50 ohms using power plane instead of ground plane?

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Hello! 

 

I have several boards with 50 ohm traces in 4-layer pcbs. my stack layer is usually top, ground, power, bottom. The 50 ohms traces should be in the top layer because is calculated between the top-prepreg-ground. 

 

But what if I have to use a 50 ohm trace in the bottom layer? Can I use the same calculation but using power layer? (assuming power plane covers all the trace area). I remember in some class that when using RF model, GND and power are short-circuited, so, maybe gnd plane and power plane are the same for RF calculations. 

 

Is that correct?

 

thank you very much

 

 

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jorge.ar wrote:

I have several boards with 50 ohm traces in 4-layer pcbs. my stack layer is usually top, ground, power, bottom. The 50 ohms traces should be in the top layer because is calculated between the top-prepreg-ground. 

 

But what if I have to use a 50 ohm trace in the bottom layer? Can I use the same calculation but using power layer? (assuming power plane covers all the trace area). I remember in some class that when using RF model, GND and power are short-circuited, so, maybe gnd plane and power plane are the same for RF calculations. 

 

Is that correct?

You could try this. In theory, Power is very nearly the same as Ground, and geometry is what sets the trace impedance.

 

In the real world, both power and ground have noise on them, from operating electronics, and you also want that noise to be insignificant at your RF frequencies.

So what is the 50 ohm trace carrying, and how sensitive is that to noise ?

You can take efforts to quiet the Power noise in your RF region, or you could do a split plane and add a local gnd on the power plane layer.

 

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Hello! Thank you for your reply.

 

Who-me wrote:

So what is the 50 ohm trace carrying, and how sensitive is that to noise ?

 

GSM antenna and GPS input (from active antenna). I always keep modules really close to the connectors / antenna. 

 

Who-me wrote:

You can take efforts to quiet the Power noise in your RF region, or you could do a split plane and add a local gnd on the power plane layer.

 

That was exactly what I had in mind. But wanted a second opinion laugh