US 5,892,408 A · Method and system for calibrating a crystal oscillator

Abstract

A method and system are provided for calibrating a batch of devices each containing a circuit which is responsive to a control signal for producing a desired output which varies in accordance with a first predetermined function of a specific ambient condition, the control signal having a magnitude which varies as a second predetermined function of the specific ambient condition, the second function being based on data stored as a look-up table in a memory of the device and which must be individually calibrated for each device. In a preferred embodiment, the device is a digital temperature controlled crystal oscillator which produces a desired output frequency and includes a voltage controlled oscillator (VCO) responsive to a control signal having a magnitude which varies as a predetermined function of ambient temperature in order to compensate for temperature variations in the oscillator output frequency. For such an application, the invention requires the connection of an accurate frequency source to each oscillator in the batch so as to enable the output frequency of the oscillator to be equalized thereto or to a multiple thereof. In calibration mode, the digital equivalent of the resulting analog control voltage is stored; whilst in compensation mode it is extracted from the memory, converted to an equivalent analog voltage and applied to the VCO. The invention is also applicable to compensate for aging of crystal oscillators in the field without requiring reconfiguring the complete look-up table.

Claims66

Dependent claims

A method for calibrating a batch of at least one device containing a circuit which is responsive to a control signal for producing a desired output which varies in accordance with a first predetermined function of a specific ambient condition, the control signal having a magnitude which varies as a second predetermined function of said specific ambient condition, said second function being based on data stored in a memory of the device and which must be individually calibrated for each device, said method comprising the steps of:

(a) subjecting all of the devices in the batch to a controlled environment wherein said ambient condition may be varied,
(b) connecting to the devices an accurate signal source whose output varies in accordance with said second predetermined function of the ambient condition,
(c) selecting a "calibration mode" of operation wherein said function is determined for a measured ambient condition and the data corresponding thereto is stored in the memory and wherein there are performed the following steps for each device in the batch:
(i) determining the value of the required control signal for producing an output signal of the desired output magnitude,
(ii) storing the respective value of the control signal or a function thereof in the memory of each device, and
(iii) repeating steps (i) and (ii) in respect of different ambient conditions until each value is stored in the memory;
and
(d) selecting a "compensation mode" of operation wherein data is extracted from the memory and used in conjunction with a measured ambient condition for adjusting the control signal.

5 more elements

A device containing a generator which is responsive to an analog control signal for producing a desired output signal, the control signal having a magnitude varies as a predetermined function of a specified ambient condition, said function being based on data stored in a memory of the device and which must be individually calibrated for each device, said device comprising:

(a) a selector means for selecting a "generation mode" or "calibration mode" of operation;
(b) a generating circuit coupled to the selector means for generating during said "generation mode" of operation the desired output signal as a function of the ambient condition, and
(c) a calibration specific circuit coupled to the selector means for determining and storing said function during said "calibration mode" of operation;
the generator circuit comprising:
(d) sensor means for measuring a magnitude of the ambient condition and producing a respective analog sensor signal,
(e) an A/D converter coupled to the sensor for converting the analog sensor signal to a corresponding digital sensor signal,
(f) a memory having at least one memory location for storing therein a respective digital control value,
(g) a signal generator having an output whose magnitude can be controlled by an analog control signal,
(h) digital processing means responsively coupled to the A/D converter and to the memory and responsive to digital values derived therefrom for producing a digital control signal representative of the analog control signal, and
(i) a D/A converter coupled to the digital processing means for converting the digital control signal to the analog control signal;
the calibration specific circuit comprising:
(j) reference signal coupling means for coupling thereto an external reference signal of the same type as the output signal,
(k) control means coupled to the reference signal coupling means and to the signal generator for determining the digital value or function thereof which, when processed by the digital processing means in the generator circuit, produces the correct digital control signal for the measured ambient condition, and
(l) data entry means coupled to the control means and to the memory for entering said digital value or said function thereof into the memory at said at least one memory location thereof.

11 more elements

A method for calibrating a batch of at least one temperature controlled oscillator (TCO) each containing an analog controlled oscillator (ACO) which is responsive to a control signal for producing a desired output frequency, said control signal having a magnitude which varies as a predetermined function of ambient temperature in order to compensate for temperature variations in the ACO output frequency, said function being based on data stored in a memory of the TCO and which must be individually calibrated for each TCO, said method comprising the steps of:

(a) subjecting all of the TCOs in the batch to a controlled environment wherein said ambient temperature may be varied,
(b) connecting a frequency source to the TCOs for producing an accurate frequency,
(c) selecting a "calibration mode" of operation wherein said function is determined for a measured ambient temperature and the data corresponding thereto is stored in the memory and wherein there are performed the following steps for each TCO in the batch:
(i) determining the value of the required control signal for producing an output signal of the desired output frequency,
(ii) storing the respective value of the control signal or a function thereof in the memory of each device, and
(iii) repeating steps (i) and (ii) in respect of different temperatures until each value is stored in the memory;
and
(d) selecting a "compensation mode" of operation wherein data is extracted from the memory an used in conjunction with a measured ambient temperature for adjusting the control signal.

