Understanding Directivity of a Vector Network Analyzer and Its Impact on Accuracy


Directivity in the context of Vector Network Analyzers (VNAs) refers to the ability of the measurement system to distinguish between the signal that is supposed to be measured and any unwanted signals, leakage or reflections, which may be present.

Generally, you could state that it is a measure of how well a directional coupler (or bridge), which is used in VNAs for signal separation, can isolate the forward-going signal from any reverse signals that occur due to impedance mismatches. However, depending on the VNA architecture, you could reason that you want the opposite as well, to ensure the reflected signals are isolated from the forward-going signal.

Since it is a ratio-based specification, high directivity indicates a greater ability to separate the incident wave from the reflected wave, leading to more accurate measurements of the device under test.

The importance of directivity is most evident when it comes to error correction where it can alter the response from the DUT. Random errors in magnitude and phase due to poor directivity result in less-than-optimal measurements. 

To illustrate, consider the following example where the magnitude and phase of the incident signal (actual) is influenced by the reflected (error). This shifts the final measurement to indicate that the impedance is less reactive (jX) and more resistive (R). 

This could go another way such that the error causes the measured impedance to show a device as having greater a reactive (jX) property than resistive (R).

While countless permutations of this illustration are possible, it is important to know that higher directivity helps to ensure that the error contribution is minimal. 

Directivity is essential in VNA measurements to ensure that the S-parameter data accurately represents the true performance of the device being analyzed, without contamination from extraneous reflected signals.


For information on Bird VNAs, see https://birdrf.com/Products/Analyzers/VNA. 

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