What do you mean by impedance matching

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.

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.

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. Few signal reflections occur. 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.

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. 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.

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. 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. Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation.

Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC industry standard. Your design goes through many hoops to become a printed circuit board.

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