Introduction

Simple network analyzer calibrations based on lumped elements are quite common for
everyday on-wafer scattering parameter measurements. Such “working” calibrations are easy to
perform and often give good results. However, you may wish to check the accuracy of one of
your working calibrations for a number of reasons. The reference impedance of the working
calibration may be set to the load impedance cf a smail lumped resistor and may not be a constant,
real 50 &. The reference plane position of the working calibration may be set by reflective
terminations whose reactance may not be accurately known. The devices may be embedded in
microstrip lines with via-hole or other transitions which are not accounted for by the working
calibration. Finally, it is possible for some lumped element calibrations to be inconsistent. For
instance, this might occur because of incorrect standard definitions (such as setting '=1 for an
open which radiates energy), performing a Line-Reflect-Match (LRM) calibration in which there
is coupling between the match standards, or using non-identical match standards in an LRM
calibration.

Even if you are presently calibrating to 50 Q using NIST’s multiline Thru-Reflect-Line
(TRL) calibration method [1] with reference impedance correction [2] you may be interested in
the techniques described in this manual. For example, you may wish to quantify the effects of
random errors due to probe placement or loose cables.

This manual describes a method for determining the accuracy of on-wafer calibrations and
includes instructions for running the NIST/Industrial Consortium software to perform the required
analysis. The verification procedure is described in [3], which we highly recommend reading.
Applications of the verification procedure are described in (3], [4], and [5]. Applications of the
method to the determination of characteristic impedance on lossy substrates are described in [6]
and [7]. Application to calibrations on lossy substrates are described in (8].

Procedure

The calibration comparison procedure is straightforward. First you must have access to artifacts
for a “benchmark” TRL calibration. Ideally these calibration artifacts should consist of a set of
lines which are identical to the lines to which your device is connected. This is best accomplished
by fabricating the calibration artifacts and the lines on the same wafer.

If you have not fabricated a set of lines on your wafer, it is possible to obtain a set of standard
lines from NIST. These lines are fabricated to standard dimensions (9] and care has been taken
to control linewidths and materials. However, even if you try to embed your devices in similar
lines, the NIST lines may differ in a number of ways from those on your wafer; caution is always
advisable when calibrating with a set of standard lines which differ from those on your test wafer.

Before beginning, you must know the capacitance per unit length C of the lines in your benchmark
standard set. If you are using NIST lines, C is printed on the glass slide to which the standard set

August 2, 1995 2.


whether the calibration standards effectively cover the frequency band desired, as discussed
in reference 2. Reference 2 also describes the normalized standard deviation, a second criteria
for judging the error sensitivity of the TRL calibrations. This quantity reflects the relative
error expected for the calibration. A value of 1 corresponds to the error of a standard TRL
calibration with a single line one-quarter wavelength longer than the thru line. When muitipie
lines are used this quantity may be significantly less than one, indicating the improvement
gained from the redundant measurements.

If the reference impedance was set real in a TRL calibration the characteristic
impedance may be plotted or saved. The characteristic impedance of the line is determined
from the propagation constant and the capacitance of the line.

The output section contains the algorithms described in (5] for determining the excess
probe-tip capacitance due by differences in the calibration and measurement substrates.

The output section contains the S-parameters of the adapter, test fixture, or probe
heads determined by the de-embedding algorithm.

The S-parameters stored on disk can also be loaded and, if desired, corrected in the
output section. If the reference impedance of the S-parameters is known, impedance and
admittance parameters may also be plotted or stored to disk. S-parameters stored in BDAT
format are ordered with frequency (in GHz) in the first column followed by the real and
imaginary parts of S,,, S,,, S,, and §, . S-parameters stored in ASCII, HPUX, or DOS
formats comply with the TOUCHSTONE compatible ordering frequency, S,,, S.,, S12, Sp.

Once the calibration data has been viewed or stored, press the Data Menu softkey to
display more data, press the Deembed Menu softkey to return to the main menu, or press
the “Q” or “X” key to terminate the program. It is also possible to get directly to the output
section from the main menu by pressing the OUTPUT softkey. At this point, no data is
available. However, you may make use of the features of the output section after ioading data
from disk using the Get data softkey.

References

1. R. B. Marks, "A muitiline method of network analyzer calibration," IEEE Trans.
Microwave Theory Tech., Vol. 39, pp. 1205-1215, July 1991.

2. R. B. Marks and D. F, Williams, “Characteristic Impedance Determination using
Propagation Constant Measurement,” JEEE Microwave and Guided Wave Letters 1, pp. 141-
143, June 1991.

3. D. F. Williams and R. B. Marks, “On-Wafer Impedance Measurement on Lossy
Substrates,” IEEE Microwave and Guided Wave Letters 4, pp. 175-176, June 1994.

August 1, 1995 14


imp+off

benchmark calibration with a 50 Q reference impedance and a
reference plane which has been translated to a fixed position which

best matches the average reference plane position of the working
calibration.

Worst case deviation after the reference impedance and reference
plane of the benchmark calibration have been adjusted to be as close
as possible to those of the working calibration. The plotted
quantities are the (Zl) or eZ, 1,) of [3]. The working
calibration is compared to the benchmark calibration with a
reference impedance which has been adjusted to best match the
reference impedance of the working calibration at each frequency
and a reference plane which has been translated to a fixed position
which best matches the average reference plane position of the
working calibration.

Fit ZO Worst case deviations of measurements of passive devices from the benchmark TRL
calibration. The 50 ohm and All options, in which worst case deviations from the 50 Q
benchmark calibration are determined, are usually more informative than this option. After
pressing this softkey you will be given options for data output format. (See Viewing,
Storing, or Plotting the Data below.) The data will be labeled as follows:

Label
initial

August 2, 1995

Data description.

Direct comparison of calibrations. The plotted quantities are the (Z,,/,)
or & (2/0 of [3]. The working calibration is compared to the

‘k calibration with the reference impedance equal to the
characteristic impedance of the lines in the benchmark calibration
set and a reference plane which has not been translated.

Worst case deviation after an impedance transformation to the best
estimate of the reference impedance of the working calibration is
applied to the benchmark calibration. The plotted quantities are the e(Z, J,)
or €4(Z,1,) of [3]. The working calibration is compared to the
benchmark calibration with a reference impedance which has been
adjusted at each frequency to best match that of the working
calibration and a reference plane which has not been translated.
This parameter is the same quantity labeled “impedance” obtained
by pressing the Fit 50 softkey.

Worst case deviation after reference plane of benchmark calibration
is adjusted to be as close as possible to reference plane of the
working calibration. The plotted quantities are the ¢(Z,,/) or
€fZy!) of [3]. The working calibration is compared to the


DESCRIPTION
> 18.685 am CPW Line

Line Lis

6.565 mm CPW Line Line Ls
3.2686 am CPW Line Line 3
2.135 am CPW Line Line 3
Ed 8.6 sm CPW Thru Thru THRU
a -@.85 mm Offset Short Refl/short SHORT

OSs een MRED ESN SBIR SEND
nannies
Cal Set none

Est. EFF. Olelectric Const. 6.95
Mass Storage Path C:\HTB386\PROG\
Ref. Plane: +@.6686 cs Output Reference Impedance: 28 of line

NIST CPW Standard Sot Hit SHIFT(KS] or SHIFT(F&] for help

SHUE menu
CREEN \ HELP 3

LOIVPE \érerfieepb bet PELE NM\LENGTE
T\ Cat SETBRIVPE \ DESCRTPIRLPS \PEF Plant i

Figure 1. The menu as seen on the screen after loading and running MultiCal. The standards
and their various descriptors are listed at the top of the menu. The calibration type, effective
dielectric constant, mass storage device, and other parameters defining the calibration are
listed below. The softkey definitions are displayed at the bottom of the screen.

