WO2008021907A2

WO2008021907A2 - Calibrated s-parameter measurements of probes

Info

Publication number: WO2008021907A2
Application number: PCT/US2007/075515
Authority: WO
Inventors: Laudie Doubrava; William Hagerup
Original assignee: Tektronix, Inc.
Priority date: 2006-08-08
Filing date: 2007-08-08
Publication date: 2008-02-21
Also published as: WO2008021907A3

Abstract

Method and apparatus for calibrating a probe (32) including calibrating a reference plane along a transmission line, the transmission line having a first port and a second port, calculating a reflection from the reference plane into the second port with the second port coupled to a load (25), applying the probe to a physical location along the transmission line substantially corresponding to the reference plane, measuring a probe response between the first port of the transmission line and an output of the probe, and de-embedding calibrated probe parameters using the probe response and the reflection from the reference plane into the second port.

Description

CALIBRATED S-PARAMETER MEASUREMENTS OF PROBES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S. C. § 119(e) from U.S. Provisional Application serial number 60/916,788, filed on May 8, 2007 and U.S. Provisional Application serial number 60/836,487, filed on August 8, 2006 the contents of which are herein incorporated by reference in their entirety.

BACKGROUND

[0002] This disclosure relates to the calibration of probes and, more particularly to calibration procedures and test fixtures for high speed probes.

[0003] High speed electronic devices are typically connected by transmission lines. To examine signals passing between the high speed electronic devices, a probe can be placed on the transmission line. However, both the probe and the presence of the probe on the transmission line can affect the signals. A calibration of the probe can be used to counteract this effect on the measurement.

[0004] For some probes, an input to a probe includes pointed tips and the output is an SMA connector, a BMA connector, or other connector type. However, standard vector network analyzer calibration kits for the probe tips are not available. As a result, to obtain some form of calibration, a special fixture setup and algorithm method is needed. [0005] For example, for a grounded coplanar waveguide (GCPW) that is the target of the probe, a GCPW structure can be characterized. The measurements can include measuring the fixture without the probe and measuring the s-parameters between an input to the fixture and an output of the probe while the probe is attached. With the probe attached, the GCPW is terminated with a characteristic impedance, typically 50 ohms. The response of the GCPW without the probe is used to approximate the contribution of the portion of the GCPW from the input of the fixture to the probe location. However, such a calibration does not take into account the effect of the GCPW when the probe is attached. Although the fixture may be physically symmetrical, the change in the response due to portions of the fixture when the probe is attached may not be electrically symmetrical. Accordingly, the approximation introduces error into the calibration.

[0006] In addition, for differential probes, a single-ended fixture is used to calibrate the probe. Accordingly, the differential probe is not calibrated on a fixture similar to the conditions of use, further introducing errors into the calibration. [0007] Accordingly, there remains a need for an improved calibration procedure and test fixture for calibration of high speed probes.

SUMMARY

[0008] An aspect includes calibrating a probe including calibrating a reference plane along a transmission line; the transmission line having a first port and a second port; calculating a reflection from the reference plane into the second port with the second port coupled to a load; applying the probe to a physical location along the transmission line substantially corresponding to the reference plane; measuring a probe response between the first port of the transmission line and an output of the probe; and de-embedding calibrated probe parameters using the probe response and the reflection from the reference plane into the second port. [0009] Another aspect includes a system for calibrating a probe including a test and measurement instrument and calibration standards. The calibration standards include a load and a transmission line standard having a first port, a second port, and a probe footprint to receive the probe located at a physical location along the transmission line substantially corresponding to a reference plane. The test and measurement instrument is configurable to calibrate the reference plane along the transmission line from measurements of the calibration standards; calculate a reflection from the reference plane into the second port with the second port coupled to a load; measure a probe response between the first port of the transmission line and an output of the probe with the probe coupled to the probe footprint; and de-embed calibrated probe parameters using the probe response and the reflection from the reference plane into the second port.

[0010] Another aspect includes an apparatus for calibrating a differential probe including a first differential transmission line having a first section and a second section; a second differential transmission line substantially identical to the first section of the first differential transmission line and having an open on one end; a third differential transmission line substantially identical to the second section of the first differential transmission line and having an open on one end; and at least one additional differential transmission line. Each additional differential transmission line includes a first section substantially identical to the first section of the first differential transmission line; a second section substantially identical to the second section of the first differential transmission line; and a third section disposed between the first section and the second section. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIGURE 1 is a block diagram of a system for calibrating probes according to an embodiment of the invention.

[0012] FIGURE 2 is a diagram illustrating an additional standard for a non-zero length probe footprint for the system of FIGURE 1.

[0013] FIGURE 3 is a block diagram of an apparatus for calibrating a differential probe according to an embodiment of the invention.

[0014] FIGURE 4 is a block diagram of an additional differential transmission line with a probe footprint for the apparatus of FIGURE 3.

[0015] FIGURE 5 is series of network diagrams illustrating the calibration of a reference plane according to an embodiment of the invention.

[0016] FIGURE 6 is a network diagram illustrating a three-port parameter matrix representing a probe location and termination of a transmission line of FIGURE 5. [0017] FIGURE 7 is a network diagram illustrating a reflection at a reference plane into the termination in FIGURE 6.

[0018] FIGURE 8 is a block diagram illustrating an example of the conversion of a three- port parameter matrix representing a probe into a two-port parameter matrix using the reflection of FIGURE 7.

[0019] FIGURE 9 is a block diagram illustrating another example of the conversion of a three-port parameter matrix representing a probe into a two-port parameter matrix using the reflection of FIGURE 7.

[0020] FIGURE 10 is a network diagram illustrating a probe characterization setup according to an embodiment of the invention.

[0021] FIGURE 11 is a network diagram illustrating the probe characterization setup of FIGURE 10 with a two-port parameter matrix substituted for the probe location. [0022] FIGURE 12 is a series of network diagrams illustrating the calibration of a reference plane on a differential transmission line according to an embodiment of the invention. [0023] FIGURE 13 is a network diagram of a common mode component of the calibration of FIGURE 12.

[0024] FIGURE 14 is a network diagram of a differential-mode component of the calibration of FIGURE 12.

[0025] FIGURE 15 is a network diagram illustrating a termination of a differential transmission line according to an embodiment of the invention. [0026] FIGURE 16 is a network diagram illustrating a common mode component of the calibration of FIGURE 15.

[0027] FIGURE 17 is a network diagram illustrating a differential mode component of the calibration of FIGURE 15.

[0028] FIGURE 18 is a network diagram illustrating a differential probe characterization setup according to an embodiment of the invention.

