Test Equipment FAQ

1. Why do I get a syntax error from the 8510 when WinCal attempts to save a calibration set?
2. How do I enter L (inductance) values for parasitics of my standards into my network analyzer cal kit definitions? All I see is the offset delay and offset impedance.
3. Why do I get a 'Calibration no longer acceptable' result when I check stability immediately after calibration?
4. Why does the HP8510 Cal Disk (P/N 101-338) specify a -0.5 ps delay for the A. LRM match standard? How do I specify LRM entries when using the HP8510 and WinCal?
5. How come I see a 180 degree step in phase in my S21 phase response when I measure my reciprocal Thru standard after an SOLR calibration?
6. How do I configure my Summit system to make a 4-port capacitance measurements using the Agilent 4284 LCR meter?

1. Why do I get a syntax error from the 8510 when WinCal attempts to save a calibration set?
This sometimes occurs when using larger numbers of frequency points. The 8510 does not have memory to support all 8 cal sets filled with maximum size arrays. This error can occur even with several cal sets deleted. Delete a few more to free enough memory.

2. How do I enter L (inductance) values for parasitics of my standards into my network analyzer cal kit definitions? All I see is the offset delay and offset impedance.
Parasitic inductance of the short and load are required for SOLT calibration, but some network analyzers do not allow one to enter these values directly. Then, the inductance can be modeled as a short piece of transmission line. Assume a maximum impedance that the network analyzer allows (typically 500 Ohms), which will give you offset impedance. Offset delay can then be calculated using equation: td = L / Z_offset. Any other information can be typically left as default. For capacitance of an open, only C_o (0th order model) is used in Cascade Calibration Kit definitions.
For more info, see "Microwave Wafer Probe Calibration Constants Instruction Manual," PN101-338

3. Why do I get a 'Calibration no longer acceptable' result when I check stability immediately after calibration?
This is an indication of poor repeatability in your measurement system. One way this can occur is when your calkit was radically misdefined or perhaps you measured a short circuit when you should have measured an open. This can create a high sensitivity to measurement system variability.

More likely is that something is wrong in the measurement system. Using your VNA with calibration turned off, compare the magnitude of measured reflection coefficient for a load and a short on each port. Expect more than 10 dB of change in order to get good calibration results.

This experiment can be repeated with coaxial standards at the ends of the cable, and at the VNA front panel to further isolate the problem. If the problem occurs when using the VNA alone there may be a problem with the VNA. Check your VNA setup. Low source power, high port attenuation, or ramp sweep will reduce VNA repeatability. Using inadequate averaging can also reduce dynamic range.

Check all cables and connections. Properly torque all connectors. Make sure that you are using good quality phase stable cables. With a poor cable any strain or inadvertent bump from the user will change the electrical behavior enough to spoil a calibration. Semi-rigid cables may be used but avoid any stresses at the connectors and properly secure the cable to minimize vibration or other motion.

If your system is performing at its best the thresholds for 'good' and 'acceptable' calibrations can be changed to provide guidance for nominal system performance.
Source: DS For more info, refer to "Calibration and Accuracy Factors Summit High-Frequency Probe Station Reference Manual," PN103-475-A

4. Why does the HP8510 Cal Disk (P/N 101-338) specify a -0.5 ps delay for the A. LRM match standard? How do I specify LRM entries when using the HP8510 and WinCal?
To implement LRM directly on an 8510 we must use the TRL algorithm. This algorithm computes the line versus Thru transmission per unit delay as a function of frequency. In the LRM case this transmission value doesn't really come into play and it is only important that we make sure that the algorithm reaches a solution. We are using a Match termination in place of the second line so our transmission is essentially zero (limited by crosstalk). This is OK since the high attenuation does not impact the TRL solution.

However if we have entered a line (Match) delay of 0 ps we end up with a divide-by-zero problem that causes the algorithm to fail. Since it is not critical, any non-zero number will work for Load delay entry. For example, -0.5 ps, 1 ps, and 5 ps would all yield the same calibration result.

The 101-338 calkit entry for LRM reflect standards (open and short) is technically not exactly correct, but will work just fine in practice. The reference plane is the center of the Thru line so really the reflect standards should be defined with a -0.5 ps offset. In practice this is not necessary since this information is only used to select a sign of a square-root and the phase shift of such a short delay is not enough to cause an improper root selection. For very long Thru's a problem may occur and it will be necessary to properly enter the delays as one-half of the Thru line length.

In all cases the reference plane may be shifted from the center of the Thru line to the probe tips by using the port extensions at each port to specify a delay equal to one-half of the Thru line standard's delay. Since the loss of the Thru line is not compensated by port extensions the loss of longer lines may introduce noticeable error (1 ps lines are very low-loss).

When using WinCal one can use this same procedure to perform an LRM calibration using WinCal's TRL algorithm. In this case all of the above comments apply. This would be unusual. Instead, it is simpler to use the LRM/LRRM algorithms in WinCal. In these cases the actual numbers are entered as described in the help file. If a zero Thru delay is desired it is necessary to enter a tiny number (e.g., 0.01 ps) to work around a minor bug (versions up through 2.23). Also uncheck the "Use computed propagation constant to move reference plane" option.

5. How come I see a 180 degree step in phase in my S21 phase response when I measure my reciprocal Thru standard after an SOLR calibration?
This kind of step behavior in the transmission phase indicates that the SOLR algorithm has not found the proper root choice in solving for the standard. Adjust your guess for the Thru delay estimate to eliminate the phase step behavior.

6. How do I configure my Summit system to make a 4-port capacitance measurements using the Agilent 4284 LCR meter?
Cascade has strived to provide an on wafer probing solution for low-level 4-port capacitance measurements. This solution includes BNC to triax adapters, cables of a specific length to accommodate accurate cable compensation, and triaxial probes capable of connecting the Hi-Pot and Lo-Pot guard voltages together close to the device under test.

A complete schematic drawing, and a list of component parts needed to make this measurement, are available in the Cascade Microtech Configuration Guide: DC/CV Parametric Probing Configuration Guide, and measurement tips are in the Application Note: Achieving High-Accuracy On-Wafer Capacitance Measurements.