The beauty of symmetricity in art, science, and nature, in general, is something of a wonder. The visual balance between elements in a painting or drawing can make or break a piece of artwork. PCB design is as much art as it is engineering, and symmetricity plays as much of a technical role as it does an aesthetic role.
Ever since its humble beginnings as a replacement for high frequency coaxial cables and waveguides, striplines are a mainstay among multi-layer RF and HDI PCB designers. These conductors can be tightly packed in the inner layers of a multi-layer PCB, where the surrounding dielectric suppresses radiation and provides dispersion compensation. Thanks Robert Barrett!
Symmetric vs. Asymmetric Stripline Arrangements
The symmetric stripline is the simplest embedded trace arrangement next to the embedded microstrip. In contrast to microstrip or embedded microstrip traces, stripline traces sit embedded in the PCB board layer, with solid copper ground planes placed above and below the traces. Inner layers in a multi-layer PCB typically contain stripline traces.
Since these traces are embedded between ground planes, they have particularly desirable EMI immunity, and other components on the PCB will be immune to any EMI produced by the striplines.
In contrast to symmetric striplines, asymmetric striplines are not centrally embedded between the ground planes. Asymmetric striplines are placed closer to one of the surrounding ground planes. When routing signals using asymmetric striplines, the closer ground plane should be used as the reference for the stripline, as this ensures a stronger return signal will be induced in the ground plane.
In a more complicated arrangement, striplines can be arranged as a coupled parallel pair of conductors within a single layer. This edge-coupled arrangement places a pair of traces in the same layer with the same distances between ground planes. This arrangement enables differential pair routing within a given layer.
A more interesting arrangement is to use a board-coupled arrangement, where two asymmetric striplines are stacked on top of each other in a symmetric arrangement. While this might require a thicker board to accommodate the stacked striplines, this saves lateral board space and allows for higher interconnect density between the two ground planes. This arrangement can also be used for differential pair routing since the two striplines are parallel.
Microstrip and via interconnections on green multi-layer PCB
Multiple Calculators, Multiple Values
There’s no shame in not memorizing every impedance equation for every possible trace arrangement. If you’ve browsed the internet looking for an impedance calculator for your stripline arrangement, you’ll need to take a close look at your results and compare them with results from other calculators.
You’ll also want to compare the equations used with various calculators. There are several common methods for calculating the impedance of a single asymmetric stripline. Some calculators use a difference between logarithmic functions, another uses a power function with approximately 6th order dependence on a number of the geometric parameters, and there are undoubtedly other formulas that can be found through an internet search.
These calculators can produce wildly different results depending on the structural parameters that define the stripline arrangement. Two different calculators can produce differences ranging from 5 to 10 Ohms. The true impedance value is likely somewhere between these values. This creates major issues with impedance matching in your PCB
When working with high speed or high frequency signals, a 5 Ohm impedance mismatch is significant enough to contribute to problems like ringing due to resonance at specific frequencies. In high frequency signals, resonance on a transmission line leads to significant radiation. With asymmetric striplines, this can create a problem in HDI boards. Thankfully, boards with lower routing density won’t be affected by this EMI due to the surrounding dielectric.
Given these potential problems that can arise from working with an impedance calculator, it is best to use a numerical simulation to determine impedance. Most folks don’t have access to this type of software, but the investment can be worth it. Alternatively, consider using another design strategy to prevent or suppress ringing.
Parameter Modulation and Differential Pairs
Due to the heavily nonlinear relationship between impedance in asymmetric stripline arrangements and their geometry, it becomes important to have a general understanding of how the impedance is affected by small changes in the stripline arrangement. Working with differential pairs of asymmetric striplines follows many of the same rules as microstrip traces.
Moving a single stripline away from the symmetric arrangement and shifting the stripline towards one of the ground planes creates a small decrease in the impedance. This shift is itself symmetric; it doesn’t matter if you move the stripline up or down, a given shift in either direction will cause the same change in impedance.
Working with differential pairs of asymmetric striplines is a bit more complicated, but some of the same rules that occur with differential microstrips apply to symmetric and asymmetric striplines. If the spacing is very large, the impedance value will saturate at a particular value, and the coupling strength will be reduced.
Once the spacing is changed, the impedance of microstrip pairs and asymmetric pairs will change in different ways If the traces have large separation, moving them closer together first increases the impedance values of the differential pair. In a microstrip arrangement, the impedance of the pair will continuously increase as the pair is moved closer together, and this continues once the spacing is reduced to less than the width of the traces.
This is not the case with stripline pairs. As the traces are moved closer together, bringing the pair closer together first increases and then decreases the even, odd, common, and differential impedance values. Once the traces are placed very close together, such that the spacing is much smaller than trace width, the odd and differential impedance values will decrease drastically and can drop as low as a few Ohms.
Traces routed on a blue PCB
If you need to ensure that your next high-precision PCB meets design specifications, the simulation tools and rules-driven design environment in Altium Designer can help you avoid signal integrity problems that arise from incorrect impedance calculations. Download your free trial and find out if Altium Designer is right for you. Talk to an Altium expert today if you want to learn more.
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