Frequently Asked Questions
Question -The de-embedding procedure uses Open and Short. But aren’t these terminations less than ideal at 67 GHz (especially the Open)? Answer - Yes this is true, however, the ISD tool creates an effective 2x Through calibration standard based on these two measurements. The key requirement is that the Open and Short are very close to 180 degrees phase-offset from each other. This means that the local maximums of the ripple for the Open measurement are at the local minimums of the ripple for the Short Measurement and vice versa. This allows a good effective 2x Through to be constructed which can be used as a de-embedding algorithm. There is then no need to implement any extra 2x-Thru reference structures since de-embedding can be accomplished with only one structure and with an accuracy close to using multiple 2x-Throughs.
Question - Is there a way to model the contact impedance without using Flight Time Scaling? Answer - This requires the coupon to provide enough electrical length to include the entire part of the transmission line where the impedance is “disturbed” by the probe contact. One possibility would be to construct a 2-port Through structure with pads for the probe to contact on each side, and then a small amount of transmission line in-between. This could be used for a 2x Through coupon measurement. With this 2x Through measurement, the ISD algorithm would know to model a distance slightly past where the probe touches the board. To implement this 2x-Through, we would need to know how far we have to de-embed past the probe tips into the trace structure. To determine the appropriate 2x-Through length, you can use this general guideline: If shorter flight time scaling, 0.9xx, if longer, flight time scaling 1.xx.