What are Waveguide Calibration Kits?
Calibration kits are an essential tool in ensuring vector network analyzer (VNA) measurements are accurate and consistent. Without calibration before testing, the algorithms used in modern VNAs would not be able to sufficiently account for the variations in interconnect and components inline to the device-under-test (DUT), and these variations would result in extensive uncertainty. Most VNAs have coaxial test ports. Hence, most VNA calibration kits are based on coaxial interconnect. This arrangement serves well for DUTs that have coaxial interconnect also but may not be ideal for DUTs with waveguide interconnect. This is why there are also Precision Waveguide Calibration.
There are some distinct differences between coaxial VNA calibration kits and waveguide VNA calibration kits. Part of the differences stems from the reality that the standard method of VNA calibration, short/open/load/through (SOLT), isn’t accessible to waveguide calibration. The short in a SOLT kit requires a very good complete reflection of incident power with S11 = 1. The same goes for an open, where a short and open ideally have 0 dB return loss. The short and the open must have a 180-degree phase difference at all frequencies for this to work properly. The load is designed to absorb all of the signal energy with zero reflection, hence infinite return loss.
It is possible to make precision loads and shorts with a waveguide. However, making a waveguide open with 0 dB return loss has yet to be achieved due to the intrinsic operation of a waveguide open as a radiator. To overcome this challenge, an offset spacer is used to offset the short from the end of the waveguide. These spacers are typically ¼ wavelength, ⅛ wavelength, and ⅜ wavelength. Hence, with a ¼ wavelength spacer, the reflected signal from the short can be phase shifter by about 180 degrees, which functionally acts as an open for the waveguide calibration. This is called a short-short-load-thru (SSLT) calibration.
Since this isn’t a perfect approach and waveguides are not ideal over all frequencies, especially considering manufacturing and assembly tolerances of the short/spacer combination, then there is some variation over frequency that is different from the ideal 180 degrees. This can often be accounted for though by VNA software algorithms and a precision calibration can be made in this way. However, the VNA will need to have a precise figure for the delay of the spacers used. A one port calibration with a short, then spacer/short combination, and lastly a load step can be used to the 1-port calibration of a VNA. A 2-port VNA calibration looks much like a SOLT calibration with a coaxial VNA calibration kit with the exception that the short/spacer combination replaces the open.
Thru-reflect-line (TRL) calibrations can also be performed with waveguide VNA calibration kits when that feature is needed. It is also often advantageous to have waveguide straights when performing laboratory testing on waveguide interconnect based DUTs. It is important that these straights are also precision waveguide components to minimize the impact of manufacturing tolerances, the same way precision adapters and cable assemblies are used for coaxial VNA calibrations.
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Peter McNeil | Nov 23, 2022 | Calibration Kits
RF VNA Calibration Kits are essential test and measurement hardware for nearly all types of VNA measurements. Though VNAs come with internal calibration from the manufacturers, there are a variety of environmental and temporal factors that could lead to significant measurement errors to take into account. This is the purpose of VNA Calibration Kits, of which, the most common type is the short, open, load, and thru (SOLT) calibration kit.
A VNA calibration kit is a set of VNA calibration standards that should be traceable to measurement standards, such as the US National Institute of Standards and Technology (NIST) standards. The calibration standards for VNAs are generally coaxial interconnect standards for a short, open, load, and through (VNA Short Open Load Thru (SOLT) Calibration Kits). Traditionally, these kits come with two shorts, two opens, two loads the same as the characteristic impedance of the transmission lines in the system, and a through (thru) standards. The most common coaxial VNA calibration kits are either 50 Ohm or 75 ohm, though there are others impedances.
SOLT Calibration Kit
It is common enough for a VNA calibration kit to also come with some precision adapters to common coaxial interconnect types. A VNA calibration kit is typically chosen to match the VNA test ports, or to match a frequently used coaxial connector type based on the devices-under-test (DUT) or what happens to be available in the test lab.
There are also VNA calibration standards that come in a combined format, such as 3-in-1 Short Open Load (SOL) Calibration Standard. These calibration standard units come with at least a short, open, and load port and are generally purchased in pairs. There are some calibration standard combined units that come with a through as well, but a precision inline adapter often provides this same function without the need to include it in a calibration standard.
RF VNA Calibration Kits also often include precision wrenches and torque wrenches specifically designed for use with VNA Calibration Kits. The wrench sizes and torque are specifically paired with the coaxial connector standard and the recommended torque specified for the coaxial connector type. In some cases, a calibration standard may be damaged or misplaced over time. In this case, individual 50 Ohm VNA Short Open Load Thru (SOLT) Calibration Components or 75 Ohm VNA Short Open Load Thru (SOLT) Calibration Components are often sold piecemeal instead of in a complete kit. This is a much more affordable approach if only one or two standards are needed instead of an entire kit.