formed in a superficial zone of said chip; and means responsive to said control signal for applying to said adjustable capacitor a voltage, the value of which is determined in dependence on said control signal; and further comprising: a decoupling capacitor connected between said adjustable capacitor and said active element, said decoupling capacitor comprising an insulating layer covering one of said regions and an electrically conducting layer for covering said insulating layer, said chip having an oxide layer, said insulating layer being a portion of said oxide layer.