Agilent 8510-13
Measuring Noninsertable Devices
Product Note
A new technique for measuring components using
the 8510C Network Analyzer
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Introduction
The majority of devices used in real-world microwave systems are noninsertable because of the connectors employed. In the past, these devices were theoretically not measurable meaning that fully traceable and verifiable data could
not be provided. Now the Agilent Technologies 8510C with an S-parameter test
set offers a new technique that provides accuracy rivaling the best insertable
device measurements. This note reviews various methods used to calibrate the
network analyzer for measurement of noninsertable devices and compares the
results and uncertainties of each method.
As shown in Figure 1, for a test device to be insertable, both connectors must
be of the same connector family, with one connector male, and the other female.
When the test device is insertable, then the measurement ports can be connected together to establish the thru connection during transmission calibration.
The configuration of the measurement test setup need not be changed between
transmission calibration and actual measurement of the test device.
Figure 1. Classes of Device Under Test (DUT). For insertable devices, Port 1 connector
type will mate with Port 2 connector type. For reversible devices, the same sex of the
same connector family are used on both Port 1 and Port 2. Transitional devices use connectors of different families on Port 1 and Port 2.
There are two general types of noninsertable test devices. The first, and probably the most prevalent, is the reversible type of device. A reversible device is
one that has both connectors of the same family and sex. Note that devices
with hermaphroditic connectors, like precision 7 mm, are both insertable and
reversible. The second category of nonisertable devices is the transitional type.
With transitional devices, the connectors are of different families, such as
coaxial and waveguide.
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In terms of measurement accuracy and convenience, it would be desirable
if all purely coaxial microwave modules and cables were configured to be
insertable. In systems using coaxial cable to interconnect the modules and
subsystems, it is common for cables to use a male connector on both ends and
for the modules to use female connectors on all their ports. Often the designer
is faced with the same measurement problem due to the fact that the device by
nature is not insertable because it is a coaxial to waveguide transition. These
non-insertable configurations lead to testing problems because the device under
test measurement specifications are seriously compromised.
Why poorer specifications? Simply, the device cannot be inserted in the measurement system using the identical configuration in which the measuring
system alone was calibrated. The difference in system configuration between
calibration and measurement produces an uncertainty in the accuracy of the
results that can only be treated as a random error having an unknown magnitude and phase. This must result in an increase in the specification guardband
causing unnecessary rejects, increased testing and analysis time, and ultimately
an increase in the cost of the device.
Accuracy enhancement
At microwave frequencies systematic effects such as leakage, test port mismatch, and frequency response will produce uncertainties in the measured
data. However, in a stable measurement environment these effects are repeatable and can be measured by the network analyzer. This process is called
measurement calibration. During measurement calibration a series of known
devices (standards) are connected to the test ports and measured. The systematic effects are determined as the difference between the measured and the
known, or modeled, responses of the standards. Now the device under test is
measured. The accuracy enhancement algorithm uses an error model of the network analyzer system to mathematically relate these errors to the results of the
device measurement and thus obtain values for the actual responses of the DUT.
Under ideal conditions, with perfectly known standards, systematic effects
would be completely characterized and removed. The accuracy to which these
standards are known establishes how well these systematic errors can be characterized and removed. In fact, a well established figure of merit for a calibrated
system is the magnitude of the residual systematic errors. These residual errors
are the portion of the uncorrected systematic errors that remain because of
imperfect modeling of the actual response of the calibration standards.
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Error model
Vector error correction techniques can greatly reduce the effects of directivity,
crosstalk, source match, and frequency response errors as well as the mismatch
uncertainties caused by the vector interaction of the input and output impedance of the DUT with the imperfect source and load match of the measuring
system. The Agilent 8510 12-term calibration and measurement model provides
the highest accuracy in common use today (Figure 2). This 12-term error model
is used in both the FULL 2-PORT and the TRL 2-PORT calibration types
offered in the 8510C.
Figure 2. 12-Term Error Model. The most sophisticated l2-term error model is required to
account for transmission and reflection signal path response and leakages, source and
load mismatch effects in transmission measurements, and the effect of an imperfect
termination in reflection measurements.
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The ideal case
The ideal case for measurement of a 2-port device provides best accuracy and
complete traceable and verifiable data (Figure 3). This calibration procedure
uses an S-parameter test set to measure an insertable device. Steps 1 and 2
measure separate error terms for directivity, source match, and reflection signal path frequency response for both test ports. Transmission isolation is
measured with both ports terminated. In step 3, the test ports are connected
together to make the thru and then measure separate error terms for forward
and reverse load match and transmission signal path frequency response.
Figure 3. Ideal Case-2-Port Cal. Measurement of insertable devise using an S-parameter
(two-path) test set is the ideal case. All error terms can be measured with the correct
test port connectors in place, and the device under test does not require physical reversal.
Since the Port 1 and Port 2 connection to accomplish the thru is the same as
will be used during the measurement, all error coefficients can be obtained
directly.
With the device connected, step 4, the stimulus is applied to Port 1 and the
forward parameters, S
11
and S21, are measured. Then the stimulus is applied to
Port 2 and the reverse parameters, S
22
and S12, are measured. These measured
values, along with the error terms, are used in the accuracy enhancement algorithm to find the actual device parameters with greatly reduced uncertainty. In
this application the thru does not actually represent any specific device, or
the special case of an absence of any device at all. This is a basic principle of
transmission measurement calibration. The transmission reference is a zerolength, zero-loss transmission line. Essentially, Port 1 of the test set is connected
directly to Port 2 of the test set, then the ports are separated and the device
to be tested is inserted. Past practice has allowed complete performance verification with verifiable and traceable specifications only when this is the case.
If the device under test is not insertable, then it is necessary to use some sort
of thru device in order to accomplish the connection for measurement of
transmission response and load match. This is the problem. With the 12-term
calibration procedure, the thru connection is used to measure the transmission signal path frequency response and the impedance of the return port. But
what good does it do to measure the frequency response and load match error
terms when the wrong adapter is installed? During calibration, the thru device
is measured but it is removed or changed for the actual device measurement.
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