August 1, 1995 5


is mounted. If you are using your own lines, C must be measured. We recommend using the
NIST procedures found in [10] to determine C for your lines. The programs CAP and
CAP NIST, available from NIST, handle the calculations and make the determination of C easy.

Once you have selected the benchmark standard set and know the capacitance per unit length of
the lines, simply perform the following steps:

3)
2

3)

4

Perform the working calibration whose accuracy you wish to verify.

Load and run program MultiCal. Load the menu appropriate to your benchmark
calibration off of the disk by pressing the GET\SAVE menu softkey. Chose “TWO-TIER
TRL” by pressing the CAL TYPE softkey. Set the first tier cal set on the main menu to
the cal set you used for your working calibration by pressing the CAL SET softkey. Set
the second tier cal set to any other unused cal set in the analyzer.

The benchmark may be performed either with respect to the characteristic impedance of
the line or with respect to 50 Q. If you select a 50 Q calibration, be sure to input the
correct capacitance and set the reference impedance to 50 Q. The capacitance is modified
by pressing the Zref softkey to enter the capacitance menu, and then pressing the SET C
softkey to modify the capacitance. The reference impedance can now be set to 50 Q by
pressing the Zref softkey. After editing the sub-menu is exited by pressing the RETURN
softkey to return to the main menu. After editing the menu, you may wish to save the
menu by pressing the SAVE menu softkey.

Contact each standard listed in MultiCal’s main menu file list and press the MEAS
softkey. MultiCal will take data for each standard and store it on the disk in the
appropriate disk file.

After storing all of the data on the disk, press the DEEMBED softkey. The program will
calculate the error coefficients, put them in the analyzer; and place you in the output
section.

Press the Data Menu softkey and store the following information in the BDAT format:
a) S-parameters of probe 1 [store in file S P1].
b) S-parameters of probe 2 [store in file S P2].
3) Effective dielectric constant (mean) [store in file EPS].

Exit MultiCal and load and run program VERIFY. After running VERIFY you will enter
the main menu. Enter the capacitance of the lines used in the benchmark calibration by
pressing the cap softkey. At this point you may also change file names and alter various
parameters of the analysis (see section on main menu below). If you have followed the
above procedure, however, you need not modify the menu. Simply press the start softkey
and the analysis will begin.

August 2, 1995 3


References

1. R. B. Marks, "A muitiline method of network analyzer calibration," JEEE Trans.
Microwave Theory Tech., Vol. 39, pp. 1205-1215, July 1991.

2. R. B. Marks and D. F. Williams, “Characteristic Impedance Determination using
Propagation Constant Measurement,” JEEE Microwave and Guided Wave Letters 1, pp. 141-
143, June 1991.

3. D. F. Williams, R. B. Marks, and A. Davidson, “Comparison of On-wafer Calibrations,”
38th ARFTG Conf. Dig. (San Diego CA), Dec. 5-6, 1991.

4. D. F. Williams and R. B. Marks, “Calibrating On-Wafer Probes to the Probe Tips,” 40th
ARFTG Conference Digest, pp. 136-143, Dec. 1992.

5. R. B. Marks and D. F. Williams, “Verification of Commercial Probe-Tip Calibrations,”
42nd ARFTG Conference Digest, pp. 37-44, Dec. 1993.

6. D. F. Williams and R. B. Marks, “Accurate Transmission Line Characterization,” JEEE
Microwave and Guided Wave Letters 3, pp. 247-249, Aug. 1993.

7. R. B. Marks and D. F. Williams, “Accurate Experimental Characterization of
Interconnects,” IEEE Transactions on Components, Hybrids, and Manufacturing Technology
15, pp. 601-602, Aug. 1992.

8. D. F. Williams and R. B. Marks, “On-Wafer Impedance Measurement on Lossy
Substrates,” IEEE Microwave and Guided Wave Letters 4, pp. 175-176, June 1994.

9. D. F. Williams and R. B. Marks, “Progress toward on-wafer standards,” 36th ARFTG
Conf. Dig. (Monterey CA), Nov. 1990.

10. D. F. Williams and R. B. Marks, “Transmission line capacitance measurement,” [EEE
Microwave and Guided Wave Letters, vol. 1, no. 9, pp. 243-245, September 1991.

August 2, 1995 9


The Output Menu

The various options accessible from the output menu:

Softkey Description

50 ohm Worst case deviations of measurements of passive devices from the benchmark
calibration. The reference impedance of the benchmark calibration is set to 50 Q,
even if the reference impedance of the benchmark calibration was initially set to
the characteristic impedance of the benchmark lines. After pressing this softkey

you will be given options for data output format. (See Viewing, Storing, or
Plotting the Data below.) The data will be labeled as follows:

Label Data description.
initial Direct comparison of calibrations. The plotted quantity is
€(50Q,1,) or €,(50Q, J.) of [3]. The working calibration is
compared to the benchmark calibration with a 50 Q reference
impedance and a reference plane which has not been translated.
Fit 50 Worst case deviations of measurements of passive devices from the benchmark

calibration. After pressing this softkey you will be given options for data output
format. (See Viewing, Storing, or Plotting the Data below.) The data will be

labeled as follows:
Label Data description.
initial Direct comparison of calibrations. The plotted quantity is

August 2, 1995

e(50Q,1,) or e,50Q, }) of [3]. The working calibration is
compared to the benchmark calibration with a 50 Q reference-
impedance and a reference plane which has not been translated.

Worst case deviation after an impedance transformation to the best.
estimate of the reference impedance of the working calibration is
applied to the benchmark calibration. The plotted quantities are the.
02,1) ore (Za f) of [3]. The working calibration is compared to

the benchmark calibration with a reference impedance which has

been adjusted at each frequency to best match that of the working
calibration and a reference plane which has not been translated.

Worst case deviation after reference plane of benchmark calibration
is adjusted to most nearly equal the reference plane of the working
calibration. The plotted quantities are the ¢(50Q,/,) or
€,(50Q,1,) of [3]. The working calibration is compared to the


all copies of the Software, and, within ten days, return to NIST the original Software distribution media
and documentation.

12. This agreement represents the complete and exclusive statements of the agreements concerning ail
license agreements to the Software between the parties and supersedes all prior agreements and
representations between them. It may be amended only by a writing executed by both parties. No
waiver of a breach of any provision of this Agreement shall constitute a waiver of any other breach,
or of such provision. Failure of either parry to enforce at any time, or from time to time, any provision
of this Agreement shall not be construed as a waiver thereof.

Signature of Software user Date
Title Name of Organization
Signature of Organization Representative Date
Title Name of Organization

In Europe return the completed agreement to Dr. David Bannister, National Physical Laboratory, c/o
DRA C1014, St. Andrews Road, Malvern, WORCESTER, WR14 3PS, United Kingdom.

