Electronics > PCB/EDA/CAD

Characteristic impedance in PCB

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dietert1:
The FR4 dielectric constant is known to some degree and this should be good enough to do an estimate.  Recently i used a digi-key online calculator and tried to make the pair impedance about 100 Ohm. If it comes out as 80 Ohm or 120 Ohm, no problem. I mean if the leading edge of a digital signal overshoots 20 % that will work. Same if 20 % is missing.
I did the estimate to get it about right. At least it helped me to select one of two 4-layer stacks offered by the PCB house.

Regards, Dieter

Doctorandus_P:

--- Quote from: joniengr081 on September 30, 2023, 03:25:53 pm ---Can we calculate at which clock frequency, the wavelength is smealler then 1, 5, or 10 inches long transmission line ? Then there would be several wavelength or cycles in the transmission line.

--- End quote ---

Sure, there are formula's for that. I think it's between 50% and 80% of light speed (in vacuum) as a rule of thumb but it depends on dieelctric constant and maybe also on track geometry. I don't know the details, but I'm sure there are formula's for that kind of stuff.

luudee:

Interesting thread ...

It just happens that we switched PCB houses, and are now using "WellPCB".
Since one of the PCBs we make uses Megtron-6, I asked our sales rep @WellPCB
for relative cost of the various materials that are recommended for "high-speed"
PCBs. I asked her to use FR-4 for as base.


normal FR4                                 1.00
Shengyi S1150G Halgon-free     1.36
Isola 370HR                                2.07
TU872 SLK                                 3.43
Megtron6_R-5775                       5.64


Hope this helps.


Best Regards,
rudi

joniengr081:
I was reading a comment which says that one problem with FR4 is that the characteristic impedance is not constant which is due to change or un-stable in dielectric constant. What is the tolerance level of dielectric constant in FR4 ?

Second question regarding FR4 comparison cost wise with other dielectric for example Rogers, Isola, Megtron-6 etc. If the price of the dielectric is 2 or 3 times then it does not mean the overall cost of the PCB is 2 or 3 times, right ? because it's only the cost of dielectric.   

luudee:

--- Quote from: joniengr081 on October 01, 2023, 10:27:12 am ---I was reading a comment which says that one problem with FR4 is that the characteristic impedance is not constant which is due to change or un-stable in dielectric constant. What is the tolerance level of dielectric constant in FR4 ?

Second question regarding FR4 comparison cost wise with other dielectric for example Rogers, Isola, Megtron-6 etc. If the price of the dielectric is 2 or 3 times then it does not mean the overall cost of the PCB is 2 or 3 times, right ? because it's only the cost of dielectric.   

--- End quote ---

Please see my post above.

Those are actual PCB prices. According to my rep, Megtron-6, is actually 5.6 times more expensive plain than FR-4.

Cheers,
rudi

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