Hi,
I am working on my board Stack-up to tune Single/Differential impedances of my board in Allegro PCB 16.6. I see that Dielectric Constant is 4.5 (FR-4) ! even for Conductor/Plane layers. I don't see if I have to change it to 1 or not? I googled and found some online calculators. Unfortunately, each one has its own approximations and consequently they're giving different results for a single set of configurations!. I am confused which one is correct - Allegro Impedance Calculator or online ones? I asked our engineers in RF department and they used AWR - Microwave Office- software which gave us different results from Allegro or online calculators. According to new calculations it seems that I have to set Dielectric Constant to 1 for Conductor/Plane layers!. If I change it to 1 I'll see that Allegro & AWR results are almost the same. However, it seems that new results are strange and different from what I see in boards of famous manufacturers? For example, Xilinx has used 9 Mil spacingas well as 3 Mil width - Mid layers - in SP605 boards for DDR-3 differential signals. These specifications result to reasonable impedances with 4.5 Dielectric Constant of Conductor/Plane layers, while differe completely from AWR results?! :-(
I am looking for comprehensive information about Allegro PCB Impedance Calculator. What shaould I do? Using Dielectric Constant of 1 for Conductor/Plne layers or continue with that default value - 4.5 - and neglect differences with other claculators.
Any comments are welcome.
Hossein![]()
I am working on my board Stack-up to tune Single/Differential impedances of my board in Allegro PCB 16.6. I see that Dielectric Constant is 4.5 (FR-4) ! even for Conductor/Plane layers. I don't see if I have to change it to 1 or not? I googled and found some online calculators. Unfortunately, each one has its own approximations and consequently they're giving different results for a single set of configurations!. I am confused which one is correct - Allegro Impedance Calculator or online ones? I asked our engineers in RF department and they used AWR - Microwave Office- software which gave us different results from Allegro or online calculators. According to new calculations it seems that I have to set Dielectric Constant to 1 for Conductor/Plane layers!. If I change it to 1 I'll see that Allegro & AWR results are almost the same. However, it seems that new results are strange and different from what I see in boards of famous manufacturers? For example, Xilinx has used 9 Mil spacingas well as 3 Mil width - Mid layers - in SP605 boards for DDR-3 differential signals. These specifications result to reasonable impedances with 4.5 Dielectric Constant of Conductor/Plane layers, while differe completely from AWR results?! :-(
I am looking for comprehensive information about Allegro PCB Impedance Calculator. What shaould I do? Using Dielectric Constant of 1 for Conductor/Plne layers or continue with that default value - 4.5 - and neglect differences with other claculators.
Any comments are welcome.
Hossein