Outside of Europe return completed agreements to Dylan Williams, NIST 813.00, 325 Broadway,
Boulder CO, 80303-3328, USA. In the case of termination of this agreement, software and manuals
should be returned to this address.

August 1, 1995 18


Refl/short

Isolation

Load

DUT

Adapt on

Adapt on 1
Adapt on Z

standard. When the isolation correction is toggled on, the isolation error terms
are calculated from the first reflection in the standard list unless an Isolation
standard is included in the standard list.

Similar to the Refl/open except that the reflection coefficient is assumed to be
approximately -1. Inductive standards can be accommodated by increasing the
length of the standard.

When the isolation correction is toggied on (this correction is toggled on and
off with the “I” key), the isolation error terms are calculated from this
standard if it is included in the list. If it is not included, the isolation error
terms are calculated from the transmission parameters of the first reflect in the
standard list.

A standard whose reflection coefficients have been measured. The file
containing the S-parameters of the Load standard is listed in the length column
on the menu. The Load standard is used oniy with the two-tier "De-embed
Transition" calibration. This calibration method is described in (9].

This is provided for the user's convenience and allows both corrected and un-
corrected device data to be stored on disk. The program will prompt for the
file name when you press the measure softkey before taking data.

An adaptor is added to both ports of the calibration model. The file containing
the S-parameters of the adaptor are listed in the DATA FILE NAME column
of the standard list. If the adaptor is a simple shunt capacitance, the value of
that capacitance in fF may be listed in the place of the file name containing the
S-parameters of the adaptor. This is useful when implementing the method of
[5], which corrects for differences in calibration and measurement substrates.

The same as Adapt on both, except that the adaptor is added only to port 1.

The same as Adapt on both, except that the adaptor is added only to port 2.
In contrast to Adapt on both, the adaptor S-parameters are reversed before
they are added to the port 2 error model; port 2 of the adaptor should be
connected to the second port of the network analyzer during both the adaptor
and DUT measurements.

August 1, 1995 10


ae

=

Measure resistance of probe heads and planar short.

Figure 2. The dc resistance of a lumped load can be measured accurately by measuring the
resistance of a short placed near the position of the load and subtracting this result from the

measured resistance of the load.

of probe heads and short line.

vom

©

\ /

Measure resistance of probe heads and long line.

Ceol

de short

a

Figure 3. The dec resistance per unit length of the line can be measured accurately by shorting one
of the probe heads and measuring the resistance of two lines through the bias port of the second
probe head. The difference of the measured resistances is divided by the difference of the lengths
of the lines to find the series resistance per unit length.

6


the file type, which can be either ASCII or BDAT.

The light bar on the file list may be moved either by pressing the arrow keys on the
keyboard, rotating the knob on your keyboard, or moving your mouse. The file list is edited by
selecting a file from the file list with the light bar and then pressing the softkeys. Use the NAME
softkey to change the file name, the LOC. softkey to change mass storage unit, the PATH softkey
to change the path name, and the TYPE softkey to change the file type. Pressing the NAME
softkey will catalog the mass storage unit and then prompt you for the file name. Pressing the shift
key and the NAME softkey will prompt you directly for the file name.

The capacitance of your line may be determined from a lumped load or the resistance of
your line per unit length. The type of calculation is toggled by pressing the MODE softkey.

When the mode is toggled so that the capacitance of the line is to calculated from a lumped
load, the dc resistance of the load is set by pressing the SET R softkey. The reference plane of
the load measurement may be shifted along your line by pressing the REF PLANE softkey.

When the mode is toggled so that the capacitance of the line is calculated from the
propagation constant and a dc measurement of the resistance per unit length of the line, the
resistance of the line may be set by holding the shift key and pressing the SET R softkey.
Pressing the SET R softkey loads the calculation sub menu. This menu allows the resistance per
unit length of the line to be calculated from a set of measurements of the total resistance of the
lines. The resistance may also be set directly by pressing the SET R softkey from within this sub
menu.

To calculate the resistance per unit length of your line from within the sub menu, the
individual measurements of line resistance must be entered into the list displayed on the screen.
Lengths and resistances may be added by pressing the ADD softkey. Lengths and resistances may
be modified by selecting the measurement with the light bar and pressing either the L(1) or the
R@ softkeys. The order of entry is immaterial.

Once the measurements have been entered into the menu, the resistance per unit length of
the line may be found by pressing the FIND R softkey. A least squares fit to the data is performed
to find the resistance per unit length of the probe heads. Incomplete data is subtracted from the
list and ignored.

In either mode, the best estimate of the capacitance of your line is found by fitting a
quadratic to the measured estimates of the capacitance and extrapolating the result to dc. The
frequencies used in fit are listed on the screen as Fmax and Fmin. These frequencies can be
modified by holding the shift key and pressing the PRINT \F(fit) softkey. As a general rule the
best measurements are low frequency measurements. The calculation is performed by editing the
menu as described above and pressing the FIND C softkey. The results may be printed by
pressing the PRINT softkey.


For second-tier calibrations. the second line also lists estimates of the lengths of the
fixtures or probes being de-embedded. The electrical length of the fixtures or probes are
calculated from the estimated effective dielectric constant and the lengths listed on this line,
and is used to determine the sign choice of the fixture or probe transmission parameters.
MuitiCal will require a very accurate estimate of this length if the electrical length of the
fixture or probe is large at the lowest calibration frequency. This difficuity can be
circumvented by including low frequencies in the calibration even when only high-frequency
data is required.

The third line below the list of standards contains the path and mass storage device
on which ail data is stored.

The fourth line lists the reference plane position with respect to the center of the thru
line standard and the output reference impedance for the calibration. Positive reference piane
positions refer to locations beyond the center of the thru line, negative reference plane
positions to locations nearer to the analyzer.

The fifth line lists the line capacitance or other line parameters used to determine the
initial reference impedance of TRL calibrations, which is equal to the lines’ characteristic
impedance.

4. Editing the Main Menu

The softkeys are used to edit the menu. These softkeys and their functions are listed

below. If the softkey affects a standard definition, only the standard selected by the arrow is
effected.

CAL TYPE Pressing this softkey toggies the calibration type.
Zref Possible types are one-tier, two-tier, and transition de-embed calibrations.

Press the shift key and this softkey simultaneously to enter the reference
impedance sub-menu. The reference impedance sub-menu defines the
properties of the transmission lines used in the calibration. If the line
capacitance and conductance [10] are specified, MultiCal uses the propagation
constant to determine the characteristic impedance (2]. The characteristic
impedance and propagation constant may also be specified directly. This
allows the reference plane of LRM calibrations to be accurately translated
even when the transmission lines are lossy and dispersive [4].

The reference impedance of the calibration is also selected from this menu.

ADD STD Pressing this softkey adds or deletes standards from the list.

August 1, 1995 7


3. An Overview of MultiCal's Main Menu

The top half of the computer screen contains a list of calibration standards. The arrow
== >) at the left of the list of calibration standards is moved by the wheel, mouse, or arrow
keys on your computer. and selects the standard to be edited or measured.

The first column of the list of calibration standards contains a general description of
the standard. The purpose of this description is solely to help the user identify the standard.

