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How to set up a cal kit with microstrip line standards.

Agilent network analyzers offer two models for the transmission medium - coax or waveguide.  They do not include a model for microstrip dispersion, where the electrical delay of a line varies with frequency.  If your calibration standards are in microstrip, you have to choose the coax model, and use average values for the electrical delays of your standards.

In the case of a TRL calibration, you do not need to know the precise delay of the line standard; this value is determined as part of the calibration process.  So you can specify an approximate value of delay from the center of your frequency range; it need only be accurate to +/- 1/4 wavelength or +/- 90 degrees over the entire range.

In the case of a LRL calibration (non-zero length thru standard), you can specify the thru standard to have zero length in the cal kit definition, so that the reference planes are established in the middle of the thru.  For the line standard's delay, enter the delay difference between it and the thru standard.  Then your device or fixture should be mounted between two lines that are each exactly one-half the length of the thru standard.  This approach is often used to make a calibrated measurement of a transistor or RFIC mounted on a PC board.  By moving the calibration reference planes to the middle of the thru standard, you can remove the errors of the PCB connectors and traces.

An SOLT calibration in microstrip is probably not a great idea, unless you have a narrow frequency range and can ignore dispersion.  If you do have a wider frequency range, you should break it up into several smaller bands.  For any cal standard that includes a length of microstrip line, you will need to define several standards in the cal kit definition, one for each frequency band.  Then approximate the dispersive delay with a different constant value for each band.


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