The VNA Master uses two types of processing to transform frequency domain data to time data (or distance data). Bandpass processing is the standard processing technique that can be applied to all frequency sweep setups. Only impulse response can be displayed in this mode. Lowpass processing is a technique that can be used only where frequency content that is fairly close to DC is available. This technique creates a pure real transform and can produce a step response in addition to an impulse response. For the same frequency sweep width, lowpass processing produces time (or distance) resolution that is a factor of 2 better than that of bandpass processing.

The VNA Master (when Domain Processing is set to Auto) always tries to use lowpass processing whenever the frequency sweep has low frequency content. For band‑limited sweeps with a starting frequency not near DC (such as for waveguide devices), the VNA Master automatically defaults to bandpass processing.

You can force the instrument to always use bandpass processing as follows: go to Application Options (press Shift 8 (System), Application Options, Time Domain). In the Time Domain Options menu, press Domain Processing to select BP Only). For most setups, however, you should take advantage of lowpass processing whenever possible.

The screen capture in Figure: Low Pass Return Loss Using Impulse Response shows a DUT with a 6 dB return loss measured in lowpass time domain mode using an impulse response. A clear 6 dB reflection is shown at approximately 6 cm from the test port.

The same DUT measured with bandpass time domain processing is shown in Figure: Band Pass Return Loss Using Impulse Response. The reduced resolution in this mode is apparent as the 6 dB reflection becomes spread out (wider peak).