[0029] FIGURE 19 is a network diagram illustrating a common mode component of the network diagram of FIGURE 18.

[0030] FIGURE 20 is a network diagram illustrating a differential-mode component of the network diagram of FIGURE 18.

[0031] FIGURE 21 is a flowchart illustrating a method of calibrating a probe according to an embodiment of the invention.

[0032] FIGURE 22 is a flowchart illustrating an example of calibrating a reference plane in the method of FIGURE 21.

[0033] FIGURE 23 is a flowchart illustrating an example of calculating the reflection in the method of FIGURE 21.

[0034] FIGURE 24 is a flowchart illustrating another example of calculating the reflection in the method of FIGURE 21.

[0035] FIGURE 25 is a flowchart illustrating an example of converting a characterization of a probe location in the method of FIGURE 21.

[0036] FIGURE 26 is a flowchart illustrating an example of characterization of the probe location in the method of FIGURE 25.

[0037] FIGURE 27 is a flowchart illustrating another example of characterization of the probe location in the method of FIGURE 21.

[0038] FIGURE 28 is a flowchart illustrating an example of the creation of common mode and differential-mode parameters in the method of calibrating a probe of FIGURE 21.

[0039] FIGURE 29 is a flowchart illustrating characterization of the termination in the method of FIGURE 28.

[0040] FIGURE 30 is a flowchart illustrating an example of the de-embedding of common mode and differential-mode probe parameters in the method of FIGURE 28.

[0041] FIGURE 31 is a flowchart illustrating the calibration of an apparatus for calibrating a probe.

DETAILED DESCRIPTION [0042] Various embodiments of the invention will next be described with reference to the drawings. These embodiments enable characterization of probes in the environment of use. [0043] FIGURE 1 is a block diagram of a system for calibrating probes according to an embodiment of the invention. In this embodiment, the system includes a test and measurement (T&M) instrument 10 and fixture 22. The T&M instrument 10 can be any T&M instrument capable of measuring scattering parameters (S-parameters). For example, a vector network analyzer (VNA) can be the T&M instrument 10. In another example, the T&M instrument can be an oscilloscope capable of making a time-domain reflectometry (TDR) measurement.

[0044] Although S-parameters are used in this description for the characterization of components, calibrations, or the like, the parameters can take any form as desired. For example, transmission parameters (T-parameters), ABCD-parameters, admittance parameters (Y-parameters), impedance parameters (Z-parameters), or the like can be used both as replacement with S-parameters or in combination.

[0045] In this embodiment, fixture 22 includes standards 27, 29, 31, and 33. In general, a standard is a structure that is characterized or can be characterized for use in a calibration or measurement. Various types of standards are described below as examples. The T&M instrument can be coupled to each of the standards 27, 29, 31, and 33 through one or more probes. Standard 27 is a transmission line standard having a first port 34 and a second port 36. A load 25 can be coupled to standard 27 at the second port 36. Standard 27 includes a portion configured to receive the probe 32 illustrated as a probe footprint 38. A probe footprint 38 is a structure capable of being coupled to a probe. In an embodiment, the probe foot print 38 can be a portion of the transmission line forming standard 27. Thus, the probe footprint 38 can be indistinguishable from the remainder of standard 27. Other examples of probe footprints 38 will be described below.

[0046] Probe locations 12, 14, 16, and 18 are locations where a probe coupled to port 1 of the T&M instrument 10 can be coupled to the fixture 22. Probe locations 24, 26, 28, and 40 are locations where port 2 of the T&M instrument 10 can be coupled to the fixture 22. The probes that can be coupled to the probe locations 12, 14, 16, 18, 24, 26, 28, 40, or any other probe locations on can, but need not be configured to receive probes that are the same as the probe 32. Furthermore, probe 32 is the probe that is to be characterized; however this does not mean that any other probes are not characterized. For example, a probe coupled to probe location 12 can be characterized as part of the characterization of the fixture 22. [0047] Although probe locations 12, 14, 16, and 18 have been described as coupling the T&M instrument 10 to the fixture 22 at various locations, other coupling can be used. For example, the fixture 22 may have connectors, waveguides, or the like that can be coupled to the T&M instrument 10. Any manner of coupling between the T&M instrument 10 and the fixture 22 can be used.

[0048] Through measurements made of the standards 27, 29, 31, and 33, the T&M instrument can calibrate a reference plane along the standard 27. Calibrating a reference plane includes the measurements, calculations, or the like that establish the reference plane along a transmission line. In an embodiment, standard 27 can be a thru standard, standard 29 can be a line standard, standard 31 can be a reflect standard, and standard 33 can be a load standard. Accordingly, a thru-reflect-line (TRL) calibration can be performed to calibrate the reference plane at a physical location substantially corresponding to the location of the probe 32 on the standard 27. Standard 33 can be used to establish the characteristic impedance of the TRL calibration.

[0049] As described above, a load 25 can be coupled to the second port 36 of standard 27. Accordingly, a reflection from the reference plane into the second port with the second port coupled to a load can be calculated. For example, a measurement from probe location 12 can be used along with the TRL calibration described above to calculate the reflection. [0050] Although illustrated as coupled to the standard 27, the above measurements with the T&M instrument are performed with the probe 32 decoupled from the standard 27. The probe 32 can be coupled to standard 27 at the probe footprint 38. The probe footprint 38 can be a zero length portion of the standard 27. That is, the probe footprint 38 is the junction between the portions of standard 27 divided by the reference plane calibrated by the TRL calibration. Since standard 27 is the thru standard of the TRL calibration, it has zero length. Accordingly, the probe footprint 38 has zero length.

[0051] In another embodiment, the probe footprint can be a physical structure. In such a case, an additional standard may be used having the probe footprint. FIGURE 2 is a diagram illustrating an additional standard for a non-zero length probe footprint. Standard 27 is an enlarged version from FIGURE 1. Standard 42 includes the non-zero length probe footprint 46. The probe footprint 46 is located between sections 52 and 54 of standard 42. Sections 52 and 54 of standard 42 are substantially identical to sections 48 and 50 of standard 27. In this embodiment, the probe 32 would be coupled to standard 42 at the probe footprint 46. [0052] Regardless of the implementation of the probe footprint 38 or 46, the probe is coupled to a standard. Referring back to FIGURE 1, the probe 32 is coupled to standard 27. Depending on the configuration of the probe, an adapter 20 may be needed to convert the connector of port 2 of the T&M instrument to the connector of the probe 32. Accordingly, the probe 32 can be coupled to the T&M instrument 10.

