Measurement calibration is an accuracy enhancement procedure that effectively removes the systematic errors (repeatable measurement variations) that cause uncertainty in measuring a device under test (DUT). During measurement calibration, the physical layer test system measures actual, well-defined standards and mathematically compares the results with ideal "models" of these standards. Calibration measurements, which characterize the test system, are made with all cables and connections in place but without the DUT.
Systematic Errors are related to signal leakage, signal reflections, and frequency response of the test system. There are six types of systematic errors.
Directivity and crosstalk related to signal leakage
Source and load impedance mismatches related to signals being reflected
Frequency response error caused by reflection and transmission tracking with the test receivers
Other factors that can impact the measurement accuracy of any measurement system are drift errors and random errors.
Drift Errors are due to the instrument or test-system performance changing after a calibration has been done. Drift is primarily caused by temperature variation and it can be removed by recalibration. The time frame over which a calibration remains accurate is dependent on the rate of drift that the test system undergoes in the test environment. A stable ambient temperature usually minimizes the rate of drift significantly. Allowing equipment to warm up and stabilize prior to calibration and properly ventilating equipment helps reduce drift errors.
Random Errors are unpredictable since they vary with time in a random fashion. Therefore, they cannot be removed by calibration. The main contributors to random error are instrument noise such as, source phase noise, sampler noise, and IF noise. The accurate source and phase-locked receiver of the network analyzer greatly minimizes these random errors. There are also external contributors to random errors such as switching power supplies, EMI, etc.
Components of the measurement setup such as imperfect connectors, cabling, and even the response of the test instruments can introduce errors into measurements. For both transmission and reflection measurements, impedance mismatches within the test setup cause measurement uncertainties that appear as ripples superimposed on the measured data. These errors can distort the signal and make it difficult to determine which reflections are from the DUT and which are from other sources.
Calibration is required for accurate measurements. Even though calibration does take a few minutes to complete, it saves time and money compared to costs associated with erroneous measurement data. Even mechanical (non-electronic) calibration is reasonably quick once you become familiar with the process.
TIP Understanding How Changes Affect Measurements
No two measurements and environmental conditions are exactly the same. The best way to understand your conditions is to experiment and see how your test equipment behaves over a period of time. A good way of doing this is to measure the same device (i.e., a known standard) hourly throughout the day. Save or print the measurement results of each measurement. Compare these results to gain an understanding of how the ambient environment and drift affect the measurements. Watch for trends with regard to the device meeting specifications or measuring within guard band limits.
We recommend that you perform a calibration on your physical layer test system when the following conditions occur:
When connectors are cleaned, repaired, or replaced.
If test cables have any changes, such as:
When a test cable is replaced
When any connection is changed except the connections to the DUT
When test cables are flexed excessively (kinked or unkinked)
If the frequency range is changed beyond the limits of the previous calibration
If the number of measurement points is increased to more than the number of points of the previous calibration
When ambient temperature changes more than 3 °C
Any other ambient environmental changes of significance
If none of the previous conditions apply, calibrate according to the intervals shown below due to drift:
Check the calibration daily at a minimum (twice daily is recommended). See How to Verify a Calibration.
Calibrate weekly (daily calibration recommended).
A good method of checking calibration is to establish a "Golden Device", which is a device that meets all specifications and is saved for comparison of the measured results in the future.
Establishing a Golden Device
Calibrate the system.
Perform the complete set of measurements on the golden device.
Save and print all of the test results from these initial measurements.
Now you can measure the golden device when you suspect that your system may need to be calibrated. Compare the results of these measurements against the results that you saved and printed from the initial measurements.
Note: At any time you can initiate a re-calibration of your measurement hardware, Characterize an Adapter, or Edit a Cal Kit. Select Utilities, then select your hardware setup.