50 Ohm calculations

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How do you calculate the PCB signals too be 50 Ohms? I have found http://www.saturnpcb.com/pcb_too...
It's a tool for making calculations, But w0067814 recommend that I should ask my PCB vendor to tell me the track widths and spaces,https://www.avrfreaks.net/index.p...

I want to know if I can trust this toolkit on calculations? I want to have a 10 layer PCB stack like this

1- Signal
2- signal
3- GND
4- signal
5- signal
6- 1.2V
7- signal
8- 3.3V
9- signal
10- signal

So If I want to have 50 ohm tracks on layer one I should reference that to layer 3? is this a good layer stack up? and to have a 50 Ohm signal on layer 10 I should reference it to layer 8?

Do you have some track widths and spaces between tracks so I can compare them with this toolkit?

Can I reference my signals to GND, 3.3V and 1.2V planes for impedance calculations?

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Controlled impedance traces should be between two supply layers ideally, preferably between two ground layers. Otherwise, the impedance will be very difficult, if not impossible, to calculate and control.

It is often possible to designate one or two layers as being impedance controlled "transmission layers", routing all transmission lines through them. For example...

1-Signal
2-Ground
3-50_OHM
4-1.2V
5-50_OHM
6-3.3V
7-Signal
8-Signal
9-Signal
10-Signal

Stacking this way makes routing easier, even an auto-router will sometimes give a decent first approximation which can be tweaked by hand to produce a good result.

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It is always wise to contact your PCB vendor. Specially if you do large stacks like you intend to do you need to know what the specified tollerances are on both layer thickness and dielectric constant. Those values you need to know to be able to calculate the 'average' impedance and the tollerance on the impedance if these are to high you have a problem, or need a special service to ensure correct impedance.....

the stack-up is always difficult. If you use shields to ensure that your lines do not radiate or to improve EMI you can best have the 50Ohms on the outer layer this will save vias and give best shielding against interference from the board. Else you better sandwich the impedant layer between 2 ground layers. But then you will need vias and might run in to problems when tuning.

I have never dealt with more then 4 layer boards wher the internal layers where ground + supply and we were lucky to manage and get all the impedant important traces on the layer opposite to ground plus that side was shielded.

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Quote:
mikericetga

your layer stack up is not symmetrical, It would cause the PCB to bent,
Do you have any values for layer spaces and trace widths so That I can compare it with saturnPCB toolkit?

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Then make those 3.3V and 1.2V layers floods. Bypass them to ground multiple places.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Am'I the only one working with more than 4 layers in here?

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You might want to ask your questions on a fpga forum. You're more likely to get an audience with people that do this everyday.

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I've done 6 in the past: signal/gnd/signal+pwr/signal+pwr/gnd/signal

You have an awful lot of signal layers there. Ideally, each signal layer has either a ground or power plane adjacent to it. Do you really need so many signal layers?

I don't think you can't get any impedance control on a signal layer if there's another signal layer beneath or above it.

A ten layer PCB will be expensive. Going to four is major step up in price compared to two layer, six is only slighty more expensive than four, and going to eight layer is another major increase in price.

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You don't necessarily need to have a signal ground layer on both bottom and top layer ( with respect to the controlled impedance line ).

In other words think through Your design requirement and reasons for controlled impedance requirements.

Then read the following http://www.microwaves101.com/enc... and approach Your PCB shop to tell You what thy can do for You.

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Thanks for your advise, I really need 10 layers and 10 layers is the minimum for my design, I can not lower the layers, Because I'm using a Big FPGA,
Has anyone used the saturnpcb toolkit?

I love Digital
and you who involved in it!