Low power temperature-controlled frequency-stabilized oscillator

1. A temperature-controlled crystal resonator oscillator, comprising:

a resonator having a resonance frequency which varies as a function of the temperature of the resonator;

a heating element in substantially direct thermal contact with the resonator, for applying heat to the resonator;

a resonator enclosure containing in its interior the resonator and the heating element, the interior of the resonator enclosure being substantially a vacuum;

a heat-sensing device in substantially direct thermal contact with the resonator enclosure, for sensing the temperature on the resonator enclosure;

an insulative layer at least partially surrounding the resonator enclosure, so as to increase the thermal resistance between the resonator enclosure and an ambient environment; and

a feedback control circuit receiving the sensed temperature of the resonator enclosure and generating a heater control signal for determining the manner in which heat is applied to the resonator by the heating element;

wherein the control circuit provides dynamic control of the resonance frequency of the resonator through feedback of the sensed temperature on the resonator enclosure in a substantially closed-loop manner.

2. The oscillator of claim 1, wherein the feedback control circuit comprises a switching mode integrated circuit.

3. A temperature-controlled crystal resonator oscillator, comprising:

(a) a power source;

(b) a resonator having a resonance frequency which varies as a function of the temperature of the resonator;

(c) a heating element in substantially direct thermal contact with the resonator, for applying heat to the resonator;

(d) a resonator enclosure containing in its interior the resonator and the heating element, the interior of the resonator enclosure being substantially a vacuum;

(e) a heat-sensing device in substantially direct thermal contact with the resonator enclosure, for sensing the temperature on the resonator enclosure;

(f) an insulative layer at least partially surrounding the resonator enclosure, so as to increase the thermal resistance between the resonator enclosure and an ambient environment;

(g) a feedback control circuit receiving the sensed temperature of the resonator enclosure and generating a control signal for determining the manner in which heat is applied to the resonator by the heating element, wherein the feedback control circuit comprises:

(1) a current sensing resistor for sensing the average current through the heating element, for generating a reference signal;

(2) a balanced bridge circuit responsive to variations in the temperature of the resonator enclosure as sensed by the heat-sensing device and to the reference;

(3) an amplifier, responsive to a signal from the balanced bridge circuit, for generating the control signal in response to the sensed temperature of the resonator enclosure;

(4) a pulse duration modulator, responsive to the control signal from the amplifier and to a synchronization signal, for generating a pulse train having a variable duty cycle; and

(5) a switching element, responsive to the pulse train from the pulse duration modulator for drawing current from the power source and applying it to the heating element in thermal contact with the resonator; and

(h) a filter network for smoothing out current pulses drawn from the power source;

wherein the control circuit provides dynamic control of the resonance frequency of the resonator through feedback of the sensed temperature on the resonator enclosure in a substantially closed-loop manner.

4. The oscillator of claim 3, wherein the feedback control circuit comprises a switching mode integrated circuit.

5. A temperature-controlled resonator oscillator, comprising:

a circuit board;

a heat-sensing device disposed at a predetermined site on the circuit board; and

a resonator oscillator package comprising a resonator inside an evacuated enclosure, the resonator oscillator package attached to the circuit board atop the site at which the heat-sensing device is situated, the oscillator package attached so that the resonator enclosure is in substantially direct thermal contact with the heat-sensing device without the heat-sensing device being adhesively affixed to sides or top of the oscillator assembly;

wherein the temperature of the resonator enclosure sensed by the heat-sensing device is fed back through a control circuit so as to control the temperature of the resonator.

6. A temperature-controlled oscillator for operation at actual ambient temperatures within an expected ambient temperature range, the oscillator comprising:

a resonator for producing a signal having a desired frequency, the resonator having a temperature coefficient curve with a range in which the slope of the curve is substantially zero, the range substantially defined by a lower limit temperature T.sub.L and an upper limit temperature T.sub.U, the lower limit temperature T.sub.L disposed substantially within the expected ambient temperature range and the upper limit temperature T.sub.U disposed approximately at or above an upper limit of the expected ambient temperature range, the resonator having an operational temperature which may vary as function of time;

a heating element for providing thermal energy to the resonator;

a thermal sensor, for sensing a temperature substantially related to the operational temperature of the resonator; and

a control circuit for receiving the sensed temperature from the thermal sensor and for governing an amount of thermal energy which the heating element provides to the resonator, the control circuit causing the heating element to provide heat to the resonator only when the operational temperature of the resonator is a given amount below the lower limit temperature T.sub.L ;

wherein the control circuit does not cause the heating element to supply thermal energy to the resonator when the ambient temperature is above approximately the lower limit temperature T.sub.L, so that energy is not expended in heating the resonator when the actual ambient temperature is above a certain temperature in the expected ambient temperature range; and

wherein the desired frequency is maintained substantially constant throughout the entire expected ambient temperature range.

7. The oscillator of claim 6, wherein the resonator comprises an FC-cut resonator.

8. An oscillator, comprising:

an FC-cut crystal resonator having a resonant frequency which may vary as a function of the temperature of the resonator; and

a heating element, disposed in direct thermal contact with the resonator, for supplying thermal energy to the resonator so as to control its resonant frequency.