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Vector Network calibration with a 75 Ohm reference impedance

Submitter: M. Ruiter & E. vd. Wal
Description: Calibration is a daunting and tedious task, also for microwave circuits. The better the calibration the better the results when measuring for example return loss, transmission phase or group delay. Therefore, we need some a priori knowledge or reference to find out what the errors are and where they are located in the measurement setup. Shown in the figure, we use a two-port vector network analyzer, a wooden box with 75 Ohm reference impedance standards for F-type connectors, An E-Cal module, cables and adapters.

Most RF measurement instruments work with 50 Ohm reference impedance, which has become the standard way of measuring devices. In fact it has become so standard, that 50 ohms is mandatory. The solution we now use is to continue measuring in the standard 50 Ohm impedance and then to transform the results afterwards.

In the measurent setup for a 2-port network, twelve error sources can be identified, all of which are also strongly frequency dependent. The calibration needs to find the complex values for all these error terms. A calibration method called SOLT can be used to solve for all twelve error terms. Part of this procedure is to attach known standards to each port. The wooden box contains 6 of these reference standards which are mechanically precision crafted. The procedure is checked with other known references. This sanity check also gives approximate numbers for the confidence level of the calibration.

Now the big trick is the E-Cal module. This little box can calibrate the complete measurement setup in a 50 Ohm system in a very reliable and repeatable fashion. The module has several unknown internal standards, which can be electronically selected. For a given calibration reference plane and reference impedance, these internal standards can be measured. These internal standards, together with the measurement results can then act as secondary standards. Using the secondary standard within the E-cal prevents the tedious work of screwing and unscrewing the expensive reference standards.

With this in place we can speed up calibration, and measure 75 Ohm devices, such as F-type adapters, LOFAR and APERTIF cables very accurately. This knowledge allows us to select the right cables and connectors for the right job, saving money at the end.

Copyright: ASTRON
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