In a case when the two lines in the differential pair are symmetric, you stimulate the differential pair with a differential stimulus in the TDR oscilloscope, and acquire the waveform on the positive switching channel (Ch1) - no math in the oscilloscope is required (this is the odd mode impedance). You acquire a reference open or a short waveform on the same channel, and use it to compute the odd mode impedance profile. Then, you multiply it by 2 to get the differential impedance. Do not use the negative channel waveforms, as they do not give you correct impedance profile. Alternatively, when you compute the impedance profile, change the characteristic impedance value (Z0, right above Threshold parameter in the impedance computation window) to 100 Ohm from 50 Ohm, and you will get differential impedance right away, bypassing the odd mode impedance step.
For a case of differential lines with mild asymmetry, proceed as follows. When the TDR is in differential mode, and the oscilloscope displays voltage, subtract the negative switching channel (Ch2) from the positive switching channel Ch1 (DUT=Ch1-Ch2). Then, acquire a reference on Ch1 and multiply it by 2 (or, acquire reference on both channels simultaneously and subtract Ch1Ref-Ch2Ref again). Now, use the DUT waveform and the computed reference waveform to compute the impedance profile and multiply it by 2, or use characteristic impedance Z0 = 100 Ohm and simply compute the impedance profile. This lengthened process allows you to account for mild non-symmetry in the lines. In any case, never use negative switching channel by itself - the IConnect TDR and VNA Software does not understand it and does not treat it properly.
This FAQ Applies to:
Product: CSA8000B, CSA8000, TDS8000, TDS8000B, CSA8200, TDS8200, 80SICON, 80SSPAR, 80SICMX
FAQ ID : 56531View all FAQs »