How do I obtain the accurate measurements of the insertion loss for the cable assembly?

How do I obtain the accurate measurements of the insertion loss for the cable assembly?

To measure the differential insertion loss of cable assembly you need to follow these steps:
1. Setup the instrument:

a. Do compensation and de-skew the channels
b. Set acquisition to volts and window out the first incident step.
c. Set the acquisition window width to ~ (4 or 5)*time delay of DUT. The window should have enough width for the waveforms to settle to their DC level.
d. Set the instrument to at least 256 averages and the maximum number of acquisition points (4000 pts for TDS8000).
e. Connect Ch1 and Ch2 the device under test (DUT) as a differential input and Ch3 and Ch4 as differential output (Ch1 and Ch3 are positive input and outputs respectively), according to the measurement setup shown in Figure 1.

2. Acquire the waveforms with IConnect:

a. Acquire the DUT TDT measurement: measure Ch3 and Ch4 and subtract them using waveform math function of IConnect: TDT_odd=Ch3-Ch4.
b. Acquire the reference. This defines a reference plane for the insertion loss measurement. If you want to de-embed the effects of the SMA connectors and the test card traces, then you need to use a thru reference trace on the test card (if available). Just connect Ch1 and Ch2 to the differential input and Ch3 and Ch4 as the differential output. Then acquire Ch3 and Ch4, and compute TDT_odd_Reference=Ch3-Ch4.
c. At this point you are ready to compute the Insertion loss (S21). Open S-parameter computational tool and load TDT_odd.wfm waveform as the “DUT” and TDT_odd_Reference.wfm waveform as the “Ref” and click on the “Compute” button. This will calculate the insertion loss for you. You can also change the maximum frequency step or the frequency range of the computed data using “Frequency Content” input field