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It is just an amplifier circuit on a single-layer PCB, and controlling impedance is not even an afterthought. Once I started working on electro-optic systems that required high sampling rates, controlling impedance was always a key design issue. The controlled impedance on the circuit board is a PCB layout problem, and I feel uncomfortable after processing the PCB for a while.

Once you start processing high-speed or high-frequency signals, you'll find that the impedance of the trace and the source and load mismatch will have a significant impact on signal integrity. Within the RF range and beyond, the impedance encountered by the control signal will ensure that your equipment operates as designed. There are some simple design strategies that can help ensure that the signal is not distorted during operation.

Impedance control and controlled dielectric

Impedance control and impedance management are two loosely interchangeable terms, referring to different methods of setting the impedance seen by the signal in the PCB. Obviously, no manufacturing process is perfect, any PCB coming off the production line will have some tracking impedance changes. The basic idea is to set the impedance encountered by the signal to a specific value, ideally limiting any impedance mismatch to a smaller value.

This can be done in two ways. First, simply choose the geometry used to form the traces, the arrangement and the materials will affect their impedance. The surrounding dielectric also affects the impedance. This case corresponds to a single trace routed on a dielectric with infinite thickness. Most simple impedance calculators assume that this situation is obvious, and this approximation is only valid in a few special cases.

You may be thinking, "Wait, why the dielectric constant of the substrate is different?" There are many reasons. First, the dielectric between adjacent traces and between the trace and the ground plane forms a capacitor, and the dielectric constant determines the stray capacitance.

Since the interface between the trace and the substrate is not a perfect reflector, the electric field actually enters the dielectric and remains coupled to the field in the trace. In short, the signal partially propagates in the dielectric and is not completely confined in the trace.

Both of these facts mean that stacking in a multilayer PCB affects the impedance seen by the signal in the trace. In fact, modifying the layer stack allows adjustment of the overall trace impedance. Adjusting the stack will change the effective dielectric constant seen by the signal, allowing impedance control in many applications.

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