For high frequency analogue or digital circuits, it is essential to protect the signals that propagate on the PCB from being damaged. In fact, signals above 100Mhz are impacted by trace impedance which, if not properly taken into account, can cause unexpected errors that are especially difficult to analyse. Luckily, impedance control allows designers and PCB manufacturers to manage the phenomenon.
What is impedance?
Impedance measures the opposition of an electric circuit when alternating current is applied to it. It is the combination of the capacitance and the induction of an electric circuit at high frequency. Impedance is measured in Ohms, similarly to resistance. However, the two values should not be confused as resistance is a characteristic of direct current. When a signal passes from a conductor with a given impedance to another of an identical impedance, the transmission is optimum. On the other hand, if the impedances are different, reflections and attenuation occur that deteriorate the signal.
How is impedance determined?
Usually, trace impedance is between 25 and 125 Ohms and depends on the following factors:
- Copper trace width and thickness
- Passage of the signal through vias
- Core thickness or prepreg material thickness on each side of the trace
- Core and prepreg material dielectric constant
- Distance from the reference copper plane
- Presence or not of solder resist
The board designer must therefore make sure that for high frequency signals, their choice of trace and stackup makes it possible to achieve a target impedance value, with a certain tolerance. The most advanced electronics CAD design tools calculate this automatically.
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What is impedance control?
Impedance control consists in measuring the impedance of certain traces when the PCB is manufactured and making sure they are within the limits given by the designer. Even though it is expensive, this technique became widespread after the turn of the century due to the continuously increasing frequencies of electronic components. It features in the following products, for example:
- Analogue and digital telecommunications
- Video signal processing
- Web boxes, TV, GPS, video games, digital cameras
- Computers, tablets, mobile phones
- Motor control modules
Why control impedance on PCBs?
When a signal needs a specific impedance to operate properly, controlled impedance should be preferred. In high frequency applications, keeping impedance constant on the complete electronic board is essential to protect the transferred data from damage and to maintain the clarity of the signal. The longer the trace or the higher the frequency, the more adaptation is needed. Any lack of rigour at this stage can increase the switching time for an electronic device or circuit and cause unexpected errors.
Uncontrolled impedance is difficult to analyse once the components are mounted on the circuit. Components have different tolerance capacities depending on their batch. Furthermore, their specifications are impacted by temperature variations which can lead to malfunctions. In such cases, replacing the component may seem to be the solution at first when, as a matter of fact, it is the unsuitable trace impedance that is the cause of the problem.
This is why trace impedances and their tolerances must be checked early on in the PCB design. Designers must work hand in hand with the manufacturer to guarantee the compliance of component values.
