The present invention relates to crystal oscillators with frequency regulation as a function of the temperature. These oscillators essentially comprise an amplifier, whose output is connected to the input across a circuit incorporating an oscillating piezoelectric crystal, the gain of the system with the thus formed feedback loop being higher than unity in order to satisfy auto-oscillation conditions. The frequency regulation as a function of temperature is obtained by connecting in series with the crystal a capacitor of appropriate value, whose capacitance varies as a function of a voltage applied to its foils and by creating said voltage, normally called the "compensating signal" in a compensating circuit incorporating a heat-sensitive element.
However, the known compensating circuits do not make it possible to industrially obtain a complete frequency regulation in the case where the oscillating crystal has been cut with a particular section, which is advantageous in certain respects and is known under the name AT. Thus, for this section, the thermal drift of the frequency of the crystal affects the shape of a third degree algebraic curve.
In the latter case, it is consequently necessary to create a compensating signal which also varies in accordance with a function of the third degree. This leads to known compensating circuits which are difficult and expensive to produce as a result of the high precision and stability required by the compensating signal to be obtained and the special characteristics required of certain components, such as heat-sensitive elements or thermistors.
In the French Patent of the present Applicant Company filed under No. 74/30338 and published under No. 2,284,219 and equivalent to U.S. Pat. No. 4,020,426 it is proposed to subdivide the curve corresponding to the third degree variation law of the compensating signal into three arcs and to ensure the formation thereof by three separate circuits each having a heat-sensitive element.
For this purpose, in the said crystal oscillator constituted by an oscillating circuit in the form of a feedback loop and incorporating an amplifier, an oscillating piezoelectric crystal and an element having a reactance which is variable as a function of electrical signals or quantities, the electrical compensating signal or quantity is created by a circuit having two terminals, one kept operating at a fixed potential by a dividing bridge and the other raised to a potential variable as a function of the temperature. First and second heat-sensitive elements are connected then by one of their ends to said other terminal, their other end being respectively connected to two dividing bridges, whereof one comprises a third of said elements, the three points being connected to the terminals of the said power supply.
Thus, at around 30 MHz, this prior art oscillator supplies a frequency stability as a function of the temperature of approximately 10.sup.-6 in the temperature range of -40.degree. to +80.degree. C.
However, for certain uses such as avionics, the temperature ranges to be covered are even wider and are typically between -55.degree. and +105.degree. C. The temperature compensating circuit of the aforementioned prior art does not satisfy such a requirement. The correction voltage curve supplied by said circuit as a function of the temperature has two inflection points with a direction change of the curvature, respectively on either side of the limit temperature (-40.degree. to +80.degree. C.).
A partial solution can be envisaged, such as an increase in the supply voltage of the circuit but has the disadvantage of increasing the potential at the terminals of the capacitor which is variable as a function of the voltage, i.e. the sensitivity thereof is reduced. Moreover, beyond 70.degree. C., the thermistors have a too limited resistance variation as a function of the temperature.