The second column contains the standard type. The standard type is used by the
software to determine how to handle the measured data. Each calibration must have a single
thru standard, and the position of the calibration reference plane is always referred to the
center of this thru. Pressing the “I” key toggies the isolation correction on and off. Uniess
an isolation standard is explicitly chosen, the first reflect is used to determine the isolation
error terms when isolation correction is toggied on. For on-wafer measurements it is usually

best to toggle the isolation correction off (the default state), which sets the isolation error
terms to zero.

The third column contains the name of the disk file in which uncalibrated S-parameter
data for that standard will be stored.

The fourth column contains information defining the standard. If the standard is a
transmission line, its length in cm is listed in this column; this length is usually the physical
length of the standard (see section 6). Other standards often list a file name defining its
properties in this column: the calibration methods of (4], [3] and [9] require such standards
with known properties.

A "*" is placed next to each standard in the fifth column if that standard has been
measured if the measurements were performed before the start of the program. A "+" is
placed next to each standard in the sixth column if the data file associated with the standard
already exists on the computer's mass storage unit.

Below the list of standards are several lines of additional information. The first line
lists the type of calibration to be performed and the first and, if applicable, second tier
network analyzer calibration sets.

The second line lists the estimated effective dielectric constant. This information,
together with the physical lengths of each standard, is required to allow the program to make
the correct root choices during TRL calibrations. Only an estimate of the effective dielectric
constant is required. Not shown is the imaginary part of the effective dielectric constant,
which is modified by holding the controi key and pressing the “EPS” softkey. For
calibrations in hollow metal waveguide, the waveguide cutoff frequency replaces the
estimated effective dielectric constant.

August 1, 1995 6


User’s Manual
Program VERIFY Rev. 1.03

This software was developed with funds from the NIST/Industrial MMIC Consortium. New features are not available
to the general public until one year after testing and release to Consortium members. Users of National Institute of
Standards and Technology (NIST) calibration software outside of the United States of America must complete a
foreign license agreement. Questions concerning the Consortium shouid be directed to Mr. Gerry Reeve at (303)497-
3557.

The authors of the software request that you refer to NIST in all publications making use of this software and that you
specifically reference the relevant publications cited in this manual. A list of relevant publications can also be found
in the help command.

While every effort has been made to ensure that this software is suitable for the purposes intended, NIST makes no
warranty, expressed or implied, as to the correctness or fitness for a particular purpose and accepts no responsibility
for direct or consequential damages which may result from its use.

HOW TO GET HELP FROM NIST
Questions regarding user and equipment interfaces, measurement methodology, or de-embedding algorithms should
be directed to Dr. Dylan Williams at (303)497-3138 (dylan@bldrdoc. gov) or to Dr. Roger Marks at (303)497-3037
(marks@boulder.nist.gov).

Microwave Metrology Group, Electromagnetic Fields Division, National Institute of Standards and Technology, 325
Broadway, Boulder Colorado, 80303.


root choices. This softkey also may be used to set the estimated lengths of the
transitions to be de-embedded in a two-tier technique.

Press the shift key and this softkey to change the position of the electrical
reference piane along the iine. (See section 6.)

SCREEN _Press this softkey to refresh the screen. Press the shift key and this softkey

HELP to see a help menu. The help menu contains a list of references for the
algorithms impiemented in this software.

Keyboard strokes are used to accomplish several other functions: press “T” to change the title
of the menu, press “A” to toggle the network analyzer interface on and off, press “D” to
toggle the debugging option (see section 10) on and off, press “I” to toggie the isolation
correction on and off, press “P” to toggie the direction of the pointer movement after each
Measurement, and press “Q” or “X” to exit the program.

5. Standard Types

The user may select from among eleven standard types. Different measurements are
performed for the thru standard type and during first and second tier calibrations, so some
care must be made when changing standard and calibration types after measurements have
been made. The possible standard types are listed below.

Thru The shortest transmission line used in the calibration. The user may only
select one thru line. The position of the calibration reference plane is always
referred to the center of this thru line. If a one-tier de-embedding scheme is
selected, switching error terms are measured when the thru measurement is
made. These are stored in a disk file with the same name as that of the thru
line, but with the prefix "G" added to the beginning of the file name.

Line A transmission line of construction identical to that of the thru line but of a
different length used for the muitiline TRL calibration method {1].

Match A two port with identical loads at each port. The output reference impedance
may be set to the impedance of the load or, if the impedance of the load is
known, it may be set to a real and constant value. In this latter case, a file
containing the reflection coefficient of the load measured in a 50 ohm system
must be placed in the standard definition column.

Refl/open A two-port with identical reflective loads connected at each port. The

reflective load is assumed to have a reflection coefficient of approximately 1.
Capacitive loads can be accommodated by increasing the length of the

August 1, 1995 9


benchmark calibration with a reference impedance equal to the
characteristic impedance of the lines in the benchmark calibration
set and a reference plane which has been translated ar each
Srequency to a position which best matches the estimated reference
plane position of the working calibration.

imp+off Worst case deviation after the reference impedance and reference
plane of the benchmark calibration have been adjusted to be as close
as possible to those of the working calibration. The plotted
quantities are the e(Z, h ) ore, (2, h of [3]. The working calibration
is compared to the benchmark calibration with a reference
impedance and a reference plane which have been translated ar each
Srequency so as to best matches the estimated reference impedance
and reference plane position of the working calibration.
R(imp) Estimated real part of the reference impedance of the working calibration as a
function of frequency.
L(imp) The inductance associated with the estimated imaginary part of reference

impedance of the working calibration as a function of frequency.

d(off) The estimated position of the reference plane of the working calibration in microns
with respect to that of the benchmark calibration as a function of frequency.

EXIT Return to the main menu. All data is lost.

August 2, 1995


Preliminary Draft

National Institute of Standards and Technology
De-embedding Software

Program MuitiCal '

Revision 1.00

USER'S GUIDE

' Trademark pending.


contacting or connecting the appropriate standard, moving the arrow to the standard on the
menu, and then pressing the MEASURE softkey. After all of the standards have been
measured, the analyzer can be de-embedded. In some situations, such as LRM, it is
convenient to measure the S-parameters of only one port at a time. Holding the control key
and pressing the MEASURE softkey allows the standard to be measured first on port 1, and
then on port 2. In this case, the program pauses between the port 1 and port 2 measurements
to allow the standard to be connected to port two before completing the measurement.

9. De-embedding the Analyzer

The analyzer is de-embedded by pressing the DEEMBED softkey. The measurement
files stored on disk are then retrieved and the calibration coefficients calculated by the de-
embedding algorithm. MultiCal can load these coefficients directly into the network analyzer
(the “A” key toggies this network analyzer interface on and off), or it can correct data stored
on disk in its ouput section (see section 10).

Because the data is stored on disk, the menu may be edited and another attempt made
at de-embedding if a mistake is made. Also, if a single connection is the reason for a bad
calibration, that measurement can be repeated and the data file for that standard written over.

Three types of de-embedding algorithms are supported. The one-tier calibration is the
most accurate and the quickest way to calibrate the analyzer. The entire analyzer is calibrated

in the one-tier method in a single step. The position of the electrical reference plane is
referred to the center of the thru line.

