The Sierra Circuits Impedance Calculator uses the 2D numerical solution of Maxwell’s equations for PCB transmission lines. It renders fairly accurate results suitable for use in circuit board manufacturing and engineering analysis. In addition to the characteristic impedance of a transmission line, the tool also calculates line parameters such as capacitance, inductance, propagation delay per unit length, the effective dielectric constant of the structure, and in the case of differential pairs, coupling coefficient, and even & odd mode characteristic parameters. This tool also computes signal losess for the chosen impedance model. Most of the free online tools for impedance calculations are generally not as accurate as they are based on empirical formulae and do not take into account the trace’s trapezoidal shape or the effect of multiple dielectric materials.
Impedance between two points in an electronic circuit can be defined as the ratio of the voltage difference and the current. It is the AC voltage difference between the two points divided by the AC current. It is assumed here that the AC voltage and currents are sinusoidal with a frequency ‘f’ cycles/sec (the radian frequency, ω, equals 2πf radians/sec). In general, impedance is a function of frequency. The unit of impedance is Ohms (Ohm= volt/ampere).
For a pure resistor of resistance R Ohms, the impedance ZR = R Ohms.
For a pure inductor of inductance L Henries, the impedance ZL= jωL Ohms.
For a pure capacitor of capacitance C Farads, the impedance ZC = 1/jωC Ohms.
Impedances can be combined in series and parallel in the same manner as resistors.
Signals propagate on a transmission line as electromagnetic waves with a propagation speed and attenuation factor per unit length. Between any point on the signal line and the corresponding point on the signal return path, the instantaneous AC voltage and the instantaneous AC current are related as follows:
Impedance of the transmission line at that point =instantaneous AC voltage instantaneous AC current
This impedance is referred to as the characteristic impedance of the transmission line. If the transmission line has the same characteristic impedance along its length, it is called a uniform transmission line.
On a transmission line, whenever a propagating signal encounters a change in characteristics at any point, some part of the signal will be reflected and signal distortion will occur. Therefore, to achieve good signal integrity, it is important to have a transmission line with uniform characteristic impedance.
A PCB transmission line comprises a signal trace and its return path- usually the nearest reference plane(s). If the geometry of the signal trace, its return path, and the material between them is unchanged throughout the length of the trace, then we have a uniform PCB transmission line. A uniform transmission line will have the same characteristic impedance along its length. Taking into account the manufacturing considerations, achieving a reasonably uniform transmission line is termed as a controlled impedance line, and its characteristic impedance is also referred to as controlled impedance.
When high-frequency signals propagate on transmission lines, a uniform controlled impedance is critical to achieve signal integrity, i.e. the transmission of the signal without distortion. Typically, you will need controlled impedance lines for high-speed digital and high frequency RF/microwave applications.
Impedance of circuit board traces is determined by:
PCBs typically use two types of transmission line structures: microstrips and striplines. There are 3 types of microstrip models to choose from- uncoated, coated, and embedded. Each of the transmission lines consists of a signal trace and a reference plane(s). Within each model, the following combinations are available:
An uncoated microstrip structure consists of a signal trace on an outer layer of a PCB. The reference layer is typically the next layer above or below the signal layer trace. Uncoated microstrips do not have a soldermask coating above the trace.
A coated microstrip structure consists of a signal trace on an outer layer of a PCB. The reference layer is typically the next layer above or below the signal layer trace. Coated microstrips have a soldermask coating above the trace.
An embedded microstrip is a structure similar to a traditional microstrip, except there is an extra dielectric layer above the signal trace. Embedded microstrips can be designed on the internal layers of a circuit board.
A stripline is a structure composed of a uniform signal trace on an inner layer of a board. It is separated by a dielectric layer followed by copper planes on each side.
Our impedance calculator features two main types of impedance models, single ended and differential, within each trace structure.
