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.
What is the working principle of impedance matching?
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.
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.