The two-tier calibration is a more complicated process, but more information is
obtained. Before beginning a two-tier calibration, the amalyzer is calibrated at some
intermediate electrical reference piane, often in a coaxial line. Then an adapter, often a test
fixture or probe head, is added to the analyzer at the intermediate electrical reference plane
and the two-tier algorithm is used to perform a second calibration on the far side of the
adaptor. The two-tier algorithm determines the S-parameters of the adapter and uses them to
move the measurement reference plane to its far side. These S-parameters are available for
output after de-embedding, and are referred to as the “S-parameters of the probe head” in
MultiCal’s output section. In order for the program to successfully resolve the signs of the
adaptor transmission parameters accurate estimates of their electrical lengths must be
provided. Two-tier calibrations can also be used to compare and determine the accuracy of
calibrations; this is discussed in (6], [7], and [8].

A third calibration type, DEEMBED TRANSITION, is aiso a second-tier calibration.
It requires only known one-port standards. This calibration uses the “load” standard type.

August 1, 1995 12


6. Reference Plane and Line Length Definitions

The electrical reference plane to which
all of the de-embedded measurements are
referred is, by default, placed in the middle of
the thru line. This reference plane can be
moved with respect to this default position
using the REF PLN softkey.

The lengths of the standards listed in
the standards menu refer to the physical
distance between probe contacts where, in the
multiline TRL aigorithm, all contact errors are
assumed to occur. These lengths define
standard lengths for purposes of de-embedding
and allow for proper averaging of errors. Since
the probes typically contact the standards at Figure 2. Physical lengths and measurement
their physical beginning, these lengths are reference planes are illustrated for a microstrip line
usually just set to the physical lengths of the with via hole transitions. ae lengths listed in the

is is i i i sl are sit en} +)
standards. This is illustrated in Figure 2. aaa i ticaned here oy ihe Ph si aus ths of the
reference pianes, which have not been translated in
this example, are shown in dashed lines.

7. Setting the Output Reference Impedance

Calibrated S-parameter measurements are always referred to an output reference
impedance. The value of this reference impedance is set in the reference impedance sub-
menu, entered by pressing the Zref softkey.

In the TRL calibration the output reference immpedance can be set equal to the
characteristic impedance of the line or, if the capacitance of the line {10] is know, to a real
and constant reference impedance (usually 50 Q) [2]. If the characteristic impedance of the
line has been determined by the calibration comparison method, the calibration reference
impedance can also be reset to a real and constant reference impedance (3]. The output
reference impedance of an LRM calibration can be either set equal to the impedance of the
match standard or, if the impedance of the match standard has already been measured, toa
real and constant impedance (4].

8. Measuring the S-Parameters of a Standard

The uncorrected S-parameters of each standard are measured and stored on disk by

August 1, 1995 re


References

[1] R. B. Marks, "A multiline method of network analyzer calibration," JEEE Trans.
Microwave Theory Tech., Vol. 39, pp. 1205-1215, July 1991.

[2] R. B. Marks and D. F. Williams, “Characteristic Impedance Determination using
Propagation Constant Measurement,” IEEE Microwave and Guided Wave Letters 1, pp. 141-
143, June 1991.

{3] D. F. Williams and R. B. Marks, “Accurate Transmission Line Characterization,” IEEE
Microwave and Guided Wave Letters 3, pp. 247-249, Aug. 1993.

[4] R. B. Marks and D. F. Williams, “Accurate Experimental Characterization of
Interconnects,” JEEE Transactions on Components, Hybrids, and Manufacturing Technology
15, pp. 601-602, Aug. 1992.

[5] D. F. Williams and R. B. Marks, “On-Wafer Impedance Measurement on Lossy
Substrates,” JEEE Microwave and Guided Wave Letters 4, pp. 175-176, June 1994.

[6] D. F. Williams and R. B. Marks, "Transmission line capacitance measurement," IEEE
Microwave and Guided Wave Letters, pp. 243-245, September 1991.

NIST transmission line capacitance determination software Rev. 1.31

Description File name Location Path Type
S-par load LOAD 2, 702 BDAT
eps eff your line EPS 3, 702 BDAT
<<< uniform weighting function will be used >>>

Find Cc from S-parameters of lumped load Print: off

R( load): 50 ohms Fmax: 20 GHz

Ref plane translation: 0 microns Fmin: 0 Hz

NAME |z00- [Pare [re MODE |SCREEN PRINT |SET R |REF PLN|FI
ND C
\MENU iFitit)| | |

Figure 1. The CAP menu. The file list is edited by selecting the appropriate file with the light bar
and using the softkeys.


Measurement of dc Parameters

The resistance of the lumped load and resistance per unit length of the line can be
measured easily through the bias ports and wafer probe heads on your analyzer. To measure the
resistance of a lumped load, first measure the resistance of the probe and section of line connected
to the load. Then measure the resistance of the probe and section of line terminated by an offset
short placed at approximately the same location as the load, as illustrated in Figure 2. The
difference of these two measurements gives the desired result.

The series resistance of the line includes not only the resistance of the center conductor,
but the resistance of the ground planes as well. There are several ways to measure this resistance.
A simple and accurate method is to terminate one probe head with a short at its coaxial port and
connect an ohm meter to the other bias port, as illustrated in Figure 3. In order to assure that the
resistance of the ground planes are properly accounted for the ground of the shorted probe head
must be isolated from the ground of the probe head to which the ohm meter is connected. Several
lines of different lengths must be contacted and the measured resistances recorded. The resistance
per unit length of the line can then be determined in the SET R sub menu.

File Formats
The ASCII format of the S-parameter file is as follows:
Freq Re(S,,) Im(S,,) Re(S,) Im(S,;) Re(S;) Im(S,,) Re(S,,) Im(S.))

where the frequency is given in GHz. This format is compatible with the Touchstone data format.
The BDAT format of the S-parameter file is as follows:

Freq Re(S;,) Im(S,,) Re(S,,) Im(S,,) Re(S,,) Im(S,,)_ Re(S,2) Im(S,)

where the frequency is given in GHz. In both cases the S-parameter data must be in the real-
imaginary format.

Both the ASCII and BDAT formats of the effective dielectric constants are as follows:
Freq Re(epsilon effective) Im(epsilon effective)

where the frequency is given in GHz. The ASCII and BDAT formats of the weighting function
files are as follows:

Freq sigma X

where the frequency is given in GHz, sigma is the normalized standard deviation of the
measurement, and X is not used.


DEL STD

FILE NM
LENGTH

GETMENU
SAVEMEN

MSI
CAL SET

DESCRIP

EPS EFF

The "Insert line" and "Delete line" keys perform the identical functions.

Press this softkey to change the file name for data
storage for the standard selected by the arrow.

Press the shift key and this softkey simultaneously to change the length or the
definition file of the standard.

Press this softkey to save or retrieve an edited menu

from mass storage. Upon saving a menu, you will be asked whether you wish
to store the volume number. If you answer "yes", the volume number of the
disk on which the data is saved will be stored. This can be useful if you use
many floppy disks with voiume labels.

Press this softkey to change the mass storage or path names used for data
storage. The mass storage path is listed three lines below the list of standards.
A storage device must be specified, but a path location is optional. If the data
disks were formatted in LIF format, the path must be omitted entirely.

