Most High-speed PCB designs can’t function properly if the Print Circuit Board designers miscalculate transmission line impedance. When bad calculations arise, traces that support high-speed edge rate signals eventually play the role of transmission lines, even though they are short. Therefore, the best practices in terms of nailing high-speed PCB designs state that you design the traces such that they are close to the set (required) impedance. Doing so minimizes return losses and helps yield a great PCB design.
To attain accurate results when calculating impedance, you can opt to perform the calculations manually or by hand. However, people have found that the most accurate formulas used to calculate impedance tend to be unwieldy when utilized in manual calculations.
Therefore, to help you start floor planning, we have compiled a list of impedance calculators that implement accurate impedance formulas. What’s more, once you have determined the rough estimation of the initial trace width needed to calculate the impedance of your PCB design. You can take things up a notch by utilizing a stack-up editor with an in-built impedance calculator to accurately calculate the required trace width.
Let’s Explore More on Impedance
Impedance affects power transmission between various PCB components, signal propagation through a PCB, and how various signals bleed inside unwanted PCB areas. Several analyses exist which help you solve impedance. However, most of these methods don’t breed realistic results unless you utilize accurate parasitic elements. Therefore, when designing a simple circuit or working with a prototype for high-frequency or high-speed operations, you must be familiar with parasitic elements.
Another crucial factor to consider when looking for impedance is how non-linear circuits are affected by various levels of input signals. That is because impedance values of non-linear circuits are merely functions of an input.
What does impedance mean?
Impedance simply describes how electronic components behave when resisting an AC. Various circuits are built using different components, which all behave quite differently when exposed to an AC.
For starters, resistors have constant resistance under varying frequencies. That is because resistors are pure DC components. Therefore, the impedance of a resistor is merely its resistance.
Meanwhile, the impedance of a capacitor is simply its capacitive reactance. The capacitive reactance of a circuit is merely the inverse of the circuit’s angular frequency.
Therefore, when the circuit’s frequency increases, the capacitive reactance decreases.
Inductors, on the other hand, also behave like capacitors. However, they do have a slight difference in that the inductive reactance of a circuit is usually directly proportional to its angular frequency.
All these basic components are present in impedance analysis calculators. However, they are not the only components found on a circuit. Other components found on circuits that include transistors, diodes, and ICs all introduce impedance. Therefore, you have to factor in all of them in your calculations.
At advanced levels, traces also offers impedance because they are not long-term inductors. The PCB board also affects the circuit’s impedance. Hence calculating impedance can be pretty tedious.
Differential impedance is another factor you should probably consider when dealing with impedance. The differential impedance of a PCB is simply the current to voltage ratio on pairs of transmission lines driven in differential mode.
How does an Impedance Calculator Function to attain the Target Impedance
All transmission line calculators operate under the same principles. They all try to determine the impedance value of a generic transmission line geometry. As per how the calculators function, they implement the following methods:
- Some of these calculators utilize the Telegrapher equations parameter (R, L, C) to accurately determine the transmission line impedance.
- Others build varying models using experimental impedance information versus trace geometry. They then utilize the data that they get back to calculate impedance efficiently.
- Other calculators utilize analytical techniques which provide dispersionless lossless impedance from famous Maxwell’s equations.
- Lastly, some of these calculators utilize the filed solver to solve Maxwell’s equation electromagnetic field. They then utilize the field to work out impedance.
Most impedance calculators, especially the ones found online, utilize #2 (for calculators based on IPC-2141) or #3 (for more accurate calculations using the first principles).
You should note that, in case you have trouble accessing a field solver, then you should opt to utilize the #3 approach from above. Utilizing this method and the correct calculator application will, undoubtedly, breed accurate results.
Free Trace Line Impedance Calculators that utilize the #3 approach
The following impedance calculators are free to utilize. What’s more, these calculators return an accurate impedance value when as per the pairs of:
- Copper weight values
- Substrate thickness
- Trace width
To utilize these to create a design for attaining desired impedance. You must cycle through different trace broadness geometric values until the impedance calculator returns your target value. The calculators include:
- Differential Trace Stripline
- Differential Trace Microstrip
- Stripline Trace impedance
- Microstrip Trace
These calculators utilize standard geometries utilized to create Print Circuit Board designs. While working with these calculators, if you choose a coplanar arrangement, you can easily approximate the trace routing by simply setting 10 mils. However, for this trick to work, the direct thickness has to be equal to or less than 10 mils.
