The longest DUT length that can be measured is a function of the propagation speed of electromagnetic waves in the DUT and is calculated in consideration of the user's application. Consider a 2-port DUT in which signals propagate from port 1 to port 2 (or vice-versa). The longest length DUT for which the SPARQ can make a complete 2-port S-parameter measurement would be:
longest length ~= (Acq _time) * (Vprop) / (N)
Acq_time = the acquisition time, which is 250ns in long DUT length mode
(Vprop) = propagation velocity of EM waves through the DUT
N = the number of electrical lengths of the DUT being acquired
For Vprop = 2/3 of speed of light (200mm/ns) and a value N=2, the longest length would be 250ns * 200mm/ns / 2 = 25 meters.
For a very long or lossy DUT, using N=2 might be reasonable in order to acquire TDR waveforms that include refections from the far-end of the DUT but in general, allowing a larger value of N will allow for more accurate measurements. The correct value of N will depend on the DUT's loss characteristics, the degree of impedance mismatches that cause reflections, and the overall requirements for accuracy desired by the user. For example, if the goal is simply to confirm that the signal has reached the other end of the cable, a value of N=1 would be sufficient, but if the user wants to fully characterize the electrical behavior of the DUT in a mostly lossless circuit, a value of N=5 is recommended.
Users who are not needing to consider what is happening at the far-end of the DUT (e.g. users who are only looking at the impedance characteristics of the near-end connector) can measure longer devices than 25m (Vprop dependent). In this case, the SPARQ will not "see" the DUT's far end connector. Note, however, that users should be careful to avoid having TDR or TDT waveforms that impinge on subsequent TDR pulses. The SPARQ's TDR pulser has a repetition rate of 1MHz in "Long" DUT length, so if the DUT's electrical length exceeds 500ns then the SPARQ will acquire the TDR reflections during the acquisition time of the next TDR pulse, causing erroneous results.