Pressing the shift key and this softkey simultaneously allows the default
calibration set on the analyzer to be selected. For one-tier calibrations, this is

simply the calibration set in the analyzer to which the calibration coefficients
will be written.

If the two-tier or the deembed transition calibration type is selected, then this
softkey allows both the first-tier and second-tier calibration sets to be chosen.
The estimated lengths of the transitions on each port between the first-tier and
second-tier calibration reference planes must also be specified for two-tier
calibrations. This is accomplished with the EPS and REF PLN softkeys.

Pressing this softkey will scroll through the standard types. Holding the
control key while pressing "TYPE” will scroll the standard types in reverse
order. The standard types are discussed in section 5.

Press the shift key and this softkey simultaneously to change the description
of the standard. This description labels and helps differentiate the standards;
itis not used in the calibration.

Press this softkey to change the estimated effective dielectric constant

displayed on the second line below the list of standards. Hold the control key
and press this softkey to set the imaginary part of the estimated effective
dielectric constant. Good estimates for the real and imaginary part of the
effective dielectric constant ensure that the algorithm correctly makes certain

August 1, 1995 8


8) After the analysis is complete, the program wiil enter an output menu. From this menu
you may print, view, or plot a variety of calibration diagnostics.

The Main Menu
All of the input parameters for program VERIFY may be modified from the main menu
by pressing the softkeys:
Softkey Effect

port 1 Specify the file containing the S-parameters of the probe connected to port 1.

port 2 Specify the file containing the S-parameters of the probe connected to port 2.

eps eff

cap

menu

select S

start

Specify the file containing the effective dielectric constant of the lines in the
benchmark calibration standard set.

Specify the capacitance per unit length of the lines used in the benchmark
calibration.

Specify the reference impedance used in the benchmark calibration. Pressing the
softkey toggles between 50 Q and Z,.

Toggle the order in which your calibration and the benchmark calibration were
performed. Usually the benchmark calibration is performed after the working
calibration and all of the measurements for the benchmark are performed with
Tespect to the working calibration. In this case choose “second tier”. Choose “first
with respect to the benchmark calibration. (In this case, the working calibration.
must determine the S-parameters of the probe head.)

The main menu may be stored or retrieved from disk.

Selects which worst case deviations are be determined. The default is the worst
case deviation of any of the four S-parameters. The worst case deviations for any
singie S-parameter may be selected with this softkey.

Refresh the screen.

Begin the calculations and then enter the output menu.

The program may be exited by pressing Q or X.

August 2, 1995 4


User's Manual

Program CAP Rev. 1.31

This software was developed with funds from the NIST/Industrial MMIC Consortium. New features are not available
to the general public until one year after testing and release to Consortium members. Users of National Institute of
Standards and Technology (NIST) calibration software outside of the United States of America must complete a
foreign license agreement. Questions concerning the Consortium should be directed to Mr. Gerry Reeve at (303)497-
3557.

The authors of the software request that you refer to NIST in all publications making use of this software and that you
specifically reference the relevant publications cited in this manual. A list of relevant publications can also be found
in the help command.

While every effort has been made to ensure that this software is suitable for the purposes intended, NIST makes no
warranty, expressed or implied, as to the correctness or fitness for a particular purpose and accepts no responsibility
for direct or consequential damages which may result from its use.

HOW TO GET HELP FROM NIST
Questions regarding user and equipment interfaces, measurement methodology, or de-embedding algorithms should
be directed to Dr. Dylan Williams at (303)497-3138 (dylan@bldrdoc.gov) or to Dr. Roger Marks at (303)497-3037
(marks@boulder.nist. gov).

Microwave Metrology Group, Electromagnetic Fields Division, National Institute of Standards and Technology, 325
Broadway, Boulder Colorado, 80303.


2. Program Organization

When MultiCal is loaded and run, a menu similar to that shown in Figure | wiil

appear on the screen. The parameters displayed on the screen define the calibration
procedure.

The MultiCal menu contains a list of the calibration standards, followed by a list of
the essential parameters defining the calibration. Softkeys displayed at the bottom of the
screen are used to edit the menu and give the user control over de-embedding and
measurement. Edited menus may be saved on disk and recalled later. This makes it easy to
completely specify a de-embedding or measurement procedure for another user. Using the
default file name "CALMENU" is convenient as this menu is automatically loaded when
MultiCal is restarted. If MultiCal cannot find the file CALMENU, it will start with a defauit
menu set up for calibration with NIST coplanar waveguide standards.

Pressing the up-arrow and down-arrow keys moves the arrow (==>) at the left of
the list of standards. Properties of the standards are changed by selecting the standard with
the arrow and using the softkeys to modify the standard attributes. Other softkeys modify the
information on the menu below the standard list.

After the menu has been edited, the uncorrected S-parameters of each standard may
be measured and stored on disk by selecting the appropriate standard with the up-arrow and
down-arrow keys and pressing the MEASURE softkey. Once the uncorrected S-parameters
of each standard have been measured and stored on disk, the analyzer may be de-embedded

by pressing the DEEMBED softkey. To exit the program and restore your CRT settings, hit
"Q" or "X" from the main menu.

Mistakes are easy to correct because ail of the intermediate measurements are stored
on disk; changing menu parameters, remeasuring a single standard, or even adding a standard
does not require restarting the calibration process from the beginning. However, some care
must be taken if type definitions are changed. Extra data is taken with the thru line to
determine the switching error terms in the analyzer. Data is also taken differently when the
one-tier and two-tier algorithms are specified.

August 1, 1995 4


4. D. F. Williams and R. B. Marks, “LRM Probe-Tip Calibrations using Nonideal
Standards,” JEEE Microwave Theory and Techniques 43, pp. 466-469, Feb. 1995.

5. D. F. Williams and R. B. Marks. “Compensation for Substrate Permittivity in Probe-Tip
Calibration,” 44th ARFTG Conference Digest, pp. 20-30, Dec. 1994.

6. D. F. Williams, R. B. Marks, and A. Davidson, “Comparison of On-Wafer Calibrations,”
38th ARFTG Conference Digest, pp. 68-81, Dec. 1991.

7. D. F. Williams and R. B. Marks, “Calibrating On-Wafer Probes to the Probe Tips,” 40th
ARFTG Conference Digest, pp. 136-143, Dec. 1992.

8. R. B. Marks and D. F. Williams, “Verification of Commercial Probe-Tip Calibrations,”
42nd ARFTG Conference Digest, pp. 37-44, Dec. 1993.

9. K. Phillips and D. Williams, “MMIC Package Characterization with Active Loads,” 36th
ARFTG Conference Digest. pp. 64-72, Nov. 1990.

10. D. F. Williams and R. B. Marks, “Transmission Line Capacitance Measurement,” [EEE
Microwave and Guided Wave Letters |, pp. 243-245, Sept. 1991.

August 1, 1995 15


of NIST. The Software user and the organizarion agree not to modify or create derivative works of the
software without written permission of NIST. Further, the Software user and the organization agree

to retain as a part of the Software the acknowiedgment that NIST is the originator and developer of the
Software.

5. Nothing in this agreement constitutes a waiver of NIST copyright that exists outside the United
States. NIST retains ail such copyrights outside of the United States of America and the exclusive Tight
to enforce those copyrights.

