This is one of the qualitative design rules of RF analog PCB and one of the EMC design rules of high-speed PCB.
Qualitative but not quantitative, I have no idea. Make a quantitative conclusion with HFSS today.
Some junior engineers violate this rule, which leads to the deterioration of the whole board index. It is estimated that the design accidents can be staged every week.
model
First, build a theoretical model of 7-layer PCB (there is generally no odd-numbered PCB) with the following cross section:
As can be seen from this cross-sectional view:
Layer 2 stripline wiring in the inner layer;
And the top layer and the bottom layer microstrip line wiring;
3. Complete plane for layer;
Every interlayer dielectric has the same thickness h;
The ground plane is indented at the edge of the plate to meet the manufacturing process specification and avoid inner layer corrosion;
The wiring layer parameterizes the contraction of the ground plane edge and the contraction size Gap.
Therefore, strictly speaking, the measurement of isolation is only valid with the contraction Gap for the ground plane.
And the isolation between the top and bottom microstrip lines.
Direct result:
The isolation between the top microstrip line and the bottom microstrip line, the greater the contraction distance Gap to the ground plane, the greater the isolation. Therefore, if the key signal line takes the microstrip line, try not to be close to the board edge.
What is the Gap distance between microstrip line and Gnd edge? There is no clear boundary in the simulation on the diagram, and it should be flexibly determined according to the characteristics and application occasions of the board. The more critical the signal, the more it shrinks.
Isolation between surface microstrip line and inner stripline
It is also the direct result:
When the inward distance Gap from the ground plane is greater than or equal to 2 times the dielectric thickness, the isolation between microstrip lines and striplines will not change, because the common ground reflux crosstalk determined by the skin effect of the ground plane plays a role at this time. For details, please refer to other original articles in “Looking at Pictures and Talking about RF” by WeChat official account, the island owner.
Isolation between inner striplines
It is also the direct result:
When the inward distance Gap from the ground plane is equal to one time of the dielectric thickness, the common ground reflux crosstalk determined by the skin effect of the ground plane just begins to play a role.
question
A careful netizen will ask, this model of the island owner doesn’t say how many mils the dielectric thickness is, and how long is the microstrip line or stripline?
It doesn’t matter how many mils the dielectric thickness is. RF microwave passive circuits have scaling characteristics, and all the articles only say that Gap is proportional to the dielectric thickness.
This model has been simulated to the worst isolation. No matter how long the microstrip line or stripline is extended, the isolation will not deteriorate. Because the microstrip line or stripline of the model has exceeded 1/2 of the guided wave wavelength. Because there is already a pit in the isolation curve, this pit corresponds to the frequency point of 1/2 guided wave wavelength. Keep paying attention to “Looking at Pictures and Talking about RF”, and you can understand why.
Basic quantitative conclusion
If the crosstalk between the top and bottom microstrip lines is considered, the larger the contraction Gap of the ground plane, the better, and it is difficult to give a quantitative standard, which is related to the function of the board and the characteristics of microstrip lines. The specific problems are analyzed in detail. The more critical the signal, the more it shrinks.
If the crosstalk between microstrip line and stripline is considered, it is enough to shrink the ground plane by 2 times the dielectric thickness.
If the crosstalk between striplines is considered, it is enough to shrink the ground plane by 1 times the dielectric thickness.