August 9, 2013
A basic capability of Vector Network Analyzers (VNAs) is to measure the S-parameters of an RF/microwave device and display the result in the frequency domain. In addition to showing performance in the frequency domain, certain VNAs have a capability that can present data in the time (or distance) domain, no doubt valuable information for design and production engineers.
The benefit of the time domain response is in providing information of the device performance at specific locations. As a result, VNAs, such as the Anritsu VectorStar™, that have this capability can provide critical performance information for the device both as it appears when installed in a system and at a specific location within the device.
VNAs typically convert frequency domain data into the time domain using the Fourier transform method. Specifically, a variation of the chirp z inverse transform is used.
For most microwave applications, the principal property of interest in time domain processing is resolution. This is inversely related to data collection bandwidth in the frequency domain. A rule of thumb defines resolution on the order of 150 mm (for air dielectric) divided by the Frequency Span (GHz). The broader the frequency span, the more information presented during the time domain analysis.
An exception to the frequency span rule is realized when measuring devices with limited bandwidth. For cases where the frequency sweep of the VNA is wider than the bandwidth of the DUT, the resulting time domain resolution will be limited by the bandwidth of the DUT.
Time domain offers a number of processing alternatives. It is important that engineers be aware of the feature set available, as selection can have a significant effect on the end result. These tools include:
• Processing – Engineers can select from two processing methods. Low pass, which has twice the spatial resolution of the alternative band pass mode, is useful in determining the characteristics of a discontinuity or component. The DC value must be approximated by extrapolation, and harmonic calibration is required. The lower the start frequency of the VNA, the more accurate the DC extrapolation calculation. With a start frequency of 70 kHz, the Anritsu MS4640 series offers the lowest available DC extrapolation point, thereby reducing the DC data point error.
Many situations, such as when measuring waveguide or band-limited DUTs, preclude using a broadband, harmonically-related frequency plan. For these cases, band pass processing should be used. Since the band pass mode does not include a DC value, only the impulse excitation is available.
• Response – If low pass is used, typical responses are either step or impulse. Impulse can also be used in band pass mode, as can Phasor Impulse, exclusive to Anritsu VNAs. Phasor Impulse enables the user to extract impedance information for a specific discontinuity from a band pass display. When Phasor Impulse processing is applied, the resulting display provides the desired impedance information.
• Window Shape – Windows are used to “condition” data prior to transformation. They are used to get around the Fourier transform’s basic (but impractical) requirement to use frequency terms stretching from –∞ to +∞, and to mitigate edge effects associated with a finite data set.
• Gating – Essentially a filter in the time domain, the gate permits observing a selected range without the influence of unwanted elements, such as connectors or multi-path signals.
This post describes the principles of time domain and how certain VNAs can conduct this important analysis. There will be more posts in the near future describing other key aspects of VNA functionality and how VNAs can provide confidence to R&D engineers working on cutting-edge designs. Until then, please visit our VNA page for more information.
