How do I obtain S11 and S21 (return loss and insertion loss) parameters from TDR measurement?
Turn on the step generator on Channel 1 in your TDR oscilloscope. For return loss, you will need a time domain reflection (TDR) waveform; turn on the acquisition on Channel 1 and acquire the resulting waveform from oscilloscope channel 1 into IConnect. For insertion loss, you will need a time domain transmission (TDT) waveform; turn on the acquisition on Channel 2 (without turning on the step generator on channel 2) and acquire the resulting waveform from oscilloscope channel 2 into IConnect. In order to obtain S-parameters in both cases, you will need a reference waveform, which defines the DUT reference measurement plane. Disconnect the cable / probe from the DUT, keep the step generator on channel 1 on, and acquire the TDR waveform. This is a reference "open" waveform; you can also use a reference "short," which is obtained if you short your cable / probe to ground; the insertion / return loss amplitude will be correct, but the phase will be shifted by 180 degrees. Then, go to IConnect Compute menu and select S-parameters. Select the reference open waveform in the Step drop-down box, and then select the TDR waveform for return loss (S11) and TDT waveform for insertion loss (S21) computation. Click compute, then right click into the S-parameter plot and select the desired data format (magnitude-phase, log magnitude-phase, real-imaginary). The repeatability of such measurement is paramount if you want to obtain clean S-parameter data. Use torque wrench for cables and the same probe pressure and pitch for best repeatability.
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