Playing catch with a young child can teach us a lot about PCB design. Alright, you don’t have to believe me but let me show you: throwing the ball to another person at the right speed and hitting the target requires hand-eye coordination, muscle strength, and awareness of fundamental physical mechanics. Most of us don't think much in throwing, but for a child, grasping these fundamentals sometimes seems like climbing an insurmountable mountain.
The PCB design skill set is, in fact, quite similar. Whether it is mapping out where target locations are or locating particularly problematic or vulnerable sections, the thought processes are parallel. Signals, too, need to arrive on time and on target. Fundamental, textbook concepts that we sometimes take for granted can loom large when ignored but can also offer solutions.
Why is Impedance Matching Important? Mismatched Impedance Causes Problems
Here’s a news flash: PCB design has become more complex. Whether in the consumer or industry market, high-speed and high-frequency devices have become the norm. And this is only the beginning.
When we work with these ultra-high-frequency designs, we must account for the fundamentals. As an example, impedance matching often became an afterthought for design teams working with lower and medium frequencies. However, impedance matching challenges RF and microwave circuit design because the window for error decreases as frequency increases. High speed digital circuits require very stable controlled impedances because of the impact on bit error rate and the potential for pulse distortion, reflection, and EMI.
Proper circuit operation depends on impedance matching—or the ability of the circuit to efficiently transfer signals from the source into the routing and then from the routing to the load. Impedance—if not treated correctly—has a remarkably negative impact on circuit performance. Without the proper impedance matching, reflections can exist along the path from the source to the load.
Until attenuation occurs, the signals happily propagate back and forth in the trace and interfere with the transmitted signal. Reflections and standing waves in high frequency lines mix with desired signals—and form a witch’s brew of amplitude and phase distortion. The direct results of this interference include data jitter and a reduction in the signal-to-noise ratio. As the distance from the source to load increases, standing waves cause impedance to ebb and flow.
Impedance Matching Fundamentals
Good PCB design requires attention to fundamentals. When considering the impact of impedance on a circuit, we need to consider the fundamental relationships between resistance, reactance, and impedance.
While you can certainly hand-test your individual boards, a SPICE simulator could help speed things along.
Everyone knows that a resistance opposes a steady electric current and—as a result—reduces energy. Reactance measures the opposition to current caused by a capacitance or an inductance. While a perfect resistance does not vary with frequency, the impact of changing frequencies on a capacitor or an inductor causes inductive (XL) or capacitive (XC) reactance to change with the frequency of an AC signal.
With all those things in mind, let’s make the jump to impedance. We know that impedance is the total opposite of a device or circuit to the flow of an alternating current. In addition, we also know that the impedance of a capacitor has an inversely proportional relationship to capacitance while the impedance of an inductor has a direct relationship with inductance.
Applying Purpose to Impedance Knowledge
Where does all this great information about textbook fundamentals connect to PCB design? No matter the size of the PCB, the traces form transmission lines. Characteristic impedance (Zo) represents the constant impedance of a line with no reflecting waves. When a circuit transmits a signal and the transmission waves reach the load, reflection waves travel back to the source and the input impedance of the transmission line changes with the addition of the reflected waves.
Our goal with impedance matching is to make the load impedance seem to look like the source impedance. To achieve efficient signal transfer, we need to hit a characteristic impedance target of 50-Ω impedance—the sweet spot for efficient signal transfer. Few signal reflections occur. Good PCB design practices seek to achieve a characteristic impedance of 50 Ω at transmission line junctions, component connections, and terminators.
Matching the impedances throughout the circuit yields a desired low voltage standing wave ratio (VSWR). Low VSWR circuits transfer the maximum amount of power from the source to the load. There’s more. Digital circuits deliver desired performance because of short transition times and high clock rates. Devices and appliances have better capabilities because of the faster, sequential transfer of signals. Constantly increasing signal switching speeds requires another look at controlling the impedance of the transmission lines/PCB traces.
Control Impedance Early with SPICE
Controlling PCB impedance begins with knowing how different variables affect circuit performance. Those variables include dielectric constant, dielectric height, trace thickness, substrate thickness, and trace width. Let’s think about those variables for a moment. Each involves physical geometry that produces the unit-length inductance.
With so many component choices available, design can be tricky to manage.
Now, everything begins to come together. The dielectric constant is the ratio of the permittivity of the dielectric to the permittivity of a vacuum. Permittivity describes the effect of the copper on an electric field and permittivity shows the ability of a material to polarize in response to an applied field. Increasing polarization in an applied field of an established strength causes the dielectric constant to increase.
With any PCB, the trace design or the materials used for the trace can cause impedance values to change. Sudden changes in trace direction cause changes in impedance. Dielectric constant can also change across the length or width of a PCB trace or because of changes in frequency and temperature. Each variance affects the characteristic impedance of an RF circuit. The changes in impedance can lower signal gain, generate noise, or cause random errors.
A controlled impedance line occurs if a PCB design has a specific characteristic impedance throughout the length of a trace. Because capacitance occurs as a function of the relative permittivity of the dielectric material, we can control the impedance with a trace that has a uniform cross-sectional geometry and consistent permittivity. With the increase in capacitance, the characteristic impedance decreases. In terms of design, the change in characteristic impedance affects our impedance matching capabilities. Materials with a lower dielectric constant require an increase in conductor width to maintain the characteristic impedance and decrease the opportunity for loss.
With a strong layout software, you’ll be able to work through impedance matching early in your design iterations and create circuits that are fundamentally more secure. Fortunately, Cadence has more than enough to ensure your design and analysis processes are integrated with as little disruption as possible. OrCAD allows you to take your designs to the next level in signal security with its strong simulation and analysis systems.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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