[0053] The T&M instrument can measure a probe response between the first port of the transmission line and an output of the probe 32 with the probe 32 coupled to the probe footprint 38. In this embodiment, a probe response is a full 2-port measurement with the first port of the transmission line as one port and the output of the probe 32 as the second port. Once this measurement has been obtained, calibrated probe parameters can be de-embedded using the probe response and the reflection from the reference plane into the second port. The de-embedding of the calibrated probe parameters is further described below. [0054] FIGURE 3 is a block diagram of an apparatus for calibrating a differential probe according to an embodiment of the invention. The apparatus for calibrating a differential probe includes a first differential transmission line 120, a second differential transmission line 126, a third differential transmission line 127 and at least one additional differential transmission line 122.

[0055] In this embodiment, the transmission lines are all differential transmission lines. For example, the differential transmission lines can be a differential grounded coplanar waveguide structure. Pads 100 and 102 are coupled to the differential transmission line 120 for coupling to a T&M instrument, load, or the like during calibration of a probe. Pads 100 and 102 are differential pads, as are pads 104, 106, 112, 114, 116, and 118. Pad 102 will be used as an example since the pads 100, 102, 104, 106, 112, 114, 116, and 118 are substantially identical. Pad 102 includes single-ended pads 142 and 144. Each single-ended pad 142 and 144 is coupled to a corresponding one of the two lines forming the differential- pair transmission line 120.

[0056] The first differential transmission line 120 includes a first section 130 and a second section 132. The first section 130 includes conductors 134 and 138 for the two lines of the differential transmission line 120. Similarly, the second section 132 includes conductors 136 and 140 for the two lines of the differential transmission line 120. The sections 130 and 132 are coupled at a reference plane 131. In this embodiment, although he differential transmission lines may be described as having sections, the sections can be lengths of the differential transmission line 120. In an embodiment, the first differential transmission line 120 can be the standard 27 of FIGURE 1.

[0057] The second differential transmission line 126 is substantially identical to the first section 130 of the first differential transmission line 120. The second differential transmission line 126 has an open on one end. Similarly, the third differential transmission line 127 is substantially identical to the second section 132 of the first differential transmission line and has an open on one end.

[0058] In addition to the first differential transmission line 120, there are one or more additional differential transmission lines. Differential transmission line 122 includes common characteristics of these additional differential transmission lines. Differential transmission line 122 includes a first section 146 substantially identical to the first section 130 of the first differential transmission line 120 and a second section 150 substantially identical to the second section 132 of the first differential transmission line 120. The first section 146 and the second section 150 are separated by a third section 148 disposed between the first section 146 and the second section 150. Any number of such lines can be used with varying lengths of section 148. Accordingly, the apparatus includes the standards for calibrating the fixture and the differential transmission line that is the test fixture for the probe.

[0059] Differential transmission lines 128 and 129 are terminated differential transmission lines. Differential transmission line 128 includes a portion 156 substantially identical to the first section 130 of the first differential transmission line 120. Differential transmission line 129 includes a portion 158 substantially identical to the second section 132 of the first differential transmission line 120. Both differential transmission lines 128 and 129 are terminated by loads 160. Although loads 160 are illustrated as resistors to ground, any load appropriate for a differential transmission line can be used.

[0060] FIGURE 4 is a block diagram of an additional differential transmission line with a probe footprint for the apparatus of FIGURE 3. In an embodiment, the apparatus for calibrating a probe includes a fourth differential transmission line 172. The fourth differential transmission line 172 includes a probe footprint 170, a first section 162, and a second section 164.

[0061] The probe footprint 170 is configured to receive a differential probe. For example, the probe footprint 170 can include solder-on terminals 166 and 168 for attaching a probe. In an embodiment, the probe footprint 170 can include a differently sized region corresponding to a probe land pattern. The first section 162 is substantially identical to the first section 130 of the first differential transmission line 120. The second section 164 is substantially identical to the second section 132 of the first differential transmission line 120. [0062] Although the fourth differential transmission line 170 has been described as separate from the first differential transmission line 120, the fourth differential transmission line 170 can be used in place of the first differential transmission line 120. Accordingly, the effect of the probe footprint 170, as implemented in a device under test (DUT) using the calibrated probe, can be calibrated out so that only the effects of the probe are included in the probe calibration. That is, the probe footprint 170 is included in the DUT. Its effect on any signal traveling along the differential transmission line containing the probe footprint 170 on the DUT will remain event after the probe is removed. Accordingly, it is not included in the calibrated parameters of the probe.

[0063] FIGURES 5-11 are network diagrams illustrating the calibration of an apparatus for calibrating a probe and the calibration of a probe using the calibrated apparatus. FIGURE 5 is a series of network diagrams illustrating the calibration of a reference plane according to an embodiment of the invention. Network diagrams 181, 183, 185, and 187 represent a series of measurements and resulting error boxes calculated from those measurements. [0064] In network diagram 181, ports of a T&M instrument are calibrated. In this example, port 1 and port 2 of a VNA are calibrated. Thus, error box 180 represents the contribution of port 1 of the VNA to a measurement to reference plane 182. Similarly, error box 186 represents the contribution of port 2 of the VNA to the measurement to reference plane 184. In this embodiment, the reference planes 182 and 184 can represent coaxial connectors on the cables of the VNA. The calibration can be performed using a short-open- load-reciprocal (SOLR) using coaxial calibration standards and a reciprocal through connection between the cables of the VNA. As a result, error boxes 180 and 186, and reference plans 182 and 184 can be calculated.

[0065] Network diagrams 183, 185, and 187 represent measurements to perform a thru- reflect-line (TRL) calibration. An objective is to calculate the error boxes C 188 and D 190, and establish the reference plane 186. Fixture 22 of FIGURE 1 described above is an example of a structure that can be used. In this example, error box C 188 represents the effect from the reference plane 182 on the connector of the cable of port 1 of the VNA 10 to reference plane 186. Similarly, error box D 190 represents the effect from the reference plane 184 on the connector of the cable of port 2 of the VNA to the reference plane 186. [0066] Network diagram 185 illustrates a line standard used in the calibration. Standard 29 is an example of the structure measured here. Since the sides of standard 29 are substantially identical to the sides of standard 27, error boxes C 188 and D 190 can be used to represent those portions. Reference planes 192 and 194 represent the reference plane 186 of network diagram 183 separated by the middle section of the standard 29. As described above, although one measurement is described here, multiple measurements using multiple length line standards can be made to improve the accuracy of the calibration. [0067] Finally, network diagram 187 represents the measurement of the reflect standards of the TRL calibration. Standard 31 is an example of such a structure. From the measurements represented in network diagrams 183, 185, and 187, in combination with the calibration from network diagram 181, error boxes C 188 and D 190 can be calculated. [0068] Accordingly, in an embodiment, what has been established is a reference plane along the standard used for the measurement in network diagram 183. This location can be the location where a probe is coupled to the standards. In the example of standard 27 of FIGURE 1, we now have a calibrated reference plane 186 corresponding to a physical location where the probe to be tested is to be attached.