The models utilized in these calculators base upon the Wadell equation, which is famous for providing pretty accurate impedance values (lossless). Even though the value they breed is not a hundred percent accurate for a couple of reasons. These models are pretty useful in terms of attaining quick trace impedance rough estimate values to utilize in the geometry equation.
The accuracy of an impedance calculator is affected by a couple of factors which include:
- Parasitics (parasitic capacitance, to be specific)
- Dispersion and dielectric losses
- Copper roughness
When operating on a high frequency, the final values from the impedance calculator are not the same as those you will find on your PCB due to the above factors. This case occurs for a digital signal when the transmission line signal’s rising edge becomes increasingly fast, concentrating signal contents on the GHz range. As per RF signals, this feature is crucial in the GHz range that exceeds 5.8 GHz.
Trace Width and its Relationship with exact impedance calculation
What exactly is a Trace?
A PCB trace is a thin line made up of copper found in the non-conductive area. You utilize the PCB trace to carry power and signal to the entire circuit.
Copper traces have different widths, and they all have a unique substrate height according to the design of the PCB.
Trace is a pretty crucial factor when it comes to designing Printed Circuit Boards. If you have designed a PCB before, you probably have faced complications when selecting the perfect trace broadness.
By default, beginners in the PCB designing industry mostly utilize the width provided by PCB designing software. At some point, this trick works. However, you must consider many factors affecting trace thickness when developing complex circuits.
How is Trace connected to impedance calculation?
Trace is directly connected to impedance calculation in more ways than one. For starters, a thin trace thickness brings about a high impedance value.
On the other hand, a large width usually brings about low impedance. Therefore, the thicker the PCB, the higher the impedance value and vice versa.
Parameters needed to Calculate Accurate Trace Impedance Values
To work out the trace impedance of a circuit, you will require the following parameters:
- The trace thickness and width
- Spacing between pair traces
- Effective Dielectric constant of the dielectric component utilized in your PCB
- The dielectric layer height between the reference planes and the signal trace
With this data, you can easily calculate the trace impedance of any PCB using an impedance calculator.
Other parameters to consider
When dealing with transmission lines, Characteristic impedance is a crucial parameter, probably the most crucial that you will encounter. Therefore, since impedance mostly circles around transmission lines, you should have prior knowledge on the same. A characteristic impedance is usually a material and a function of geometry. It is also a dynamic value that is independent of line length.
Carrying out Impedance Calculations using an Impedance Calculator
When working with an impedance calculator, you need first to select the correct impedance calculator mode. You should set the calculator mode according to the geometry of the relevant reference planes and the signal layer.
Most calculators draw trace geometries according to which layer the trace is found and how it is routed. So whether the PCB trace is on the top or bottom surface of the PCB or embedded inside different layers, you can easily choose the calculator mode that suits your design.
If you know your target impedance, then you can easily calculate the impedance value. You must type in your target impedance in the calculator, hit calculate, and the software does the rest.
Similarly, if you know the target trace broadness, then calculating the PCBs impedance is just as simple as calculating the width of the trace.
All you have to do is insert your target trace thickness inside an impedance calculator (mostly, the “insert target trace width text box” will be provided. Once you have keyed in your target trace width, you must click the “calculate impedance” button, and voila! You instantly get the impedance value.
And that is how you can easily calculate the impedance and trace width of any PCB with ease. What’s more, every impedance calculator you find out there utilizes accurate formulas to ensure correct results each time.
Why should PCB Designers Strive to Control PCB Impedance?
The need for Increased Signal Power
A Print Circuit Board trace function by transferring signals from the source to the receiving end. For this transmission to take place, the signal must flow through a given trace length to reach the receiving end.
However, the PCB has to have matching impedance to achieve maximum signal power. Therefore, during corresponding layer construction you must utilize equal impedance to attain max signal power.
Improved Performance
PCB designs that employ impedance control perform faster than other PCBs and use less energy. Due to these features, the PCB tends to function better for longer. The electronics industry heavily utilizes impedance calculators to ensure electronics stay functional for longer.
Conclusion
Impedance calculation involves many parameters, and you must consider many factors to achieve accurate results. Due to these reasons, many engineers mess up the calculations and ultimately gain incorrect results that tamper with the PCB’s quality. However, using an impedance calculator, you can easily tackle impedance problems and always attain accurate results each time.
