Impedance meter capable of performing measurements at high precision over wide impedance and frequency ranges

• US 5,345,182 A
• Filed: 10/23/1992
• Issued: 09/06/1994
• Est. Priority Date: 10/31/1991
• Status: Expired due to Term

• Current Assignee: Agilent Technologies, Inc.
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Wakamatsu, Hideki
• Primary Examiner(s): Strecker, Gerard R.
• Assistant Examiner(s): DO, DIEP N
• Application Number:
  US07/965,795
• Time in Patent Office: 683 Days
• Field of Search: 324/638, 324/648, 324/649, 324/654, 324/658, 324/691, 324/713, 324/715, 324/115, 324/522
• US Class Current: 324/649
• CPC Class Codes: G01R 27/06 Measuring reflection coefficients; Measuring standing-wave ratio
• CPC Class Codes: G01R 27/08 Measuring resistance by measuring both voltage and current

Abstract

A system for measuring a broad range of impedance values with high precision and over a broad frequency band. Both the broadrange impedance measurement capability of the V-I method and the broadband frequency measuring capability of the reflection coefficient method are provided. A remote measurement capability is also available. Based upon the V-I method, a selection is made between a circuit to achieve an ideal open and a circuit to achieve an ideal short circuit in response to the impedance values. A boundary for selecting the impedance is, for instance, 50Ω. To measure a high impedance, the ideal open type circuit is selected; to measure a low impedance, the ideal short circuit is selected. The source and measuring instruments are extended by a coaxial cable. A floating measuring instrument is obtained by using a balun. Impedance measurements for 1 MHz to 2 GHz are possible.

Claims

1. An impedance meter, comprising:

(a) a signal source comprising means for exciting a device under test (DUT);

(b) a first measuring instrument for measuring a first voltage indicative of a voltage drop across said DUT;

(c) a second measuring instrument for measuring a second voltage indicative of a current through said DUT; and

(d) switching means for switching the interconnection of said signal source and said first and second measuring instruments between an ideal open type circuit and an ideal short circuit type circuit in response to a DUT impedance determined on the basis on said first and second voltages, said ideal open and ideal short circuit type circuits being pre-defined circuit configurations;

wherein the DUT impedance is a function of at least said first and second voltages.

2. An impedance meter according to claim 1, wherein the DUT, said signal source, and said first and second measuring instruments are connected with each other via coaxial cables having a characteristic impedance Z₀, and internal impedances of said signal source and said measuring instruments are approximately equal to Z₀.

3. An impedance meter according to claim 2, further comprising a balun associated with at least one of said coaxial cables, said balun preventing at least one of said measuring instruments from receiving a common mode voltage associated with said source.

4. An impedance meter according to claim 2, wherein said first measuring instrument comprises a voltmeter arranged to measure a voltage between an inner and an outer conductor of a first coaxial cable;

said second measuring instrument comprises a voltmeter arranged to measure a voltage between an inner and an outer conductive of a second coaxial cable; and

said signal source is arranged to apply a voltage across an inner and an outer conductor of a third coaxial cable;

said first, second and third coaxial cables being operatively coupled to said DUT.

5. An impedance meter according to claim 4, wherein said switching means comprises means for connecting the outer conductor of said first cable to the outer conductor of said third cable in response to a high DUT impedance, and for connecting the outer conductor of said first cable to a common potential in response to a low DUT impedance.

6. An impedance meter according to claim 4, wherein said switching means comprises means for connecting the inner conductor of said first cable to the inner conductor of said third cable and outer conductor of said second cable in response to a high DUT impedance, and for connecting the inner conductor of said first cable to the inner conductor of said second cable in response to a low DUT impedance.

7. An impedance meter according to claim 4, wherein said switching means comprises means for connecting the inner conductor of said second cable to the outer conductors of said first and third cables in response to a high DUT impedance, and for connecting the inner conductor of said second cable to the inner conductors of said first and third cables in response to a low DUT impedance.

8. An impedance meter according to claim 4, wherein said switching means comprises means for connecting the outer conductor of said second cable to the inner conductor of said first cable in response to a high DUT impedance, and for connecting the outer conductor of said second cable to a common potential in response to a low DUT impedance.

9. An impedance meter according to claim 4, wherein said switching means comprises means for selectively connecting the inner conductor of said first cable to the inner conductor of said third cable and the outer conductor of said second cable.

10. An impedance meter according to claim 4, wherein said switching means comprises a diode switch coupled between said first, second, and third coaxial cables.

11. An impedance meter according to claim 4, wherein said balun and said switching means are arranged within a single housing and are remote from said signal source and said measuring instruments.

12. A method for measuring an impedance of a device under test (DUT), wherein the DUT is operatively interconnected with first and second voltage measuring instruments and a signal source, comprising the steps of:

(a) exciting a DUT with a signal source;

(b) measuring, with a first measuring instrument, a first voltage indicative of a voltage drop across said DUT;

(c) measuring, with a second measuring instrument, a second voltage indicative of a current through said DUT;

(d) determining a DUT impedance on the basis of said first and second voltages; and

(e) switching the interconnection of said signal source and said first and second measuring instruments between an ideal open type circuit and an ideal short circuit type circuit in response to the DUT impedance determined on the basis on said first and second voltages, said ideal open and ideal short circuit type circuits being predefined circuit configurations;

wherein the DUT impedance is a function of at least said first and second voltages.

13. A method of measuring an impedance of a DUT according to claim 12, comprising the step of connecting a measuring port to which the DUT is connected, said signal source, and said first and second measuring instruments with each other via coaxial cables having a characteristic impedance Z₀, wherein internal impedances of said signal source and said measuring instruments are approximately equal to Z₀.

14. A method for measuring an impedance of a DUT according to claim 13, comprising the steps of coupling first, second and third coaxial cables to said DUT;

arranging said first measuring instrument to measure a voltage between an inner and an outer conductor of said first coaxial cable;

arranging said second measuring instrument to measure a voltage between an inner and an outer conductor of said second coaxial cable; and

arranging said signal source to apply a voltage across an inner and an outer conductor of said third coaxial cable.

15. A method for measuring an impedance of a DUT according to claim 14, wherein said switching step (e) comprises connecting the outer conductor of said first cable to the outer conductor of said third cable in response to a high DUT impedance, and connecting the outer conductor of said first cable to a common potential in response to a low DUT impedance.

16. A method for measuring an impedance of a DUT according to claim 14, wherein said switching step comprises connecting the inner conductor of said first cable to the inner conductor of said third cable and outer conductor of said second cable in response to a high DUT impedance, and connecting the inner conductor of said first cable to the inner conductor of said second cable in response to a low DUT impedance.

17. A method for measuring an impedance of a DUT according to claim 14, wherein said switching step comprises connecting the inner conductor of said second cable to the outer conductors of said first and third cables in response to a high DUT impedance, and connecting the inner conductor of said second cable to the inner conductors of said first and third cables in response to a low DUT impedance.

18. A method for measuring an impedance of a DUT according to claim 14, wherein said switching step comprises connecting the outer conductor of said second cable to the inner conductor of said first cable in response to a high DUT impedance, and connecting the outer conductor of said second cable to a common potential in response to a low DUT impedance.

19. A method for measuring an impedance of a DUT according to claim 14, wherein said switching step comprises selectively connecting the inner conductor of said first cable to the inner conductor of said third cable and the outer conductor of said second cable.