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, leading 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 adequately accounting for the imaginary analog for resistance, including how this shapes the overall board design. Impedance refers to the opposition that a circuit presents to the flow of alternating current, while controlled impedance involves the design and manufacturing process to achieve a precise, consistent impedance in a circuit.
Impedance and Controlled Impedance
Comparison of Impedance and Controlled Impedance |
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|---|---|---|
| Aspect | Impedance | Controlled Impedance |
| Definition | Opposition a circuit presents to the flow of alternating current. | Intentional design and manufacturing process to achieve a precise, consistent impedance. |
| Nature | Can be inherent or unintentional in a circuit. | Deliberately engineered and controlled. |
| Application | Found in all electronic circuits. | Primarily utilized in high-speed digital and high-frequency analog circuits. |
| Importance | Critical for proper circuit functioning, but tolerances may not be as stringent. | Crucial for signal integrity in high-frequency applications; tight tolerances are necessary. |
| Components | Comprises resistance, inductance, and capacitance. | Involves precise control of trace width, thickness, substrate material, and other parameters. |
| Examples | Simple resistor circuits, passive components. | High-speed PCB traces, transmission lines, and connectors in communications |
As mentioned above, impedance is a measure of opposition to current flow. This definition or something similar is what most people hear for resistance when learning introductory network analysis. Resistance is a particular 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, specific 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.
Controlled impedance is critical in high-speed digital circuits and high-frequency analog circuits to ensure signal integrity and minimize losses. Engineers can precisely define impedance by controlling the PCB trace width, thickness, dielectric constant of the substrate material, and other parameters.
Common PCB Impedance Standards
The impact of controlled impedance on design occurs 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 use 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 typical cable interface is coaxial, with the same 50-ohm value 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:
- 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 equal.
- 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 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 Controlled Impedance Into Design
With an understanding of controlled impedance, discussing the overall effect on the board is still necessary. Let’s review. Impedance is the opposition to the current, considering real resistance and imaginary reactance. Some useful design factors arise from impedance measurements and their effects on circuit topology:
- Max power transfer. The max power transfer theorem ensures the impedance at the load is the complex conjugate to the impedance inherent at the source (current or voltage generator). 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. This matching eliminates reflections, which degrade signal quality via destructive interference until 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. 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 controlled 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 so, 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.