6. This agreement shall be construed, interpreted, and governed under the laws of the United States.
If any provisions of this agreement shall be held by a court of competent jurisdiction to be illegal,
invalid or unenforceable, the remaining provisions shail remain in full force and effect.

7. This software is delivered without any warranty of any kind, including without limitation the
warranties of merchantability, fimess for a particular purpose and non-infringement. The entire risk
as to the quality and performance of the Software is borne by the Software user and the
organization, including but in no way limited to, indirect, special, incidental, or consequential damages
of any kind including, without limitation, damages for loss of goodwill, work stoppage, computer
failure or maifunction, or any other commercial damages or losses. In no event will NIST be liable
for any damages, even if NIST has been informed of the possibility of such damages, or for any claim
by any other party. Shouid the Software prove defective, the Software user and the organization, and
not NIST, assume the entire cost of any cost of service and repair. This disclaimer of warranty
constitutes an essential part of the agreement. This is a complete statement of NIST warranty. No NIST
employee or other third party is authorized to make any modifications or additions to this warranty.

8. Both the Software user and the organization shall indemnify and hold harmiess the U.S. Government
for any loss, claim, damage, or liability of any kind arising out of the use of this Software by the

Software user, the organization, or any party using the Software on their behalf or under their
authorization.

9. Neither the Software user or the organization may download or otherwise export or reexport the
Software or any underiying information or technology except in full compliance with ail United States
and other applicable laws and regulations. In particular, but without limitation, none of the Software
or underlying information or technology may be downloaded or otherwise exported or reexported (1)
into (or to a national or resident of) Cuba, Haiti, Iraq, Libya, Yugoslavia, North Korea, Iran, Syria,
or any other country to which the United States has embargoed goods; or (ii) to anyone on the United
States Treasury Department's list of Specially Designated Nationals or the United States Department
of Commerce’s Table of Deny Orders. By downioading or using the Software, the Software user and
the organization are agreeing to the foregoing and are representing and warranting that neither the
Software user nor the organization are located in, under the control of, or a national or resident of any
such country or on any such list.

10. The Software user or organization may terminate this Agreement at any time by discontinuing the
use of the Software, destroying all copies of the Software, and returning to NIST the original Software
distribution media and documentation.

11. NIST may terminate this software license at any time by notifying the Software user. Upon
notification of termination, Software user will immediately discontinue the use of the Software, destroy

August 1, 1995 17


lL. Introduction

MuitiCai performs scattering parameter (S-parameter) calibrations on the HP 8510B,
HP 8510C, the HP 8700 series vector network analyzers, and the Wiltron 360 vector network
analyzer. MuitiCal runs on most Hewiett Packard 9000 series 200/300 computers under HP
BASIC and on PCs under the Windows and DOS environments. MultiCal is not a standalone
Windows or DOS product; to run on a PC it requires the commercial interpreter HTBasic,
available from TransEra Corp., Orem, Utah at (801)224-6550 or HP Basic for Windows,
available from Hewlett Packard, Inc. at (800)829-4444. While the software was developed
for use with on-wafer standards, it can also be used with other planar transmission lines, with
coaxial transmission lines, and with hollow metal waveguide.

The menu-driven software makes calibrating vector network analyzers using de-
embedding algorithms developed at the National Institute of Standards and Technology
(NIST) extremely easy. The basic de-embedding aigorithm used in the software is based on
the multiline Thru-Reflect-Line (TRL) algorithm developed by Marks (1]. If more standards
than the minimum number are available, MultiCal will automaticaily and opumaily weight
the results for maximum accuracy; there is no need to partition the frequency band to
eliminate the instability of the TRL algorithm near those frequencies where the difference in
line fengths of one line pair approaches a multiple of a half wavelength. MultiCal’s multiline
TRL algorithm also optimally measures the propagation constant of the lines used in the
calibration. MultiCal can determine and reset the calibration reference impedance using the
method of [2], applicable to low loss substrates, or the method of [3], which is applicable to
lossy and dispersive substrates.

MuiltiCal supports the line-reflect-match (LRM) calibration. If a line standard of
moderate length is available, MultiCal can also accurately translate the reference plane and
accurately reset the reference impedance to 50 Q, even when the transmission lines are lossy
and dispersive and the resistor standard has a significant reactance {4]. This requires the
auxiliary program LRMCal.

MultiCal supports the off-wafer calibration method of [5], which corrects for
differences in substrate dielectric constants.

MuiltiCal performs both one-tier and two-tier de-embedding. The two-tier algorithms
can be used to electrically characterize probe heads and other components, and to implement
the calibration comparison and verification methods described in (6], [7], and (8]. MuitiCal
also supports a second-tier “transition de-embed” calibration based on 3 or more known but
otherwise arbitrary one-port standards [9].

After de-embedding, MultiCal can plot and analyze various data, including

propagation constant, line-standard characteristic impedance, S-parameters, impedance
parameters, and admittance parameters.

August 1, 1995 3


Viewing, Storing, or Plotting the Data

After selecting the data type by pressing one of the softkeys you will enter a second
sub-menu. From this menu you can view, plot, or store the data with the following softkeys.

Softkey

PLOT

BDAT

ASCII

Description

The data will be plotted on the screen. After scaling and other manipulations, it
can be plotted on a pen plotter or printed on your printer.

The data will be saved in a BDAT file.

The data will be saved in an ASCII file. This format is not recommended for
transferring files between systems.

The data will be saved in an HP-UX ascii file. This format is recommended for
transferring files between systems.

The data will be printed on the screen.
The data will be printed on the printer.

Return to the previous menu.

File Formats

Input parameters must be in the BDAT format. The order of the parameters in the S-
parameter file is:

Freq Re(S,,) Im(S,1) Re(S,,) Im(S2,) Re(Sz) Im(Sz) Re(S,) Im(S,2)

where the frequency is given in GHz. The order of the parameters in the effective dielectric
constant file is:

Freq Re(eps eff) Im(eps eff)

where the frequency is given in GHz.

August 2, 1995 8


Contents

1. Introduction 3
2. Program Organization 4
3. An Overview of MultiCal's Main Menu 6
4. Editing the Main Menu 7
5. Standard Types 9
6. Reference Plane and Line Length Definitions 11
7. Setting the Output Reference Impedance ll
8. Measuring the S-Parameters of a Standard ll
9. De-embedding the Analyzer 12
10. MuitiCal’s Ourput Section 13
Appendix 1. Foreign License Agreement 16

MultiCal' was developed with funds from the NIST/Industrial MMIC Consortium. New features are not
available to the general public until one year after testing and release to Consortium members. Users of MultiCal
and other National Institute of Standards and Technology (NIST) calibration software outside of the United States
of America nmst complete the license agreement in Appendix 1. Questions concerning the Consortium should
be directed to Mr. Gerry Reeve at (303)497-3557.

‘The authors of the software request that you refer to NIST in all publications making use of this software and
that you specifically reference the relevant publications cited in this manual. A list of relevant publications can
also be found by executing MultiCal’s help command.

While every effort has been made to ensure that this software is suitable for the purposes intended, NIST makes
no warranty, expressed or implied, as to the correctness or fitness for a particular purpose and accepts no
responsibility for direct or consequential damages which may resuit from its use.

