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Model 1222B50-100J
Rev A
Figure (1a) and (1b) shows the return loss of a typical balun
depicted in both single ended and differential modes.
Combining the measurements of S22 and S33 from Figure
(1a) with measurements of S23 and S32 using equation (1)
the differential mode return loss is shown in Figure (1b)
Phase and amplitude measurements using a
2-port analyzer
To evaluate the phase and amplitude balance of a balun it is
important to note that the system, in which it is measured,
must be repeatable to within the tolerance of which the
measurements are required.
So if a mechanical switch is employed to connect between
the two differential ports and the analyzer it has to be
repeatable enough not to perturb the results. Likewise, if a
simple approach of manually unhooking and re-hooking
coaxial lines, good RF-measurements techniques should be
followed.
Amplitude and phase balance is evaluated using the following
equation:
21
31
1020 S
S
LogAB = (2)
21
31
S
S
angPB = (3)
It should be noted that if the parameters are used in the
reverse order the results are still correct, but the balance will
have an opposite sign. However, in most case the user is
only interested in the absolute value and therefore the sign is
of no importance.
Back-to-Back measurements
This technique involves mounting two identical units in a
Back-to-Back configuration. This enables the user to evaluate
the insertion loss of both units in series and calculate the loss
by dividing the results by 2. The drawback in evaluating the
insertion loss of baluns in this manor is that balun #1 is used
to match into balun #2 and assuming good production
tolerances the result will become “too” good. The Back-to-
Back measurement technique gives valid results. However,
the results are a measurement of insertion loss in the case of
perfect match.
More representative insertion loss measurements are based
on adding the measurements of S21 and S31 after each of
the measurements have been deembedded and renormalized
to the target impedances.
22 13121020 SSLogIL += (4)
Equation (4) is used to determine the insertion loss of a
balun. This Equation only looks at the transmitted
energy and therefore the insertion loss due to miss-
matched ports is accounted for.
Evaluating balun performance of non 50
Ohm units
The technique of deembedding is a significant obstacle
in evaluating the actual performance of any microwave
system. For microwave devices like baluns the test
board must be deembedded correctly to achieve
correct S-parameters when performing a
renormalization of the port impedances.
If the impedance or line lengths are incorrect in the
deembed files, the performance measured could differ
from the performance of the part in an actual system.
Here is a list of parameters that are directly affected by
the deembed files:
1. For the phase and amplitude balance, the
deembed files have to represent the test board
used within the accuracy of the test being
performed.
2. For the return loss, the correct length of the test
board is crucial or the consequent
renormalization will fail.
3. For overall insertion loss both the length and
insertion loss of the test board is important to
know accurately.
Anaren will inform any customer, upon request, of the
deembed files and algorithms used in obtaining the
published results.
Multi-ports analyzer techniques
Multi-port analyzers have made the evaluation of balun
significantly easier, but still not without pitfalls. When
evaluating the overall performance of a balun one can
look at the balun as either a 3 ports device or a logical
2-port device, where one of the ports is balanced. The
results that the analyzer will present are significantly
different and will be covered in the following section.
Evaluating the performance of a balun as a 3 port
device
On a multi-port analyzer this is a straightforward
technique, which gives full S-parameters of the balun.
Each of the measurements must be combined in the
same fashion as described in the previous section. All
the same discussions apply and the same equations
should be used to evaluate performance parameters.