You should use this function when creating a signal integrity model for an interconnect that normally operates at rise times slower than the fast rise time of the TDR oscilloscope. If you try to create a model that operates at rise times much faster than the operating rise time of your driver, such a model may be excessively complex, and may slow down the simulations unnecessarily. A model valid up to more realistic rise times will be easier to extract and simulate. The fast corner of your device driver should determine the range of validity for your interconnect model and what the final signal rise time should be after you apply the filter. If you acquire fast data from the oscilloscope and save it in IConnect, then when a need for a model operating at a faster rise time arises, you can always to go back to that original data and extract a new model valid up to this faster rise time.
When you apply the filtering function to the signal in IConnect TDR and VNA Software, the software aims to produce a signal that you would have observed on the oscilloscope if the oscilloscope bandwidth were limited by the rise time t_filter. If your original signal rise time measured at the DUT reference plane was t_rise, then the final rise time will be t_final = sqrt (t_rise ^2 + t_filter^2). If you need to achieve a specific t_final number, you should measure the rise time of your TDR signal at the DUT reference plane (end of your cable, probe or fixture), and then the desired filter rise time will be found as t_filter = sqrt (t_final^2 - t_rise^2). For example, to achieve 80ps final rise time with 40ps rise time at the end of the cable, you would use a 69ps rise time filter in IConnect (69 = sqrt [80^2 - 40^2]).