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Manufacturing.

Model X3C25F1-02S

Rev D

The effects of the test fixture on the measured data must be minimized in order to accurately determine the

performance of the device under test. If the line impedance is anything other than 50

 and/or there is a discontinuity

at the microstrip to SMA interface, there will be errors in the data for the device under test. The test environment can

never be “perfect”, but the procedure used to build and evaluate the test boards (outlined below) demonstrates an

attempt to minimize the errors associated with testing these devices. The lower the signal level that is being

measured, the more impact the fixture errors will have on the data. Parameters such as Return Loss and

Isolation/Directivity, which are specified as low as 27dB and typically measure at much lower levels, will present the

greatest measurement challenge.

The test fixture errors introduce an uncertainty to the measured data. Fixture errors can make the performance of the

device under test look better or worse than it actually is. For example, if a device has a known return loss of 30dB and

a discontinuity with a magnitude of

–35dB is introduced into the measurement path, the new measured Return Loss

data could read anywhere between

–26dB and –37dB. This same discontinuity could introduce an insertion phase

error of up to 1

.

There are different techniques used throughout the industry to minimize the affects of the test fixture on the

measurement data. Anaren uses the following design and de-embedding criteria:



Test boards have been designed and parameters specified to provide trace impedances of 50

1. Furthermore, discontinuities at the SMA to microstrip interface are required to be less than

–35dB and insertion phase errors (due to differences in the connector interface discontinuities

and the electrical line length) should be less than

0.50 from the median value of the four

paths.



A “Thru” circuit board is built. This is a two port, microstrip board that uses the same SMA to

microstrip interface and has the same total length (insertion phase) as the actual test board. The

“Thru” board must meet the same stringent requirements as the test board. The insertion loss

and insertion phase of the “Thru” board are measured and stored. This data is used to

completely de-embed the device under test from the test fixture. The de-embedded data is

available in S-parameter form on the Anaren website (www.anaren.com).

Note: The S-parameter files that are available on the anaren.com website include data for frequencies that are

outside of the specified band. It is important to note that the test fixture is designed for optimum performance through

4.0GHz. Some degradation in the test fixture performance will occur above this frequency and connector interface

discontinuities of

–25dB or more can be expected. This larger discontinuity will affect the data at frequencies above

4.0GHz.

Circuit Board Layout

The dimensions for the Anaren test board are shown below. The test board is printed on Rogers RO4350 material

that is 0.020

” thick. Consider the case when a different material is used. First, the pad size must remain the same to

accommodate the part. But, if the material thickness or dielectric constant (or both) changes, the reactance at the

interface to the coupler will also change. Second, the linewidth required for 50

 will be different and this will introduce

a step in the line at the pad where the coupler interfaces with the printed microstrip trace. Both of these conditions will

affect the performance of the part. To achieve the specified performance, serious attention must be given to the

design and layout of the circuit environment in which this component will be used.

If a different circuit board material is used, an attempt should be made to achieve the same interface pad reactance

that is present on the Anaren RO4350 test board. When thinner circuit board material is used, the ground plane will

be closer to the pad yielding more capacitance for the same size interface pad. The same is true if the dielectric

constant of the circuit board material is higher than is used on the Anaren test board. In both of these cases,

narrowing the line before the interface pad will introduce a series inductance, which, when properly tuned, will

compensate for the extra capacitive reactance. If a thicker circuit board or one with a lower dielectric constant is used,