RF Test Equipment Series: Calibration Kits
RF calibration kits are a key tool in ensuring that Vector Network Analyzer (VNA) and other RF instruments provide accurate measurements to the testing plane of the device. Due to the intrinsic addition of phase and impedance variation over RF test equipment interconnect, measurements from a VNA made without calibration will include the S-parameter and time-domain characteristics of the interconnect test system. A calibration kit is used to provide a reference plane that starts at the interconnect to the DUT. This way, complex de-embedding can be automatically performed by the RF test equipment (mainly VNAs) without additional manipulation of the data.
Purpose of an RF Calibration Kit (or Module)
- Provide corrective factors for broadband impedance matching of a VNA to the test plane
• Generate corrective factors for the limited directivity of VNA internal hardware
• Remove the frequency response introduced by test system interconnect to the test plane
There are a few variations of calibration kits. Mainly, there are two types, mechanical calibration kits and electronic calibration (ECal) modules. Mechanical calibration kits typically have a short, open, load, and through (SOLT) for typical coaxial testing, but there are a variety of other calibration kit types for various other calibration algorithms. The type of calibration kit and algorithm used is determined by the engineer and should be a best fit for the type of system the engineer is testing.
Calibration Algorithm Summary
- Short-Open-Load-Through (SOLT): common with coaxial interconnect
• Short-Short-Load-Through (SSLT): common with waveguide
• Short-Short-Short-Through (SSST): common for high frequency waveguide or coaxial measurements (shorts with different offset lengths)
• Reciprocal instead of through (SOLR/SSLR/SSSR): used when a good through isn’t available
• Line-Reflect-Line (LRL): high precision coaxial, waveguide, or wafer-level
• Advanced Line-Reflect-Match (ALRM): high precision
With a mechanical calibration kit a user needs to connect and disconnect each of the calibration standards to the test interconnect attached to the VNA ports in a precise process indicated by the calibration algorithm. As this process can be considered cumbersome, several companies have introduced eCal modules that include internal standards and automation that performs each step of the calibration algorithm with only one interconnect. This can help to mitigate the uncertainty introduced by variations in a manual connection process.
Calibration also introduces some level of uncertainty to the measurements. This is why there is an emphasis on the quality of a calibration kit. As these uncertainties often worsen at higher frequencies, calibration kits are typically specified to a maximum frequency. In some cases, the calibration kit interconnect may not be a perfect match to the VNA output ports and precision adapters are required to mate the calibration kit to the VNA interconnect. It is generally recommended to limit the interconnect to as short and lowest number of connections possible to ensure higher precision measurements, which is why some calibration kits are ordered alongside high-precision adapters.
Useful Calibration Kit Accessories and Equipment