Following is the table of 82 impedance calculators that are designed and categorized based on the different trace geometries:
Uncoated | Coated | Embedded | Embedded (Inverted) | Stripline |
---|---|---|---|---|
Uncoated Microstrip Single Ended | Coated Microstrip Single Ended | Embedded Microstrip Single Ended | Embedded Microstrip (Inverted) Single Ended | Stripline Single Ended |
Uncoated Microstrip Single Ended Composite | Coated Microstrip Single Ended Composite | Embedded Microstrip Single Ended Composite : A | Embedded Microstrip (Inverted) Single Ended Composite : A | Stripline Single Ended Composite : A |
Embedded Microstrip Single Ended Composite : B | Embedded Microstrip (Inverted) Single Ended Composite : B | Stripline Single Ended Composite : B | ||
Embedded Microstrip Single Ended Composite : C | Embedded Microstrip (Inverted) Single Ended Composite : C | Stripline Single Ended Composite : C |
Uncoated | Coated | Embedded | Embedded (Inverted) | Stripline |
---|---|---|---|---|
Uncoated Microstrip Differential Pair | Coated Microstrip Differential Pair | Embedded Microstrip Differential Pair | Embedded Microstrip (Inverted) Differential Pair | Stripline Differential Pair |
Uncoated Microstrip Differential Pair Composite | Coated Microstrip Differential Pair Composite | Embedded Microstrip Differential Pair Composite : A | Embedded Microstrip (Inverted) Differential Pair Composite : A | Stripline Differential Pair Composite : A |
Embedded Microstrip Differential Pair Composite : B | Embedded Microstrip (Inverted) Differential Pair Composite : B | Stripline Differential Pair Composite : B | ||
Embedded Microstrip Differential Pair Composite : C | Embedded Microstrip (Inverted) Differential Pair Composite : C | Stripline Differential Pair Composite : C | ||
BroadSide Coupled Stripline Pair | ||||
BroadSide Coupled Stripline Pair Over Core |
Uncoated | Coated | Embedded | Embedded (Inverted) | Stripline |
---|---|---|---|---|
Coplanar Uncoated Microstrip Single Ended | Coplanar Coated Microstrip Single Ended | Coplanar Embedded Microstrip Single Ended | Coplanar Embedded Microstrip (Inverted) Single Ended | Coplanar Stripline Single Ended |
Coplanar Uncoated Microstrip Single Ended Composite | Coplanar Coated Microstrip Single Ended Composite | Coplanar Embedded Microstrip Single Ended Composite : A | Coplanar Embedded Microstrip (Inverted) Single Ended Composite : A | Coplanar Stripline Single Ended Composite : A |
Coplanar Embedded Microstrip Single Ended Composite : B | Coplanar Embedded Microstrip (Inverted) Single Ended Composite : B | Coplanar Stripline Single Ended Composite : B | ||
Coplanar Embedded Microstrip Single Ended Composite : C | Coplanar Embedded Microstrip (Inverted) Single Ended Composite : C | Coplanar Stripline Single Ended Composite : C |
Uncoated | Coated | Embedded | Embedded (Inverted) | Stripline |
---|---|---|---|---|
Coplanar Uncoated Microstrip Differential Pair | Coplanar coated Microstrip Differential Pair | Coplanar Embedded Microstrip Differential Pair | Coplanar Embedded Microstrip (Inverted) Differential Pair | Coplanar Stripline Differential Pair |
Coplanar Uncoated Microstrip Differential Pair Composite | Coplanar Coated Microstrip Differential Pair Composite | Coplanar Embedded Microstrip Differential Pair Composite : A | Coplanar Embedded Microstrip (Inverted) Differential Pair Composite : A | Coplanar Stripline Differential Pair Composite : A |
Coplanar Embedded Microstrip Differential Pair Composite : B | Coplanar Embedded Microstrip (Inverted) Differential Pair Composite : B | Coplanar Stripline Differential Pair Composite : B | ||
Coplanar Embedded Microstrip Differential Pair Composite : C | Coplanar Embedded Microstrip (Inverted) Differential Pair Composite : C | Coplanar Stripline Differential Pair Composite : C |
Uncoated | Coated | Embedded | Embedded (Inverted) | Stripline |
---|---|---|---|---|
Coplanar Uncoated Microstrip Single Ended Without Ground Plane | Coplanar Coated Microstrip Single-Ended Without Ground Plane | Coplanar Embedded Microstrip Single-Ended Without Ground Plane | ||
Coplanar Uncoated Microstrip Single-Ended Without Ground Plane Composite | Coplanar Coated Microstrip Single-Ended Without Ground Plane Composite | Coplanar Embedded Microstrip Single Ended Without Ground Plane Composite : A | ||
Coplanar Embedded Microstrip Single Ended Without Ground Plane Composite : B | ||||