[0069] The calibration has characterized the transmission line to the reference plane 192. A probe could be coupled to an open end of the transmission line of standard 31 in order to calibrate the probe. However, the TRL calibration described above is effective only if the transmission line of a probe under test that is coupled to reference plane 192 has a physical structure matching the transmission line used for the standards. For example, if a grounded coplanar waveguide was used as the transmission line, the probe tips could be coupled to an open end of the standard 31 of FIGURE 1. The TRL measurement characterizes the reference plane 192 where the probe is attached. However, since the end of the transmission line of standard 31 is an open, the transition effects between the open end of the transmission line and the probe are artifacts that will not be present in actual usage. For example, the probe can be coupled to a middle of a microstrip line between two devices. This environment does not match the environment in which the probe was calibrated. Accordingly, errors can be introduced into the calibration if a probe is measured when coupled to an open end of a transmission line.

[0070] FIGURE 6 is a network diagram illustrating a three-port parameter matrix representing a probe location and termination of a transmission line of FIGURE 5. The network diagram can represent the thru standard 27. A termination 191 can represent the load 25 coupled to the standard 27. A three-port error box E 198 is introduced between reference planes 192 and 194. Two of the three-ports correspond to the two-ports of the transmission line at the probe location 38. The third port 196 corresponds to the connection of the probe to the transmission line.

[0071] In an embodiment, the error box E 198 represents an infinitesimal point on the transmission line. Accordingly, it can be modeled by the S-parameters described in equation 1, as follows:

[0073] When a probe is coupled to the transmission line at the point represented by the third port 196, it will introduce a discontinuity. However, that discontinuity is attributable to the loading introduced by the probe itself. Those effects are of interest in calibrating out the effect of the probe on a measurement. Since the probe is placed at a known reference plane between reference planes 192 and 194, the S-parameters of the probe can be calculated. [0074] In an embodiment, the three-port error box E 198 is reduced to a two-port error box Er with the port at reference plane 192 being the first port and the previous third port 196 becoming the second port. FIGURES 7-9 illustrate examples of how to reduce the three-port error box E 198 to a two-port error box Er.

[0075] FIGURE 7 is a network diagram illustrating a reflection at a reference plane into the termination in FIGURE 6. A termination T 191 has been placed at reference plane 184. An objective is to calculate the reflection Fin looking into reference plane 194 towards the termination T 191. Using the example above, the VNA port 2 is decoupled and the load 25 is coupled to the standard 27. A measurement can be made from VNA port 1 to calculate the reflection. This measurement is made before the probe 32 is placed on the standard 27. Accordingly, a reflection out of reference plane 182 of FIGURE 6 is measured. Error box C 188 can be extracted from the measurement, leaving the reflection Fin looking into reference plane 194 of FIGURE 7.

[0076] In an embodiment, the termination T 191 can be a characterized termination with the same connector as used on VNA port 2. Accordingly, the measurement of the termination T 191 can be mathematically combined with the error box D 190 to calculate the reflection Fin looking into reference plane 194.

[0077] Regardless of how it is obtained, the reflection Fin looking into reference plane 194 is now characterized. FIGURE 8 is a block diagram illustrating an example of the conversion of a three-port parameter matrix representing a probe into a two-port parameter matrix using the reflection Fin of FIGURE 7. Network diagram 201 represents the three-port error box E 198 with one-port terminated with a load characterized by the reflection Fin. Port 200 on error box E 198 represents the connection at reference plane 192. [0078] Using the reflection Fin, equation 1 can be reduced into a two-port matrix with port 200 as a first port, and port 196 as a second port. Equation 2 represents the resulting two-port matrix.

[0080] Error box Er 202 represents the reduced two-port matrix described in equation 2. sExx represent the three port parameters of the port location error box E 198 where xx represents the input and output ports.

[0081] In an embodiment, the termination T 191 is a termination with the characteristic impedance of the transmission lines used in order to reduce reflections in the measurement of the probe characteristics. Although termination T 191 has been described as a termination, the termination T 191 can be any form of load.

[0082] FIGURE 9 is a block diagram illustrating another example of the conversion of a three-port parameter matrix representing a probe into a two-port parameter matrix using the reflection of FIGURE 7. Once the reflection Fin is obtained, it can be represented by a one- port load 205. Looking into port 204, the reflection will appear as Fin. However, any one- port network can be interpreted as a shunt on a transmission line where the other port 206 of the transmission line is terminated with an open. Accordingly, the one-port load with the reflection Fin can be converted into two-port parameters 207 with ports 204 and 206. Since the connection point of the probe without the probe being attached is modeled as an open on a third port, the two-port parameters 207 with ports 204 and 206 correspond to the reduced two-port error box Er 202 with port 200 at reference plane 192 and port 196 at the probe. [0083] FIGURE 10 is a network diagram illustrating a probe characterization setup according to an embodiment of the invention. Using FIGURE 1 as an example, the probe 32 has been coupled to the standard 27 and the VNA 10 port 2. Error box F 214 represents the response of the probe 32. Error box G 216 represents any adapter between the probe and the VNA port 2. Termination T 191 represents the load 25 coupled to an end of the standard 27. Thus, in an embodiment, this network diagram corresponds to the physical configuration of the probe characterization setup.

[0084] FIGURE 11 is a network diagram illustrating the probe characterization setup of FIGURE 10 with a two-port parameter matrix substituted for the probe location. Taking the network diagram of FIGURE 10 and substituting the error box Er 202 of FIGURE 8, FIGURE 11 is created characterizing the path in FIGURE 1 from VNA port 1 , through the standard 27, past the probe footprint 38, into the probe 32, through the probe adapter 20 and into the VNA 10 port 2. The probe adapter 20 can be characterized in a separate measurement. Since the parameters of the error box F 214 for the probe are the only unknown, parameters for the probe can be de-embedded from the measurement. [0085] FIGURE 12 is a series of network diagrams illustrating the calibration of a reference plane on a differential transmission line according to an embodiment of the invention. Network diagram 301 represents the calibration of reference planes 308 and 310 for a differential transmission line setup. Error boxes 300, 302, 304, and 306 represent the characterizations of the contributions of each of the four ports.

