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Differential pair parameters calculation

MrSlavic

Good day to everyone! 

I'm using Cadence Allegro PCB Editor 17.2 with high speed license. 

Is there a way to see which formulas Cadence uses in Transmission line calculator+diffpair calculator in PCB stack-up? 

Microstrip and striplines are the main points of interest. 

The reason I'm asking is that the Cadence results are a bit different from formulas described in Eric Bogatin's "Signal Integrity" book. 

I apologize if such question was already answered before. Also I hope I've picked the correct forum this time. 

Thank you. 

  • Hi.

    There is no way that I know of to see the formula used by Cadence. Generally speaking the Cross Section Manager - AKA stackup does give close results to other tools. The thing about the formulas are they assume that the info entered is correct but in the real world one is dealing with physical properties of materials that may not be known exactly. Height, copper roughness, ER etc.

    For example things such as the ER Constant of the fiberglass, Side wall etch, Fiber weave of the board etc can have a profound effect on the real world analysis of a microstrip. For low frequency designs on FR4 material good results can be achieved in the the Sub 500Mhz range but beyond that the math can go out the window. This is because the property of the material is not accounted for in the classic equations over frequency.

    The only way of knowing the un known when it comes to impedance of a board is to build test coupons and actually measure the results on something like a TDR or VNA. Without such instruments it is a "Shot In the Dark" ! in particular for high frequency designs.

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