METROLOGY 101
Verifying the Integrity of S-Parameter Measurements Christopher L. Grachanen National Instruments
S-Parameters
ensemble i.e. VNA, cables and adapters and fixtures if used, to within acceptable limits. For transmission measurements these errors components are associated with measurement port isolation, load matching and the frequency response of transmission tracking. For reflection measurements, these error components are associated with measurement port directivity, source matching and the frequency response of reflection tracking. VNA SVC is performed using RF standards with known characteristics and/or known relationships. These standards are characteristically grouped together into a kit commonly referred to as a ‘VNA Cal Kit’ depending on the type of VNA SVC being performed. One of the most popular VNA SVC types is called SOLT (ShortOpen-Load-Thru). A SOLT VNA Cal Kit with have RF standards with characteristics of a shorted transmission line, an open transmission line, a termination (typically 50 ohms) as well as an adapter (thru) for connecting measurement ports together (not needed for a single port SVC). Another popular VNA SVC type is the LRL (Line-
Vector Network Analyzers (VNA) are used to measure a variety of RF parameters such as transmission loss, reflection loss, phase and group delay for a variety of passive RF devices and signal interconnections. RF devices span from attenuators and terminations to power splitters and directional couplers. Interconnections include RF cables, adapters, fixtures, probes as well as PCB signal traces. These RF devices and interconnections as measured by VNAs yields s-parameters. S-parameters describe the response of a RF device or interconnection to a stimulus (incident signal) applied to any or all of its connections (ports). S-parameters are essentially ratios comparing port stimuli to port responses to the applied stimuli. VNAs are configured with one or more ports depending on its intended purpose i.e. making singleend measurements (one or two ports), making differential measurements (four ports) and making multiple aggressor/ victim crosstalk measurements (greater than four ports). S-parameter naming nomenclature is port based denoting stimuli port and response port. For a two-port s-parameter, the naming nomenclature starts with an ‘S’ followed by the response port and then stimuli port: S11 = Stimuli on Port 1, Response on Port 1 (Reflection) S12 = Stimuli on Port 2, Response on Port 1 (Transmission) S21 = Stimuli on Port 1, Response on Port 2 (Transmission) S22 = Stimuli on Port 2, Response on Port 2 (Reflection)
VNA Calibration Prior to making s-parameter measurements, the VNA should be within its calibration interval if measurements need to be traceable, a prerequisite for accredited based measurements, and allowed sufficient warmup time. VNA calibrations are typically performed annually and should not be confused with VNA S-parameter Vector Cal (SVC) performed prior to making s-parameter measurements which are also often referred to as VNA calibrations. VNA SVC is used to remove error components from within the VNA measurement
Cal Lab: The International Journal of Metrology
Maury Microwave Type N, VNA Calibration Kit, model 8850CK40. Photo courtesy of Maury Microwave.
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Apr • May • Jun 2020
