Trail construction, like roads or other similar pathing, has to contour to the features of the ground beneath it. These trails are usually built to minimize the impact of human activity, meaning the lay of the land takes precedence over ease of use (to a point). Trail designers avoid extended steep climbs, but sometimes it’s unavoidable. When there is the option, most trails will incorporate switchbacks, a longer but less taxing ascent than a direct route over the surface.
Just as it is for tired hikers and liquid flow, electron flow acted upon by a field’s potential will attempt to do so by circumnavigating through the path of least resistance. Almost everything in nature is driven by a need to maximize energy loss, as it leads to more stable equilibrium states. Current is no exception: circuit designers can understand how and what happens to the current flowing within a circuit by properly accounting for the imaginary analog for resistance, including how this shapes the overall board design.
Impedance: The Analog, Time-evolving Resistance
Impedance is a measure of opposition to current flow. Stop if that sounds familiar! This definition or something similar is what most people hearfor resistance when learning introductory network analysis. Resistance is a special condition of impedance when the reactance – the imaginary counterpart to resistance – equals zero. Like an introductory kinematics problem glossing over friction or angular motion, resistance is a simpler stand-in for impedance, especially concerning DC. It’s important to note that pure resistance and reactance are theoretical concepts: real-world impedance always includes elements of both. However, certain lumped circuit models may exclude one for ease of analysis when appropriate.
Impedance, in general, introduces the concept of time to a circuit. DC conditions exist as an infinite steady state without the use of switches to drive changes in the circuit. Instead of operating as respective opens and shorts, capacitors and inductors can perform in a time-variant manner or opposite to their DC steady state. The voltage or current waveforms shift a quarter-period depending on the field storage method of an inductor or capacitor. These features open up new circuit functions even with the same topologies.
Common PCB Impedance Standards
The impact of impedance on design is felt immediately when beginning a new project. One of the first things designers have to devise is a stackup that meets the performance goals of the board. With as many unique functions as possible, it’s necessary to pick materials with the underlying properties for optimization. Part of a board’s functionality requires traces designed to meet certain impedance values for various data and transmission standards. General traces are typically designed using 50 ohms as the single-ended target impedance and 100 ohms for the double-ended target impedance.
PCBs are an interface between device-level components and external connections to standalone off-board electronics via cabling, edge connectors, etc. Therefore, impedance standards for cables and board conductors can vary wildly. A common cable interface is coaxial, which also possesses the same 50-ohm value present in traces found on a board. The 50-ohm value has no particular significance for coaxial cable design besides being a practical middle ground between multiple fabrication processes that modify trace impedance. At the highest level, the two largest countervailing forces are:
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- Minimal loss: Determined by the ratio of the resistance per meter to the characteristic impedance. For a shielded conductor, the ratio of the inner and outer conductors must be balanced.
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- Maximum power rating. Greater power correlates to a stronger electric field originating from the conductor. There is a tradeoff with the insulator’s effectiveness at different distances and thicknesses from the conductor, with the breakdown voltage of the material performing optimally at a radius between the maxima and minima positions.
The 50-ohm value is the geometric mean of the two impedance values that perform best in terms of loss and power rating, making it suitable for both tasks without sacrificing one characteristic for another. There are a great many additional standards for both cables and conductors:
| Impedance (Ω) | Use case/standard |
| 2, 4, 8, 16, 32 | Lower end: speakers
Upper end: headphones |
| 25 | Finds use in some electromechanical devices and wiring, e.g., inductors, splitters, couplers. |
| 75 | An alternative coaxial standard that prioritizes loss over power. Generally used for any digital signal, including audio and video. |
| 90 | USB standard |
| 100 | Associated with networking, including ethernet (CAT5) |
| 120 | Found in high-frequency communication standards like CAN bus |
| 600 | Telephone line |
Integrating Impedance Theory Into Design
With an understanding of what impedance is and how it’s utilized, it’s still necessary to discuss the overall effect on the board. Let’s review. Impedance is the opposition to the current, taking into account real resistance and imaginary reactance. Some useful design factors arise from impedance measurements and their effects on circuit topology:
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- Max Power Transfer. By ensuring the impedance at the load is the complex conjugate to the impedance inherent at the source (current or voltage generator), the Max Power Transfer theorem is upheld. It is crucial to understand that the max power transfer external to the load is not maximum efficiency. The complex conjugate of the load, given an impedance of Z = a + bi, will be Z* = a – bi; in other words, the reactance is brought as close to zero as possible to eliminate reflections, and the resistance of the load and source form a voltage divider.
- Impedance Matching. Like the Max Power Transfer theorem, impedance matching is also concerned with the load impedance designed as the complex conjugate of the source impedance. The reason for this is to eliminate reflections, which degrade signal quality via destructive interference, up to the point of complete signal incoherence.
- Characteristic Impedance. The characteristic impedance zigs where the other listed design criteria zag: it does not use the complex conjugate of the source at the load. Instead, the impedance at the load is equivalent to that of the source (and the wire). This is because the input impedance equals the characteristic impedance equals the output impedance. Essentially, no reflections can form as the transmission line is effectively infinite in length compared to the signal wavelength.
Your Contract Manufacturer Can Handle Impedance, from Design to Fabrication
Understanding PCB impedance and its role in design offers designers a detailed operation at both the circuit and functional level. Remember that the current has to be guided by impedance for even the most basic circuit, and its pivotal role only builds from there. Suppose your design is suffering from issues related to conductors missing impedance by significant values or other manufacturing issues. If that’s the case, VSE can draw out the best performance for your board. Here, we’re a team of engineers driven to build exceptional electronics for our customers. Coupled with you and our professional manufacturing partners, we’ll have your design optimized and your board produced for high reliability and functionality.
