Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Calibration
Reexamination Certificate
2002-01-15
2003-11-18
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Calibration
C324S638000
Reexamination Certificate
active
06650123
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for characterizing interface devices, such as adapters, test fixtures and other networks, used with a vector network analyzer (VNA). More particularly, the present invention relates to methods for determining scattering-parameters (S-parameters) of such interface devices.
2. Description of the Related Art
A vector network analyzer (VNA) is an instrument that is typically used to measure complex transmission and reflection characteristics of devices under test (DUTs). Different DUTs can have different types of connection configurations at their ports. For example, a two-port DUT can have one of many of possible configurations of connectors at its two ports. Some DUTs, known as “insertable” DUTs, have two connectors that are from the same connector family and of opposite sex, one connector being male and the other being female. An insertable two-port DUT is configured such that calibration can be performed by connecting ports of a VNA together with the aid of a cable to establish a thru-connection, during calibration, and without having to change the configuration measurement setup for the actual measurement of the DUT.
In contrast, a “non-insertable” DUT cannot be connected to the two ports of a VNA without use of adaptors or other test fixture arms (referred to collectively hereafter as interface devices). An example of a non-insertable device is a “reversible device,” which has two connectors of the same family, but also of the same sex (i.e., both being male or female). Another example of a non-insertable DUT is a transitional device that has two connecters that are of different families (e.g., one connector being a coaxial cable and the other being a waveguide). Stated generally, interface devices (e.g., an adapters or test fixture arms) are required when using a VNA to make measurements of certain devices (e.g., non-insertable devices).
When interface devices are used to connect DUTs to a VNA, there is a need to remove the effects of such interface devices. This can be accomplished by determining the scattering-parameters (S-parameters) of the interface devices in question and then de-embedding (i.e., mathematically removing) them from measured data. The present invention relates to characterizing of interface devices. Accordingly, the focus hereafter shall be directed to characterizing interface devices, and any discussion of de-embedding (methods of which are well known in the relevant art) will be limited.
S-parameters of a multi-port device characterize how the device interacts with signals presented to the various ports of the device. An exemplary S-parameter is “S
12
.” The first subscript number is the port that the signal is leaving, while the second is the port that the signal is being injected into. S
12
, therefore, is the signal leaving port 1 relative to the signal being injected into port 2. The four S-parameters associated with an exemplary two-port device
102
are represented in
FIG. 1
, where:
S
11
is referred to as the “forward reflection” coefficient, which is the signal leaving port 1 relative to the signal being injected into port 1;
S
21
is referred to as the “forward transmission” coefficient, which is the signal leaving port 2 relative to the signal being injected into port 1;
S
22
is referred to as the “reverse reflection” coefficient, which is the signal leaving port 2 relative to the signal being injected into port 2; and
S
12
is referred to as the “reverse transmission” coefficient, which is the signal leaving port 1 relative to the signal being injected into port 2.
A number of procedures have been developed for determining the S-parameters of interface devices (e.g., adapters). In a first technique, referred to hereafter as method A, S-parameters are determined using a simple one port calibration and some measurements with an interface device in place. Method A (often known as the “reflective-termination” method) shall be described with reference to
FIG. 2
, which shows a first port of a two-port interface device under test
204
coupled to a calibration port of a VNA
202
and a second port of interface device under test
204
coupled to reflective standards
206
. Reflective standards
206
normally have reflective coefficient magnitudes of unity and phases differing by pi (&pgr;). Method A, however, can only determine one reflection coefficient (i.e., S
11
). Further, the high dependence on the standards quality (i.e., the quality of reflective standards
206
), among other reasons, tends to increase the uncertainty of this method and can lead to larger errors than may be acceptable.
A second technique, referred to hereafter as method B, is similar to method A but uses multiple line lengths after the interface device to acquire more information. Method B (often known as the “multiline one-port” method), illustrated in
FIGS. 3A and 3B
, can be used to extract all S-parameters and is less dependent on standards quality. However, method B is relatively complicated, even for a single port. In method B, a VNA
302
is used to make a series of measurements of several standards (typically a load
306
plus several line lengths
308
,
310
with reflective standards
312
,
314
attached) both with an interface device under test
304
present (as shown in
FIG. 3B
) and absent (as shown in FIG.
3
A). Accordingly, for a multi-port structure (i.e., an N-port structure, where N≧2), method B can be quite time consuming. Further, in a fixtured or probing environment, a non-coaxial side of interface device
304
may be extremely space constrained so it may not be practical to construct the different line lengths required for method B. Since the differences in line length must be of the order &lgr;/4 at the operating frequency, this problem is particularly acute in the radio frequency (RF) ranges where the manufacturing need for such a method is greatest. Additionally, more line lengths than two are needed for broader frequency ranges.
A third technique, referred to hereafter as method C, uses two port calibrations at both ends of an interface device under test to determine the full S-parameters of the interface device. Method C is illustrated in
FIG. 4
for a single interface device under test
404
(e.g., a fixture arm). While this process can work for a single interface device, it becomes quite time-consuming for N interface devices (e.g., an N-port fixture structure) and many calibrations maybe required. The following is a description of how method C would be used in a common two port problem.
Consider a wafer-probe environment where the desire is to find the S-parameters of the probes themselves (i.e., each probe is considered a two-port interface devices in this example). To implement method C, a pair of calibrations are required for each port, resulting in a total of four calibrations (although, all may not be needed in some cases). This basic example is shown in FIG.
5
. Referring to
FIG. 5
, two interface devices
504
a
and
504
b
(i.e., wafer probe A and wafer probe B) are connected, respectively, to port 1 (P1) and port 2 (P2) of a VNA (not shown). As just mentioned, the desire is to find the S-parameters for the wafer probes (i.e., interface devices
504
a
and
504
b
) themselves.
The use of method C to find the S-parameters for the probes will now be described with reference to
FIGS. 6A-6C
. Using method C, a two port calibration at coaxial connector
506
a
is first performed. For this to make sense, the two wafer probes
504
a
and
504
b
must be connected by a thru
602
, as shown in FIG.
6
A. This is a critical step since this must be a true thru (i.e., no extra parasitics and reference planes in the wafer domain must be aligned). Further, extra length cannot be easily corrected for since there is an effect on the mismatch at the two wafer probes
504
a
and
504
b
. The next calibration is performed at the wafer level using commonly available calibration substrates
604
, as shown in FIG.
6
B. As just suggested, the refer
Anritsu Company
Fliesler Dubb Meyer & Lovejoy LLP
Le N.
Nguyen Vincent Q.
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