[0086] Although error boxes for a VNA with four ports have been illustrated, the four-port parameters can be generated using a two-port network analyzer. For example, two-port measurements can be made between the various combinations of ports 1-4. Those measurements can be converted into a four-port measurement. Similarly, other four-port measurements described in this description can be made using two-port T&M instruments. [0087] Network diagrams 303, 305, and 307 represent measurements made on differential thru, line, and reflect standards similar to 120, 122, and 126 of FIGURE 3. Similar to the generic transmission line example of FIGURE 5, a TRL calibration is made to establish reference planes 318 and 320. Accordingly, error boxes C 314 and D 316 can be calculated from the measurements, characterizing two sections of the differential transmission line. For example error box C 314 and D 316 can correspond to sections 130 and 132 of FIGURE 3, respectively.

[0088] Once obtained, the four-port parameters can be separated into differential-mode parameters and common mode parameters. Equation 3 describes the differential-mode and common mode components of a four-port matrix.

Diff .ModeStim. ComModeStim. sDD sDD,, sDC, sDC, 12 SDD₁ sDD_l2 sDC_u sDC_l2

Diff ModeStim. sDD 21 sDD SDC₂₁ sDC sDD_n sDD_nn sDC_2l sDC₂₂ sCD, sCD ₁₂ SCC_U sCC 12 sCD_n sCD_l2

ComModeStim. sCD₀, sCD_nn sCC₀, sCC 'y22 _. sCD_Oλ sCD, 22_. sCC₇, sCCy 22_.

(3)

[0089] Parameters described as sDD are differential responses from a differential stimulation. Similarly, sCC parameters are common mode responses from common mode stimulation. Parameters sCD and sDC represent cross-mode responses. In an embodiment, the differential transmission lines, differential standards, or other differential apparatuses can be substantially top-to-bottom symmetric. That is, differential apparatuses can be substantially symmetric about a surface dividing the two single ended halves of the differential apparatus. Accordingly, the cross-mode responses can be substantially zero. [0090] In an embodiment, the four-port parameters can be measured. Then each set of four- port parameters for each standard is converted into differential-mode parameters and common mode parameters. Accordingly, the TRL calibration can be performed for each mode resulting in a differential TRL calibration and a common mode TRL calibration. [0091] In another embodiment, a four-port TRL calibration can be calculated from the four- port parameters. The four-port TRL calibration can be separated into a differential TRL calibration and a common mode calibration.

[0092] FIGURE 13 is a network diagram of a common mode component of the calibration of FIGURE 12. Error box A 322 represents the common mode contribution of the VNA ports 1 and 2 represented by error boxes 300 and 302 in FIGURE 12. In this description, VNA ports 1 and 2 can be referred to as port 1 and VNA ports 3 and 4 referred to as port 2 when referring to a differential or common mode component. Error box B 328 represents the common mode contribution of the VNA ports 3 and 4 represented by error boxes 304 and 306 in FIGURE 12. Error boxes C 324 and D 326 represent the common mode contribution of error boxes C 314 and D 316 of FIGURE 12.

[0093] FIGURE 14 is a network diagram of a differential-mode component of the calibration of FIGURE 12. Similar to the common mode component described in FIGURE 13, error boxes A 330, B 338, C 322, and D 334 correspond to the differential component of the VNA ports 1 and 2, VNA ports 3 and 4, section 130, and section 132. It should be noted that in an embodiment, in both FIGURE 13 and 14, the resulting error boxes are in the form of two-port networks.

[0094] FIGURE 15 is a network diagram illustrating a termination of a differential transmission line according to an embodiment of the invention. Similar to network diagram 303 of FIGURE 12, FIGURE 15 is related to the physical structure with a load replacing connections for VNA ports 3 and 4, similar to the generic transmission line example of FIGURE 6. Error boxes Tl 340 and T2 342 represent the loads coupled to the single-ended ports of the differential transmission line. For example, the loads can be coupled to the ports 142 and 144 of differential port 102 in FIGURE 3. [0095] FIGURE 16 is a network diagram illustrating a common mode component of the calibration of FIGURE 15. FIGURE 17 is a network diagram illustrating a common mode component of the calibration of FIGURE 15. Similar to the decomposition of the four-port measurements described above, the two-port measurements of the terminated differential transmission line can be converted into error boxes described in the differential and common mode components. Error box Tcm 344 represents the common mode component and error box Tdm 354 represents the differential-mode component.

[0096] Similar to the generic transmission line case described above, a reflection can be measured or calculated from reference plane 312 into the termination. Since there are both differential and common mode terminations, there are now differential and common mode reflections at the reference plane 312.

[0097] FIGURE 18 is a network diagram illustrating a differential probe characterization setup according to an embodiment of the invention. FIGURE 18 corresponds to the physical layout when a differential probe is coupled to the calibration apparatus. In an embodiment, the differential probe has a single-ended output. Accordingly, only one VNA port, VNA port 3 represented by error box B 304 is needed. Similar to the generic transmission line operation, error box G 364 represents any adapter needed to couple the probe to the VNA and error box F 362 represents the probe. Although a differential to single ended probe is used as an example, the probe can have differential inputs and differential outputs, In such an embodiment, VNA port 4 306 could be used with VNA port 3 304 for differential measurements at the output of the probe.

[0098] In this embodiment, a three-port measurement is made using VNA ports 1, 2, and 3. Again, as with the four-port measurements, the three-port measurement can be performed with a two-port VNA with the results combined together to form the three-port parameters. Error box E 357 represents the probe footprint 131 of FIGURE 3.

[0099] FIGURE 19 is a network diagram illustrating a common mode component of the network diagram of FIGURE 18. From the three-port measurements between the differential port at reference plane 308 and the probe output port at reference plane 358 or 360, differential and common mode parameters can be extracted. In this embodiment, as the measurement was a differential to single-ended measurement, error boxes G 364 and B 304 can remain the same.

[00100] In contrast, the differential probe location error box E 357 can be divided into common mode and differential-mode components. In an embodiment, for the common mode, the error box Er 370 can be created as described with reference to the generic transmission line case. For example, the common mode component of differential probe location error box E 357 can be modeled as a three-port network. As the common mode reflection at reference plane 312 was determined as described in FIGURE 16, the common mode component of differential probe location error box E 357 can be converted into two- port common mode probe location error box Er 370. Accordingly, as all error boxes besides the differential probe common mode error box F 368 are known, the common mode response of the differential probe can be de-embedded from the measurements. [00101] FIGURE 20 is a network diagram illustrating a differential-mode component of the network diagram of FIGURE 18. Similar to the common mode described with reference to FIGURE 19, the differential probe location differential-mode error box Er 374 can be calculated from the reflection from reference plane 312 described with reference to FIGURE 17. Accordingly, as all error boxes besides the differential probe differential-mode error box F 372 are known, the differential-mode response of the differential probe can be de- embedded from the measurements.

