Electronic measurement instrument probe accessory offset, gain, and linearity correction method

• US 6,321,171 B1
• Filed: 04/03/1998
• Issued: 11/20/2001
• Est. Priority Date: 04/03/1998
• Status: Expired due to Fees

• Current Assignee: Tektronix Incorporated (Fortive Corp.)
• Original Assignee: Tektronix Incorporated (Fortive Corp.)
• Inventors: Baker, Clifford E.
• Primary Examiner(s): Hoff, Marc S.
• Assistant Examiner(s): MILLER, CRAIG S
• Application Number:
  US09/055,014
• Time in Patent Office: 1,327 Days
• Field of Search: 702/104, 702/57, 702/64-66, 702/75-77, 702/85-88, 702/90, 702/91, 324/76.11, 340/870.11
• US Class Current: 702/104
• CPC Class Codes: G01D 18/008 with calibration coefficients stored in memory
• CPC Class Codes: G01D 3/022 having an ideal characteristic, map or correction data stored in a digital memory

Abstract

A correction method for an electronic instrument accessory probe utilizes an error correction equation wherein at least one term contains an exponent less than unity. One simple such equation is: S=Cs²+Bs+A+b|s|^x (where 0<x<1), but additional terms may be added, either with integer exponents greater than 2, or with other fractional exponents less than one. In the most simple embodiment, there are only four coefficients and the only term with a fractional exponent has an exponent of ½ (i.e., x=0.5). A second set of coefficients may be needed for the correction of negative values.

Claims

1. A method for compensating for offset, gain, and linearity errors in a measured signal, s, sent from an probe accessory to an electronic measurement instrument via a conductive pathway, said probe accessory serving the function of monitoring one or more of electrical parameter such as voltage, current, impedance, or frequency, said measured signal communicating an uncorrected indication of the monitored electrical parameter'"'"'s values, the method producing an error corrected signal, S, having reduced offset, gain, and linearity errors over a predetermined range of calibrated values, the method comprising the steps of:

calibrating the probe accessory and conductive pathway with respect to a particular one of the electrical parameters to find four or more coefficients for a correction function having the form;

S=Cs²+Bs+A+b|s|^x, where 0<

x<

1 and further coefficients beyond four are associated with either higher powers of s, such as Ds³, or additional terms with different values of x still satisfying the condition 0<

x<

1, such as c|s|^x′

, such that when the correction function is applied to the communicated signal, s, for values within the predetermined range of calibrated values, an enhanced accuracy signal, S, is produced;

monitoring one of the electrical parameters with the probe accessory and communicating the values of the measured signal s over the conductive pathway to the electrical measurement instrument, the values of the measured signal s being within the predetermined range of calibrated values; and

applying the correction function using the predetermined four or more coefficients to the values of the measured signal s, thereby producing values of S.

2. A method according to claim 1 wherein the four or more coefficients are only found and applied for positive values of the measured signal s and the predetermined range of calibrated value is entirely composed of positive values, and an additional four or more coefficients are found and applied for negative values of the measured signal s, and a second predetermined range of calibrated values associated with the additional four or more coefficients is entirely composed of negative values.

3. A method according to claim 2 wherein the step of calibrating comprises the steps of:

monitoring a particular one of the electrical parameters and communicating a plurality of positive sample values of the measured signal s over the conductive pathway to the electrical measurement instrument, the positive sample values being distributed over the positive predetermined range of calibrated values;

using calculation and iteration, finding a best fit solution for the value of the four or more coefficients, using the plurality of positive sample values and corresponding ideal expected positive values of the enhanced accuracy signal, S′

;

monitoring the particular one of the electrical parameters and communicating a plurality of negative sample values of the measured signal s over the conductive pathway to the electrical measurement instrument, the negative sample values being distributed over the negative predetermined range of calibrated values; and

using calculation and iteration, finding a best fit solution for the value of the additional four or more coefficients, using the plurality of negative sample values and corresponding ideal expected negative values of the enhanced accuracy signal, S′

.

4. A method according to claim 2 wherein the x'"'"'s in the step of calibrating have the value of 0.5.

5. A method according to claim 2 further comprising the step of

storing the four or more coefficients and the additional four or more coefficients in the probe accessory.

6. A method according to claim 5 further comprising the step of:

accessing, from the electrical instrument, the four or more coefficients and the additional four or more coefficients stored in the probe accessory.

7. A method according to claim 2 wherein the electrical measurement instrument is a digital multimeter.

8. A method according to claim 2 wherein the electrical measurement instrument is a digital oscilloscope.

9. A method according to claim 2 wherein the electrical measurement instrument is a spectrum analyzer.

10. A method according to claim 1 wherein the step of calibrating comprises the steps of:

monitoring a particular one of the electrical parameters and communicating a plurality of sample values of the measured signal s over the conductive pathway to the electrical measurement instrument, the sample values being distributed over the predetermined range of calibrated values; and

using calculation and iteration, finding a best fit solution for the value of the four or more coefficients, using the plurality of sample values and corresponding ideal expected values of the enhanced accuracy signal, S′

.

11. A method according to claim 1 wherein the x in the step of calibrating has the value of 0.5.

12. A method according to claim 1 further comprising the step of:

storing the four or more coefficients in the probe accessory.

13. A method according to claim 12 further comprising the step of:

accessing, from the electrical instrument, the four or more coefficients stored in the probe accessory.

14. A method according to claim 1 wherein the electrical measurement instrument is a digital multimeter.

15. A method according to claim 1 wherein the electrical measurement instrument is a digital oscilloscope.

16. A method according to claim 1 wherein the electrical measurement instrument is a spectrum analyzer.