Method and apparatus for a crystal oscillator using piecewise linear odd symmetry temperature compensation

A method for temperature compensating a crystal oscillator across a predetermined temperature range, comprising the steps of:

• providing a substantially linear temperature sensitive diode, a source driver, a summation circuit, a crystal oscillator including a varactor tuning circuit and a plurality of operational amplifiers having inputs and outputs and including adjustable gain and clipping, at least one of the amplifiers having an inverse gain relationship to the remaining amplifiers;
• driving the diode with the source driver such that a substantially linear temperature-dependent signal is provided;
• coupling the signal to the inputs of the plurality of operational amplifiers;
• adjusting the gain and clipping of each amplifier such that the output of each amplifier is clipped at upper and lower temperatures that are substantially equally spaced above and below the inflection temperature, respectively, the outputs providing a piecewise linear signal having odd symmetry about an inflection temperature of a Bechmann curve of the crystal oscillator;
• summing the outputs over the temperature range in the summation circuit such that a compensation signal proportional to the Bechmann curve is provided;
• and
• applying the compensation signal to the tuning circuit such that the crystal oscillator is substantially temperature compensated.

A temperature compensated crystal oscillator having a predetermined temperature range, comprising:

• a substantially linear temperature sensitive sensor, a source driver, a summation circuit, a crystal oscillator including a tuning circuit and a plurality of operational amplifiers each having an input, an output and an adjustable gain and clipping;
• the source driver is connected to the sensor such that a substantially linear temperature-dependent signal is provided from the sensor;
• the sensor is connected to the inputs of the plurality of operational amplifiers, the signal being coupled to the inputs of the amplifiers;
• the gain and clipping of each amplifier being adjusted such that the output of each amplifier provides a piecewise linear signal extending over the temperature range and having odd symmetry about an inflection temperature of a Bechmann curve of the crystal oscillator;
• the outputs of the amplifiers being coupled to the summation circuit to provide a compensation signal over the temperature range, the compensation signal being proportional to the Bechmann curve;
• and
• the compensation signal from the summation circuit being coupled to the tuning circuit such that the crystal oscillator is temperature compensated.