[00102] Although reference has been made to differential transmission lines in FIGURE 3 for describing the calibration of a differential probe, that particular structure is not required. Any differential transmission line structure can be used.

[00103] FIGURE 21 is a flowchart illustrating a method of calibrating a probe according to an embodiment of the invention. In an embodiment, a method of calibrating a probe includes calibrating a reference plane along a transmission line having a first port and a second port in 400, the transmission line having a first port and a second port, calculating a reflection from the reference plane into the second port with the second port coupled to a load in 402, applying the probe to a physical location along the transmission line substantially corresponding to the reference plane in 404, measuring a probe response between the first port of the transmission line and an output of the probe in 406, and de-embedding calibrated probe parameters using the probe response and the reflection from the reference plane into the second port in 408.

[00104] Calibrating a reference plane along a transmission line in 400 can be performed by a variety of techniques. For example, calibrating the reference plane in 400 can include performing a TRL calibration using the transmission line as a through calibration standard. As described above the reference plane can correspond to the physical location where a probe will be placed on the transmission line.

[00105] Calculating a reflection from the reference plane into the second port with the second port coupled to a load in 402 can include measurements, calculations, or a combination of such techniques. For example, as described above, the load can be characterized. Using the calibration of the reference plane and the corresponding error boxes, the reflection from the reference plane can be calculated. Alternatively a measurement can be made of the reflection into the first port of the transmission line. Again, using the error boxes from the calibration of the reference plane, the reflection from the reference plane can be calculated.

[00106] Once the measurements, calibrations, calculations, or the like have been made on the transmission line without the probe, the probe can be applied to the physical location along the transmission line substantially corresponding to the reference plane in 404. As described above, the reference plane can correspond to the physical location where the probe was applied. However, mechanical inaccuracies, mechanical measurement errors, probe imperfections, or the like can cause the actual probe location to be different from the reference plane on the transmission line. Such variations are included within a physical location substantially corresponding to the reference plane. Thus, a probe with slight mechanical imperfections that is placed slightly off from the actual reference plane on the transmission line will still be considered as substantially corresponding to the reference plane.

[00107] As described above, a two-port measurement can be made on a single-ended transmission line. In another example, a three-port measurement can be made on a differential transmission line with the probe having a single-ended output. Each of these are examples of measuring a probe response between the first port of the transmission line and an output of the probe in 406. In addition, although the measurement has been described as a measurement of a probe response, it can include the response of the transmission line, a VNA, or other components of the test setup. In another embodiment, the measurement can have characterizations of portions of the test setup removed from the measurement. For example, the error boxes describing the VNA can be removed from the measurement by the VNA during the measurement of the probe response.

[00108] As described above, once the transmission line and reflection into the reference plane are characterized and the probe response is measured, the calibrated probe parameters can be de-embedded in 408. In an embodiment, the calibrated probe parameters are extracted from the measured probe response using the probe response and the reflection from the reference plane into the second port in 408.

[00109] FIGURE 22 is a flowchart illustrating an example of calibrating a reference plane in the method of FIGURE 21. In an embodiment, the method includes calibrating a first port and a second port on a T&M instrument in 410. As described above, this can be a SOLR calibration. Short-open-load-termination (SOLT), TRL, line-reflect-line (LRL) calibration, or the like that can calibrate the ports of the T&M instrument can be used in 410. [00110] As described above, a TRL calibration can be performed using the transmission line as the thru calibration standard. Performing the TRL calibration in 412 can include measurements on other standards as well. For example, multiple line standards can be used as desired for the desired accuracy.

[00111] In 414, error parameters from the first port of the T&M instrument to the reference plane are calculated in response to the TRL calibration. In 416, error parameters from the second port of the T&M instrument to the reference plane are calculated in response to the TRL calibration. The error parameters can be the exact error boxes described above. In another example, the error parameters can be in a different format and converted as needed when de-embedding the probe response, calculating the reflection from the reference plane, or the like. Accordingly, contributions of the transmission line to probe measurement can be de-embedded.

[00112] FIGURE 23 is a flowchart illustrating an example of calculating the reflection in the method of FIGURE 21. This example includes coupling the load to the second port of the transmission line in 418 and measuring a reflection from the first port of the transmission line into the load in 420. As the transmission line and the reference plane have been characterized, the reflection from the reference plane into the second port can be de- embedded from the reflection from the first port of the transmission line into the load in response to the calculation of the reference plane in 422.

[00113] FIGURE 24 is a flowchart illustrating another example of calculating the reflection in the method of FIGURE 21. In this example, the reflection of the load is characterized in 424. This characterization can be performed at the time of the calibration of the probe or at an earlier time. Regardless of when it is made, the reflection from the reference plane into the second port can be calculated in response to the reflection of the load and the calculation of the reference plane in 426.

[00114] FIGURE 25 is a flowchart illustrating an example of converting a characterization of a probe location in the method of FIGURE 21. An embodiment includes calculating a three- port parameter matrix for the physical location of the transmission line in 428. The calculation of the three-port parameter matrix can be accomplished through a variety of techniques. For example, an electromagnetic simulation of the structure of the physical location can be performed. From the simulation, the three-port parameter matrix can be generated.

[00115] As described above, the three-port parameter matrix can be converted into a two- port parameter matrix in 430 using the reflection from the reference plane into the second port as a load on a port of the three-port parameter matrix. Accordingly, the probe location can be characterized at the reference plane. The two-port parameter matrix can be used to de-embed the calibrated probe response in 432.

[00116] FIGURE 26 is a flowchart illustrating an example of characterization of the probe location in the method of FIGURE 25. As described above, a model can be made of the probe location. In particular, if there is a structure that will be on a DUT to be tested using the calibrated probe, the structure can be calibrated out so that the probe response includes the effects of the probe and not the remaining structure. Accordingly, the physical location along the transmission line can be modeled in 434. Modeling can include any variety of representation. As described above, a EM simulation can be used to generate the model. In another example, a lumped component model can be generated for the physical location. Any such representation can be used as a model.

