4 If an adapter with an input match SWR = 1.03 is used, a coupler directivity of 40 dB can be degraded to 32 dB. It is the vector sum of all leakages and reflections including residual reflection effects of adapters and test cables. The directivity term is most significant for devices having a small reflection. To illustrate the measurement uncertainty introduced by a test adapter, consider the directivity term, which is independent of the device to be measured. 4-6 This equation does not take into account the phase of the error terms therefore, it is the most pessimistic prediction of the uncertainty. Here, Equation 1 shows that the measured value of S11M includes the actual S11A, the directivity, reflection tracking, source match and load match error terms. To understand the effect of these terms on the measurement of a device, a simplified equation can be used for the worst case uncertainty of a reflection measurement. Forward and reverse isolation E XF, E XRįigure 1 shows the six error terms associated with the forward calibration of a two-port circuit.Forward and reverse source match E SF, E SR.Forward and reverse transmission tracking E TF, E TR.Forward and reverse load match E LF, E LR.Forward and reverse reflection tracking E RF, E RR.Forward and reverse directivity E DF, E DR.A 12-term error correction like the one implemented with a two-port TRM calibration corrects for the following systematic error terms: Since the systematic errors can be measured, however, they can also be corrected using calibration standards. They introduce most of the measurement uncertainties. The systematic errors that occur during microwave measurements are those that are repeatable and can be measured by a network analyzer. The TRM calibration technique establishes the 12-term error correction for each frequency point.
2-3 The name TRM represents the three standards that are used in the calibration - thru, reflect and match. 2 The TRM offers equivalent accuracy and has the advantage that fewer and simpler standards are required. The TRM calibration is an alternative to the traditional two-port error correction that uses short, open, load and thru standards (SOLT). Its performance is comparable to an SMA connector up to 12.4 GHz. Although the ZMA connector can be optimized to work up to 22 GHz, current requirements ask for a calibration kit up to X-band. The ZMA interface is a high coupling force bayonet connector used in microwave applications. The design was performed with the aid of Ansoft HFSS simulation software to characterize and optimize the calibration standards. This article discusses the design of a TRM calibration kit for the ZMA interface.
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