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Seperation between ground plane and microstrip.

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viperpaki007

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Hi,

Can someone educate me how to choose the separation distance between ground plane and microstrip line. I had read somewhere on internet that smaller seperation between ground plane and microstrip line reduces the inductance and makes the high frequency response better. Is it really the case?. What are the drawbacks of choosing larger seperation between ground plane and microstrip line?
 

It basically has to do with the return currents. If you have a larger loop, the flux linkage is large, resulting in a large inductance. This will limit your high frequency propagation of signals. It is also bad in terms of coupling.

A smaller loop has lower flux linkage, thus a smaller inductance. For high frequency designs, it is always best to minimize the return loops.
 
Presume you are asking about micro strip lines with specific impedance e.g. 50 ohm. For a certain substrate permittivity, lets say FR4 with 4.3 to 4.6, the inductance and capacitance per length unit will be always the same, independent of substrate height respectively ground plane "spacing". Height and width are simply linearly scaled. The main advantage of thinner substrate is smaller trace width, on the other hand, losses will slightly increase.

You can use the Saturn PCB toolkit to calculate impedances and specific L and C for strip line geometries.
 
Depends on how many layers your board has, this can often be the determining factor, decide on the number of layers then ask your fabricator for their preferred builds.... Use Saturn to determine track sixes to achieve desired impedance if required.
 

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