[00117] From the model, the three-port parameter matrix describing the physical location can be generated in 436. The calibration of the reference plane can establish error boxes up to the interface to the physical location. The model of the physical location characterizes the probe footprint. Accordingly, the calibration apparatus can be characterized to the probe location. [00118] FIGURE 27 is a flowchart illustrating another example of characterization of the probe location in the method of FIGURE 21. In this example, as described above, the reflection from the reference plane into the second port is converted into a shunt impedance in a two-port parameter matrix in 438. Accordingly, the calibrated probe response can be de- embedded in response to the two-port parameter matrix in 440.

[00119] FIGURE 28 is a flowchart illustrating an example of the creation of common mode and differential-mode parameters in the method of calibrating a probe of FIGURE 21. As described above a transmission line used in the calibration can be a differential pair transmission line. The first port of the differential pair transmission line can include first and second differential ports. Similarly, the second port can include third and fourth differential ports. Accordingly, with a differential transmission line, multiple differential transmission line standards can be measured in 442 where the differential pair transmission line is a through standard of the differential transmission line standards. [00120] As described above, the measurements of the differential transmission line standards can be converted into differential-mode parameters and common mode parameters in 444. Accordingly, from the differential transmission line standards, a differential-mode reference plane and a common mode reference plane can be calibrated in response to the differential- mode parameters and common mode parameters in 446. This calibration of the differential and common mode reference planes includes the characterization of the differential and common mode components of the calibration apparatus.

[00121] FIGURE 29 is a flowchart illustrating characterization of the termination in the method of FIGURE 28. In an embodiment, the error box for the probe location is characterized for both the differential-mode and common mode. Accordingly, as described above, a differential-mode reflection from the differential-mode reference plane into a differential load can be calculated in 450. In addition, a common mode reflection from the common mode reference plane into the differential load can be calculated in 452. Accordingly, the differential-mode and common mode characterizations of the probe location error box can be generated.

[00122] FIGURE 30 is a flowchart illustrating an example of the de-embedding of common mode and differential-mode probe parameters in the method of FIGURE 28. In an embodiment using a differential pair transmission line, a probe response between the first port and the output of the probe can be measured in 454. As described above, this measurement can be a three-port measurement using the two single-ended ports of the differential pair transmission line and the output port of the probe. Accordingly, the probe response can be this three-port measurement.

[00123] As described above, the probe response can include measurements of other components in addition to the probe. Regardless of the contribution, in 456, the probe response is converted into a common mode probe response and a differential-mode probe response. Accordingly, since the differential and common mode error boxes have been characterized and a differential and common mode measurement is available, the common mode probe parameters of the calibrated probe parameters can be de-embedded in response to the common mode probe response in 458, and the differential-mode probe parameters of the calibrated probe parameters can be de-embedded in response to the differential-mode probe response in 460.

[00124] FIGURE 31 is a flowchart illustrating the calibration of an apparatus for calibrating a probe. An embodiment includes calibrating an apparatus for calibrating a probe. Calibrating the apparatus includes measuring a plurality of calibration standards in 470 where the calibration standards include a transmission line on the apparatus. This measurement can include calibration of a T&M instrument, measuring the calibration standards with the T&M instrument, or the like.

[00125] From the measurements of the calibration standards, a first section of the transmission line can be characterized in 472. The first section of the transmission line is the section between a first port of the transmission line and a reference plane along the transmission line. In addition, a second section of the transmission line can be characterized in 474 using the measurements of the calibration standards where the second section is between a second port of the transmission line and the reference plane.

[00126] In addition, the calibration of the apparatus can include coupling a load to the second port of the transmission line in 476 and characterizing a reflection from the reference plane into the load in 478. As a result, the components of the apparatus from the reference plane to the load can be characterized.

[00127] Using that characterization of the reflection, a probe location on the transmission line substantially corresponding to the reference plane can be characterized in 480. Accordingly, the apparatus is characterized for the calibration of a probe. The components of the apparatus are characterized so that when a probe is coupled to the probe location, the components of the apparatus can be de-embedded from a measurement of the response of the probe.

[00128] In an embodiment, three-port parameters for the physical location on the transmission line can be calculated. The three-port parameters can be converted into two-port parameters using the reflection from the reference plane into the load on a port of the three- port parameter matrix. In another embodiment, the reflection from the reference plane into the load can be converted into a shunt impedance represented by a two-port parameter matrix. Accordingly, the probe location can be characterized such that it may be de-embedded from a measurement of a probe.

[00129] An embodiment includes a system for calibrating a probe. The system includes a test and measurement instrument and multiple calibration standards. The T&M instrument includes means for calibrating the reference plane along the transmission line from measurements of the calibration standards; means for calculating a reflection from the reference plane into the second port with the second port coupled to a load; means for measuring a probe response between the first port of the transmission line and an output of the probe with the probe coupled to the probe footprint; and means for de-embedding calibrated probe parameters using the probe response and the reflection from the reference plane into the second port.

[00130] The means for calibrating the reference plane includes any processor configured to perform a calibration in response to measurements. The means for calculating a reflection from the reference plane into the second port with the second port coupled to a load includes any processor configured to calculate a reflection from a measurement. The means for measuring a probe response between the first port of the transmission line and an output of the probe with the probe coupled to the probe footprint includes any signal generation and measurement devices capable of measuring the probe response. The means for de- embedding calibrated probe parameters includes any processor configured to manipulate measurements and calibrations to extract, de-embed, or otherwise isolate parameters from measurements.

[00131] A processor can be a variety of devices. Such devices include general purpose processors, special purpose processors, application specific integrated circuits, programmable logic devices, distributed computing systems, or the like. In addition, a processor may be any combination of such devices. In addition, the processors described above for various means can be independent processors, the same processor configured to execute a variety of code, or the like.

[00132] Furthermore, an embodiment can include means for performing any of the above described operations. Examples of such means include the devices, systems, apparatus, configurations, or the like described above.

[00133] Another embodiment includes an article of machine readable code embodied on a machine readable medium that when executed, causes the machine to perform any of the above described operations. As used here, a machine is any device that can execute code. Microprocessors, programmable logic devices, multiprocessor systems, digital signal processors, personal computers, or the like are all examples of such a machine. [00134] Although particular embodiments have been described, it will be appreciated that the principles of the invention are not limited to those embodiments. Variations and modifications may be made without departing from the principles of the invention as set forth in the following claims.

Claims

1. A system for calibrating a probe, comprising: a test and measurement instrument; and a plurality of calibration standards including a load and a transmission line standard having a first port, a second port, and a probe footprint to receive the probe located at a physical location along the transmission line substantially corresponding to a reference plane; in which the test and measurement instrument is configurable to: calibrate the reference plane along the transmission line from measurements of the calibration standards; calculate a reflection from the reference plane into the second port with the second port coupled to a load; measure a probe response between the first port of the transmission line and an output of the probe with the probe coupled to the probe footprint; and de-embed calibrated probe parameters using the probe response and the reflection from the reference plane into the second port.