Coplanar Embedded Microstrip Single Ended Without Ground Plane Composite : C |
Uncoated | Coated | Embedded | Embedded (Inverted) | Stripline |
---|---|---|---|---|
Coplanar Uncoated Microstrip Differential Pair Without Ground Plane | Coplanar Coated Microstrip Differential Pair Without Ground Plane | Coplanar Embedded Microstrip Differential Pair Without Ground Plane | ||
Coplanar Uncoated Microstrip Differential Pair Without Ground Plane Composite | Coplanar Coated Microstrip Differential Pair Without Ground Plane Composite | Coplanar Embedded Microstrip Differential Pair Without Ground Plane Composite : A | ||
Coplanar Embedded Microstrip Differential Pair Without Ground Plane Composite : B | ||||
Coplanar Embedded Microstrip Differential Pair Without Ground Plane Composite : C |
These composite models are also available for single-ended, differential pair, and coplanar configurations.
The dielectric that is closer to the trace has more effect on the trace’s impedance than the dielectrics that are afar. Composite geometry is most useful when multiple dielectric layers are present with different dielectric constants (Er). Instead of calculating effective dielectric constant, composite models allow us to use the parameters as it is to get more accurate impedance values. So, composite models are more practical in obtaining precise impedances than their non-composite counterparts.
The Impedance Calculator evaluates single-ended and differential impedance for various impedance models. The tool also calculates signal losses of any given trace.
After selecting the relevant impedance model, you’ll have to key in the input parameters. For instance, if you select uncoated microstrip single-ended, the input parameters are:
Dielectric information:
Trace information:
The table below helps you to choose the right Δw value for different copper weights. It also provides the minimum trace width for inner and outer layers.
Copper weight Vs ΔW guidelines |
|||
---|---|---|---|
Copper weight | ΔW (W - W1) | Minimum trace width inner layer | Minimum trace width outer layer |
¼ oz | 0.5 mils | 2.5 mils | 3 mils |
½ oz | 0.5 mils | 3 mils | 4 mils |
1 oz | 1 mil | 4 mils | 6 mils |
2 oz | 3 mils | 6 mils | 8 mils |
3 oz | 6 mils | 8 mils | 12 mils |
4 oz | 7 mils | 9 mils | 14 mils |
Once all the input parameters are entered, hit calculate to view the results.
The output parameters are:
Please note that the input parameters might change depending on the impedance model you choose.
If you require any assistance with the Dk and Df value of the material, click on Show PCB Dielectric Material Construction table. This table shows the Dk and Df of the selected material for different thicknesses and resin content.
If you’d like to update the dielectric height and the corresponding dielectric constant, click on the plus button next to dielectric height.
Here, you can choose the dielectric height based on PCB type, board frequency, material, material type, and material thickness.
If you click on update, the dielectric height, dielectric constant and dissipation factor will be automatically updated in the input section of the tool. Similarly, you can update the values of Δw and trace thickness using the respective plus buttons. After making these changes, click on the calculate button adjacent to the trace width field to view the new results.
You can also calculate the signal loss for the chosen impedance model. For now, let’s calculate the signal losses for uncoated microstrip single-ended.
The input parameters are:
As mentioned earlier, the input parameters are subject to change depending on the chosen impedance structure.
Once all the input parameters are entered, click on calculate loss. You can see the values of conductor loss, dielectric loss, total insertion loss, and attenuation factor. If you require additional information on any of these input or output parameters, click on the corresponding help button.
The units can be changed as per your requirement. You can take the printout of your final readings by clicking the print button.
The impedance calculators are ideal for high-speed traces that require specific impedance for lossless data transfer. You need to understand your PCB design to get the most out of it. You can also check out our Stackup Designer to build an effective stack-up.
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