HOW TO GET HELP FROM NIST
Questions regarding user and equipment interfaces, measurement methodology, or de-embedding algorithms
should be directed to Dr. Dylan Williams at (303)497-3138 (dylan@bldrdoc.gov) or to Dr. Roger Marks at
(303)497-3037 (marks@bouider.nist.gov).

Microwave Metrology Group, Electromagnetic Fields Division, National Institute of Standards and Technology,
325 Broadway, Boulder Colorado, 80303.

August 1, 1995 2


Introduction

Program CAP determines the dc capacitance of on-wafer quasi-TEM transmission lines
fabricated on lowloss substrates from measurements of lumped loads or from measurements of
the dc resistance per unit length of the line. The dc capacitance of the lines is used in program
MultiCal! to determine their characteristic impedance, and to reset the reference impedance of
MultiCal’s multiline Thru-Reflect-Line (TRL) calibration [1] with the method of [2].
Transmission lines printed on high loss substrates can be characterized by the methods described
in [3], [4], and [5].

If the de capacitance of your line is determined from the measurement of a lumped load
the program requires as input the measured S-parameters of the load and the measured effective
dielectric constant of your line. If the capacitance of your line is determined from the measured
dc resistance per unit length of the line then only the measured effective dielectric constant of
your line is required. The two solution algorithms are described in [6].

Program CAP runs on most Hewlett Packard 9000 series 200/300 computers under HP
BASIC and on PCS under the Windows and DOS environments. MultiCal is not a standalone
Windows or DOS product; to run on a PC it requires the commercial interpreter HTBasic,
available from TransEra Corp., Orem, Utah at (801)224-6550 or HP Basic for Windows,
available from Hewlett Packard, Inc. at (800)829-4444. While the software was developed for use
with on-wafer standards, it can also be used with other planar transmission lines and with coaxial
transmission lines.

Running CAP

Before running program CAP you must load the BASIC operating system. Once the
BASIC operating system has been loaded, you can load program CAP by placing the program
disk in the system drive and typing LOAD "CAP". The program is executed by typing RUN. The
menu shown in Figure 1 will then be displayed on the screen. The menu contains all of the
information required to determine the dc capacitance of your lines. The menu may be refreshed
by pressing the SCREEN softkey. The menu may be saved on disk or retrieved from disk by
pressing the MENU softkey (shift SCREEN softkey).

The upper portion of the menu consists of a file list. Two files are shown. In the first row
is the file with the measured S-parameters of your lumped load. In the second row is the file
containing the effective dielectric constant of your line. In the third row is the file containing the
weighting function for the capacitance data. Usually this is the relative standard deviation file
produced by program MultiCal. If no file is specified, uniform weighting is used, as is the case
upon loading and running the program.

The first column contains a description of the file. The second column contains the file
names of each file. The third column lists the mass storage unit on which the file is or will reside
and the fourth column gives the path name (HFS formatted disks only). The fifth column gives

2


10. MuiltiCai’s Output Section

The output section may be entered immediately after the de-embedding process is
finished by pressing the data menu softkey. At this point, you can piot or store to disk the
line’s effective dielectric constant, attenuation constant, relative phase constant, or
characteristic impedance. You may also pilot or save the S-parameters of either of the
adaptors if a two-tier procedure was used. You can also plot the normalized standard
deviation and effective phase delay of the best line pair [1]. This data can be useful in
evaluating the quality of the calibration as a function of frequency. The output section aiso
has a submenu which calculates the capacitances needed to correct for differences of
calibration and measurement substrates using the method of [5]. Finally, it is possible to read
in data, calibrate it, and display or store it back to disk.

After the desired data has been selected, the data can be viewed on the screen’s of
some network analyzers by pressing the ANA softkey, in a frequency table on the computer's
CRT by pressing the CRT softkey, or on paper by pressing the paper softkey. Data can be
plotted on the screen by pressing the Plot softkey. The data can also be stored on disk in
either BDAT format by pressing the BDAT softkey, in a TOUCHSTONE compatible HP
format by pressing the ASCII softkey, in the HPUX file format by pressing the HPUX
softkey, or in a DOS format by pressing the DOS softkey.

Stored data files may be read from disk using the Get data softkey. The number of
frequency points in the file must match the number of frequencies in the de-embedded data.

MultiCal’s multiline TRL algorithm will calculate the propagation constant of the lines
used in the calibration, which MultiCal expresses as an effective dielectric constant. The
values are frequency-dependent, but shouid be smooth and close to the expected value of the
effective dielectric constant of the transmission line for reasonably high frequencies. Often
problems in the calibration can be detected by comparing the different propagation constants
calculated for.each line pair. The “d? key toggies between an optimum averaging scheme (the
defauit mode) and a debugging mode, in which each line pair consists of a fixed line and the
thru line. This debugging mode is useful for detecting errors in a particular line measurement
as it allows the propagation constants determined for each line pair to be compared.

The choice "Propagation constant and attenuation" includes the relative phase constant
(the "slow-wave factor") and the loss in dB/cm of the line standards. The propagation
constant is sent to the ANA as the real part of the "user 1" parameter, the attenuation constant
as the imaginary part.

Another valuable parameter is the effective phase delay. This quantity is also
frequency dependent. Its value is the effective phase delay at each frequency of the optimum
set of lines used in the calibration. This quantity is always between 0 degrees and 90 degrees:
90 degrees provides the minimum sensitivity to error. This quantity can be used to determine

August 1, 1995 13


Appendix 1. Foreign License Agreement

Foreign License Agreement for NIST On-Wafer Calibration Software

Use of the National Institute of Standards and Technology (NIST) On-Wafer Calibration
Software, which includes but is not limited to the programs MultiCal, FINDLEN, VERIFY, and
CAP, in whole or in part (hereafter referred to as “the Software”), is prohibited outside of the
United States of America except under terms of this license agreement. If you do not agree to be
bound by the agreement and have not returned a signed copy of the agreement to NIST, you may
not use the Software and must promptly destroy any copies of it. You must also promptly return
to NIST the original Software distribution media and any documentation.

(Software user)

(hereafter referred to as “the Software user”) agrees to be bound by the terms of this Agreement.

(Organization Representative)

agrees to commit the user’s employer, organization, or company,

(Organization)

(hereafter referred to as “the organization”) to be bound by this Agreement and represents that he/she
is authorized to make such a commitment on behalf of the organization.

1. This agreement binds both the Software user and the organization to its terms and conditions. This
agreement must be signed, dated, and returned to NIST before the Software user may use the Software.

2: This agreement grants the Software user limited rights to use the Software. This is a grant of a no-
fee, non-exclusive, non-transferable license to use the Software, and is personal to the Software user.
A separate agreement must be completed for each Software user, even if there are several users in the
organization. The Software user and the organization are responsible for installation, management, and
operation of the Software; it is their responsibility to ensure that the Software is used only in such a
manner so as to comply with the terms of this agreement.

3: The Software user is granted the right to publish the results obtained from using the Software
provided that, with the results, references appropriately the NIST publications which describe the
methods implemented in the Software or that he identifies the Software and the ownership of the
Software.

4. Title, ownership, and all intellectual property rights in and to the Software and Documentation shall

remain in NIST. This Agreement does not include the right to sublicense the Software and, therefore,
may not be assigned (by operation of law or otherwise) or transferred without the prior written consent

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