2. The system of claim 1 , in which the test and measurement instrument is further configurable to: measure a reflection from the first port with the load coupled to the second port; and calculate the reflection from the reference plane into the second port in response to the measured reflection from the first port.

3. The system of claim 1 , in which the test and measurement instrument is further configurable to: convert the reflection from the reference plane into two-port parameters; and de-embed calibrated probe parameters using the two-port parameters.

4. An apparatus for calibrating a differential probe, comprising: a first differential transmission line having a first section and a second section; a second differential transmission line substantially identical to the first section of the first differential transmission line and having an open on one end; a third differential transmission line substantially identical to the second section of the first differential transmission line and having an open on one end; and at least one additional differential transmission line, each additional differential transmission line having: a first section substantially identical to the first section of the first differential transmission line; a second section substantially identical to the second section of the first differential transmission line; and a third section disposed between the first section and the second section.

5. The apparatus of claim 4, in which the first, second third, and additional differential transmission lines are at least one of coplanar waveguides, grounded coplanar waveguides, and coupled microstrip.

6. The apparatus of claim 4, further comprising: a fourth differential transmission line including: a probe footprint to receive the differential probe; a first section substantially identical to the first section of the first differential transmission line; and a second section substantially identical to the second section of the first differential transmission line.

7. A method of calibrating a probe, comprising: calibrating a reference plane along a transmission line, the transmission line having a first port and a second port; calculating a reflection from the reference plane into the second port with the second port coupled to a load; applying the probe to a physical location along the transmission line substantially corresponding to the reference plane; measuring a probe response between the first port of the transmission line and an output of the probe; and de-embedding calibrated probe parameters using the probe response and the reflection from the reference plane into the second port.

8. The method of claim 7, further comprising: performing a TRL calibration using the transmission line as a through calibration standard.

9. The method of claim 7, further comprising: calibrating a first port and a second port on a test and measurement instrument; performing a TRL calibration with the first port and the second port of the test and measurement instrument using the transmission line as a through calibration standard; calculating error parameters from the first port of the test and measurement instrument to the reference plane in response to the TRL calibration; and calculating error parameters from the second port of the test and measurement instrument to the reference plane in response to the TRL calibration.

10. The method of claim 7, further comprising: coupling the load to the second port of the transmission line; measuring a reflection from the first port of the transmission line into the load; and de-embedding the reflection from the reference plane into the second port from the reflection from the first port of the transmission line into the load in response to the calculation of the reference plane.

11. The method of claim 7, further comprising: characterizing a reflection of the load; and calculating the reflection from the reference plane into the second port in response to the reflection of the load and the calculation of the reference plane.

12. The method of claim 7, further comprising: calculating a three-port parameter matrix for the physical location the transmission line; converting the three-port parameter matrix into a two-port parameter matrix using the reflection from the reference plane into the second port as a load on a port of the three-port parameter matrix; and de-embedding the calibrated probe response in response to the two-port parameter matrix.

13. The method of claim 12, further comprising: modeling the physical location along the transmission line; generating the three-port parameter matrix from the model of the physical location.

14. The method of claim 7, further comprising: converting the reflection from the reference plane into the second port into a shunt impedance in a two-port parameter matrix; and de-embedding the calibrated probe response in response to the two-port parameter matrix.

15. The method of claim 7 in which the transmission line is a differential pair transmission line, and the first port includes first and second differential ports and the second port includes third and fourth differential ports, the method further comprising: measuring a plurality of differential transmission line standards, the differential pair transmission line being a through standard of the differential transmission line standards; converting the measurements of the differential transmission line standards into differential-mode parameters and common mode parameters; calibrating a differential-mode reference plane and a common mode reference plane in response to the differential-mode parameters and common mode parameters.

16. The method of claim 15 , further comprising : calculating a differential-mode reflection from the differential-mode reference plane into a differential load; and calculating a common mode reflection from the common mode reference plane into the differential load.

16. The method of claim 7 in which the transmission line is a differential pair transmission line, and the first port includes first and second differential ports and the second port includes third and fourth differential ports, the method further comprising: measuring a probe response between the first port and the output of the probe; converting the probe response into a common mode probe response and a differential- mode probe response; de-embedding common mode probe parameters of the calibrated probe parameters in response to the common mode probe response; and de-embedding differential-mode probe parameters of the calibrated probe parameters in response to the differential-mode probe response.

17. A method of calibrating an apparatus for calibrating a probe, comprising: measuring a plurality of calibration standards, the calibration standards including a transmission line on the apparatus; characterizing a first section of the transmission line using the measurements of the calibration standards, the first section being between a first port of the transmission line and a reference plane along the transmission line; characterizing a second section of the transmission line using the measurements of the calibration standards, the second section being between a second port of the transmission line and the reference plane; coupling a load to the second port of the transmission line; characterizing a reflection from the reference plane into the load; characterizing a probe location on the transmission line using the characterization of the reflection from the reference plane into the load, the probe location substantially corresponding to the reference plane.

18. The method of claim 17 , further comprising : calculating three-port parameters for the physical location on the transmission line; and converting the three-port parameters into two-port parameters using the reflection from the reference plane into the load on a port of the three-port parameter matrix.

19. The method of claim 17 , further comprising : converting the reflection from the reference plane into the load into a shunt impedance represented by a two-port parameter matrix.

20. A system for calibrating a probe, comprising: a plurality of calibration standards including a load and a transmission line standard having a first port, a second port, and a probe footprint to receive the probe located at a physical location along the transmission line substantially corresponding to a reference plane; and a test and measurement instrument including: means for calibrating the reference plane along the transmission line from measurements of the calibration standards; means for calculating a reflection from the reference plane into the second port with the second port coupled to a load; means for measuring a probe response between the first port of the transmission line and an output of the probe with the probe coupled to the probe footprint; and means for de-embedding calibrated probe parameters using the probe response and the reflection from the reference plane into the second port.

21. The system of claim 20, the test and measurement instrument further comprising: means for measuring a reflection from the first port with the load coupled to the second port; and means for calculating the reflection from the reference plane into the second port in response to the measured reflection from the first port.

22. The system of claim 20, the test and measurement instrument further comprising: means for converting the reflection from the reference plane into two-port parameters; and means for de-embedding calibrated probe parameters using the two-port parameters.