Single insertion trimming of highly accurate reference oscillators

Abstract

A highly integrated monolithic self-compensated oscillator (SCO) with high frequency stability versus temperature variations is described, together with a cost effective single insertion point trimming (SPT) algorithm. The SPT is utilized to adjust the phase and frequency of the SCO to meet frequency stability versus temperature and frequency accuracy requirements for a reference clock. The techniques used in the SPT algorithm provide a robust, fast and low testing cost for the SCO. Moreover, the concepts and techniques utilized in the SCO SPT can be used effectively for any temperature compensated oscillator (TCO) including TCXO, MEMS, FBAR and RC oscillators. Additionally, the described SPT algorithm is capable of measuring the temperature sensitivity of any oscillator, estimating suitable temperature compensation parameters and adjusting the oscillator frequency to the required value simultaneously.

What is claimed is: 
     
         1 . An apparatus comprising:
 an oscillator comprising a frequency determining element;   circuitry responsive to a digital frequency setting value to cause a change in an output frequency of the oscillator;   circuitry responsive to a digital phase setting value to cause a specified phase across the frequency determining element;   circuitry to determine frequency differences at different instants of time between the output frequency of the oscillator and a reference frequency;   control logic;   a first control loop controlled by the control logic to, under a temperature condition, apply temperature modulation to the oscillator, determine a temperature sensitivity of the output frequency of the oscillator, and search for and identifying a final digital phase setting value that results in a temperature sensitivity that matches a predetermined criterion; and   a second control loop controlled by the control logic to, under the temperature condition, determine a final digital frequency setting value that minimizes the frequency differences;   wherein the final digital phase setting value and the final digital frequency setting value simultaneously satisfy a frequency accuracy requirement and a requirement of frequency stability over temperature.   
     
     
         2 . A method of calibrating an oscillator circuit fabricated on an integrated circuit and responsive to a frequency setting signal to cause a change in an output frequency of the oscillator circuit, comprising:
 applying temperature modulation to the oscillator;   measuring results of the temperature modulation;   processing the measured results to estimate at least first and second temperature derivatives of the output frequency;   using the estimates of the temperature derivatives of the output frequency to determine a plurality of coefficients of a temperature compensation function with an objective of causing temperature sensitivity of the output frequency to match a predetermined criterion; and   storing the plurality of coefficients;   wherein the oscillator circuit is configured to, during operation, use the plurality of coefficients and a temperature measurement to produce a compensated frequency setting signal that compensates the output frequency in accordance with the temperature measurement.   
     
     
         3 . The method of  claim 2 , wherein:
 applying temperature modulation comprises modulating a temperature of the oscillator in accordance with a triangle or sawtooth waveform;   measuring results of the temperature modulation comprises measuring the output frequency using a frequency-to-digital converter to obtain digital frequency measurements; and   processing the measured results comprises estimating the at least first and second temperature derivatives using the digital frequency measurements.   
     
     
         4 . An oscillator circuit comprising an oscillator and calibration circuitry and responsive to a frequency setting signal to cause a change in an output frequency of the oscillator circuit, the calibration circuitry comprising:
 temperature modulation circuitry comprising heaters and a feedback control loop for applying temperature modulation to the oscillator;   measurement circuitry to measure results of the temperature modulation;   calculation circuitry to use results of the temperature modulation to determine a plurality of coefficients of a temperature compensation function;   storage to store the plurality of coefficients;   temperature measurement circuitry; and   compensation circuitry to, during operation, use the plurality of coefficients and a temperature measurement to produce as a compensated frequency setting signal that compensates the output frequency in accordance with the temperature measurement.   
     
     
         5 . The apparatus of  claim 4 , wherein said results comprise at least a first temperature derivative of the output frequency. 
     
     
         6 . The apparatus of  claim 4 , wherein said results comprise at least first and second temperature derivatives of the output frequency. 
     
     
         7 . The apparatus of  claim 4 , wherein the oscillator circuit is one of the following types: LC oscillator, RC oscillator, MEMS oscillator and FBAR oscillator. 
     
     
         8 . A method of validating temperature performance of an oscillator circuit and responsive to a frequency setting signal to cause a change in an output frequency of the oscillator circuit, comprising:
 applying temperature modulation to the oscillator;   measuring results of the temperature modulation;   processing the measured results to estimate at least first and second temperature derivatives of the output frequency; and   using the estimates of the temperature derivatives of the output frequency to determine whether or not temperature sensitivity of the output frequency matches a predetermined criterion.