5 more elements

A temperature compensated oscillator (TCO) for generating a desired frequency which is stabilized over a predetermined temperature range, said TCO comprising:

a compensation circuit for compensating in compensation mode for changes in temperature in accordance with at least one pre-stored compensation value or a function thereof,
a calibration specific circuit active only in calibration mode for determining and storing said at least one compensation value or function thereof, and
a selector means coupled to the compensation circuit and to the calibration specific circuit for selecting a corresponding compensation mode or calibration mode of operation;
the compensation circuit comprising:
an analog controlled oscillator (ACO) having an output frequency controllable by an analog control signal V.sub.c,
a temperature sensor for sensing the ambient temperature of the ACO and producing a corresponding analog signal,
an A/D converter coupled to the temperature sensor for converting the analog signal to a corresponding digital temperature value,
a memory having at least one memory location for storing therein a respective digital control value,
digital processing means coupled to the A/D converter and to the memory and responsive to the respective digital control value and to the digital temperature value for producing a digital signal representative of the analog control signal V.sub.c, and
a D/A converter coupled to an output of the digital processing means for converting said digital signal to said analog control signal V.sub.c ;
the calibration specific circuit comprising:
reference signal coupling means for coupling thereto an external signal having an accurate reference frequency,
control means coupled to the reference signal coupling means and to the compensation circuit for determining the digital control value which, when processed by the digital processing means in the compensation circuit, produces the control signal V.sub.c corresponding to the measured ambient temperature of the ACO, and
data entry means for entering said digital control value into the memory at said at least one location thereof.

11 more elements

A temperature compensation oscillator (TCO) for generating a desired frequency which is stabilized over a predetermined temperature range, said TCO comprising:

a compensation circuit for compensating for changes in temperature in accordance with a pre-stored compensation function,
a dedicated aging calibration circuit for determining the storing at least one value for operating in conjunction with the compensation circuit so as to compensate for aging of the oscillator, and
a selector means coupled to the compensation circuit and to the dedicated aging calibration circuit for selecting a corresponding compensation mode or aging calibration mode of operation;
the compensation circuit comprising:
a signal controlled oscillator (ACO) having an output frequency controllable by an analog control signal V.sub.c,
a temperature sensor for sensing the ambient temperature of the ACO and producing a corresponding analog temperature signal, and
a transducer means responsive to the temperature signal for producing the control signal V.sub.c ;
the aging calibration circuit comprising:
reference signal coupling means for coupling thereto an external signal having an accurate reference frequency,
control means coupled to the reference signal coupling means and to the ACO for determining an offset correction value or a function thereof which operates in conjunction with the compensation circuit so as to produce the control signal V.sub.c corresponding to the measured ambient temperature of the ACO,
storage means for storing said offset correction value or said function thereof, and
offset correction means coupled to the storage means and to the compensation circuit and being responsive to said control signal V.sub.c for operating the compensation circuit in accordance with said offset correction value or a function thereof.

9 more elements

A TCO comprising a calibration specific circuit active only in calibration mode for determining and storing at least one compensation value or function thereof for operating in conjunction with a compensation circuit including therein an analog controlled oscillator (ACO) having an output frequency controllable by an analog control signal V.sub.c, to compensate for variations in temperature, the TCO further comprising a dedicated aging calibration circuit for determining and storing at least one value for operating in conjunction with the compensation circuit so as to compensate for aging of the oscillator, wherein:

the calibration specific circuit comprises:
reference signal coupling means for coupling thereto an external signal having an accurate reference frequency,
control means coupled to the reference signal coupling means and to the compensation circuit for determining the digital control value which, when processed by the digital processing means in the compensation circuit, produces the control signal V.sub.c corresponding to the measured ambient temperature of the ACO, and
data entry means for entering said digital control value into the memory at said at least one location thereof;
the dedicated aging calibration circuit comprises:
reference signal coupling means for coupling thereto an external signal having an accurate reference frequency,
control means coupled to the reference signal coupling means and to the ACO for determining an offset correction value or a function thereof which operates in conjunction with the compensation circuit so as to produce the control signal V.sub.c corresponding to the measured ambient temperature of the ACO,
storage means for storing said offset correction value or said function thereof, and
offset correction means coupled to the storage means and to the compensation circuit and being responsive to said control signal V.sub.c for operating the compensation circuit in accordance with said offset correction value or a function thereof;
and wherein
the calibration specific circuit and the dedicated aging calibration circuit employing shared components.

8 more elements

Dependencies are inferred from the claim text.

Method and system for calibrating a crystal oscillator

Life of this patent

Method and system for calibrating a crystal oscillator

Life of this patent

Abstract

Claims66