Waveguide media suffers from frequency dispersion, which basically means that signals at different frequencies that are launched at the same time into a waveguide will arrive at different times at the output of that waveguide. This phenomenon is not evident when looking at the frequency response of the waveguide. When looking at the time or distance response, however, you will see the effect of this dispersion. The VNA Master uses a special frequency‑to‑time conversion technique to compensate for this dispersion in waveguide media. This compensation is applied only in the distance domain.

Figure: S11 Measurement of 32 cm Long Waveguide Showing Dispersion Compensation shows the reflection response (S11) from a 32 cm‑long waveguide. The distance response shows a sharp peak at 32 cm, as expected. The time domain response shows a spread‑out peak at approximately 1.17 ns, which is the equivalent of 35 cm distance. Ideally, the S11 time domain response should be equivalent to the distance domain response. Because of the dispersion, however, the time domain response is spread out and inaccurate. The distance domain response, which has dispersion compensation, produces the corrected response. FGT is used in this screen capture to again illustrate how gating a DUT in the time domain will smooth out its response in the frequency domain. (Compare the measured response in TR4 with the FGT modified version shown in TR1).

 

 

 

Similarly, the effect of the dispersion compensation can be seen in the transmission response (S21) of a 15 cm long waveguide, as shown in Figure: S21 Measurement of 15 cm Long Waveguide Showing Dispersion Compensation. Note how the distance response is sharp and centered at 15 cm, whereas the time response is spread out and inaccurate.