Apparatus and method for calibrating equipment for high frequency measurements
View Patent ↗Calibration standards for accurate high frequency or wide bandwidth calibration measurements. A “short” or “reflect” standard is formed in a printed circuit board from a conductive coating on a generally planar surface. The conductive coating connects a signal trace to one or more ground planes. The generally planar surface is at least as wide as the signal trace and is preferably several times wider than the signal trace to provide a short standard with properties uniform over a wide frequency range. The short standard is incorporated into a printed circuit upon which a device under test is to be mounted. Connections to the short standard are made through components equivalent to components used to connect a device under test. When a through and line standard are added to the same board, the test board contains all the standards needed for a TRL calibration.
1. A printed circuit board having a calibration standards, the calibration standard comprising:
a) a ground plane;
b) a signal trace running parallel to the ground plane, the trace having a width;
c) a generally planar surface having a width wider than the width of the trace; and
d) a conductive coating disposed on the generally planar surface, the conductive coating making a direct electrical connection between the trace and the ground plane.
2. The printed circuit board of claim 1 wherein the generally planar surface is a wall of an opening formed in the printed circuit board.
3. The printed circuit board of claim 2 wherein the generally planar surface extends from the center of the trace for at least the width of the trace in all directions.
4. The printed circuit board of claim 3 wherein the generally planar surface has a non-planarity of less than 5% of the width of the generally planar surface.
5. The printed circuit board of claim 3 wherein the generally planar surface has a curvature of an arc of a circle spanning less than 20 degrees.
6. The printed circuit board of claim 3 wherein the generally planar surface is normal to the ground plane where the generally planar surface intersects the trace.
7. The printed circuit board of claim 3 wherein the generally planar surface is a portion of a non-planar member.
8. The printed circuit board of claim 7 wherein the opening is a slot.
9. The printed circuit board of claim 1 wherein the generally planar surface is a wall bounding an opening in the printed circuit board.
10. The printed circuit board of claim 1 wherein the generally planar surface is an edge of the printed circuit board.
11. The printed circuit board of claim 10 wherein the generally planar surface extends from the center of the trace for at least the width of the trace in all directions.
12. The printed circuit board of claim 11 wherein the generally planar surface has a non-planarity of less than 5% of the width of the generally planar surface.
13. The printed circuit board of claim 11 wherein the generally planar surface has a curvature of an arc of a circle spanning less than 20 degrees.
14. The printed circuit board of claim 11 wherein the generally planar surface is normal to the ground plane where the generally planar surface intersects the trace.
15. The printed circuit board of claim 11 wherein the generally planar surface is a portion of a non-planar member.
16. A test fixture for a device under test comprising:
a) a position for mounting a device under test;
b) a first separable connectors;
c) at least one signal connection path between the position for mounting a device under test and the first separable connector;
d) a first calibration standard comprising:
i) a conductive generally planar surface;
ii) a second separable connector;
iii) a first signal trace electronically connecting the second separable connector and the conductive generally planar surface;
e) a ground plane, parallel to the first signal trace, the ground plane electrically connected to the conductive generally planar surface;
f) a second calibration standard comprising:
i) a third separable connector;
ii) a fourth separable connector;
iii) a second signal trace electrically connected between the third and the fourth separable connectors and parallel to the ground plane, the second signal trace being at least twice as long as the signal connection path.
17. The test fixture of claim 16 additionally comprising a second ground plane parallel to the first signal trace and disposed on an opposite side of the ground plane from the first signal trace.
18. The printed circuit board of claim 16 wherein the generally planar surface extends from the center of the first signal trace for at least the width of the first signal trace in all directions.
19. The printed circuit board of claim 18 wherein the generally planar surface has a non-planarity of less than 5% of the generally planar surface.
20. The printed circuit board of claim 18 wherein the generally planar surface has a curvature of an arc of a circle spanning less than 20 degrees.
21. The printed circuit board of claim 18 wherein the test fixture comprises a printed circuit board and the generally planar surface is formed on a wall of a slot in the printed circuit board.
22. A method of making calibrated measurements on a component mounted to a substrate using a network analyzer, comprising:
a) connecting the network analyzer to a plurality calibration standards on the substrate through separable connectors, at least one of the calibration standards being a reflect standard including a trace, a ground plane and a generally planar conducting member wider than the trace and disposed on a surface of the substrate, the conducting member connecting the ground plane and the trace;
b) making measurements on the plurality of calibration standards with the network analyzer;
c) connecting the network analyzer to the component through separable connectors;
d) adjusting measurements of the component based on measurements made of the calibration standards.
23. The method of claim 22 wherein the substrate comprises a printed circuit board.
24. The method of claim 23 wherein the component comprises an electrical connector.
25. The method of claim 22 wherein the measurements are made at frequencies in excess of 3 GHz.