Apparatus and method for a temperature-controlled frequency source using a programmable IC

A temperature-controlled frequency source, comprising:

• a substantially isothermal substrate;
• an oscillator including a piezoelectric crystal having a frequency-temperature response curve, the crystal being thermally coupled to the substrate;
• a thermal element including a resistive heating element and a current-pass transistor being thermally coupled to the substrate, the thermal element for driving the substrate substantially to a turning point temperature of the frequency-temperature response curve of the crystal defined by at least one point on the curve where a slope is zero;
• a bridge circuit having a first and second leg, the bridge circuit being electrically coupled to a bridge amplifier which provides a signal for driving the thermal element;
• a thermistor being thermally coupled to the substrate and the crystal, the thermistor being electrically coupled in the first leg of the bridge circuit;
• a programmable integrated circuit (IC) including at least one memory and first and second D/A converters, the at least one memory applies a first digital signal to the first converter such that the first converter provides an analog control signal for controlling a temperature of the substrate to the second leg of the bridge circuit, the at least one memory applies a second digital signal to the second converter such that the second converter provides an analog frequency adjustment signal to the crystal oscillator;
• and
• a package encompassing the element, substrate, crystal oscillator, thermistor, bridge circuit and IC within a substantially sealed environment, the package includes a sealed electrical interface through which programming signals address the at least one memory of the IC.

A method for a temperature-controlled frequency source, comprising the steps of:

• providing a substantially isothermal substrate whereupon a crystal oscillator, a thermistor, a thermal element, a bridge circuit with first and second legs and a programmable IC with a memory are thermally and electrically coupled, and an encompassing package substantially sealing the source;
• testing the source at different substrate temperatures so as to find a turning point on a crystal frequency-temperature response;
• programming the IC with a control signal value corresponding to the turning point;
• converting the control signal value to a control signal and applying the control signal to a second leg of the bridge circuit and a signal from the thermistor to the first leg of the bridge circuit such that a thermal feedback through the substrate maintains a substantially constant temperature at the crystal turning point resulting in a substantially stable output frequency from the frequency source;
• programming the IC with a frequency adjustment signal value corresponding to a desired output frequency;
• and
• converting the frequency adjustment signal value to a frequency adjustment signal and applying the frequency adjustment signal to the oscillator such that the desired output frequency is obtained.