We are performing a time-domain measurement of S21 at a particular frequency by changing the sweep time option of the VNA. So, for the purpose of the following questions, assume that a VNA is configured as follows for a measurement:
1) No averaging;
2) Fixed IF bandwidth;
3) One sample point (one frequency point is sampled);
4) One network parameter measurement (S21).
Question 1: What is the effect of increasing the sweep time beyond the minimum determined by the configured IF bandwidth? Does the VNA measure the transmitted voltage for a longer amount of time with increasing measurement time (i.e. does the VNA use more periods of the continuous wave in order to determine S21)? Or, does the VNA measure the transmitted voltage for the same amount of time and just "wait" for the remaining time (i.e. regardless of the "sweet time" setting, does the VNA use the same number of periods of the continuous wave in order to determine S21)?
Question 2: How does the VNA know when to start measuring the transmitted voltage without knowing the propagation delay? Does it trigger on an edge? If so, can the VNA get "tricked" by radiation coupling to a device under test or some other external signal at the same frequency as the CW?