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If you are new to electronic circuit design, or even if you have been designing printed circuit board (PCB) for a while, you have heard all over again about Impedance.
The real question here is, which type of impedance are we talking about?
In this lesson, we are going to talk about the different types of impedance, what we need to care about, and why are important for us, as electronic engineers and printed circuit board (PCB) designers.
Impedance fundamentals
Before we start, we have to define three important fundamentals:
Definition of a net,
Definition of impedance,
Definition of round trip-time of flight.
A net in electronics is an electrical path that joins two or more points, or pins, on a circuit board. It consists of conductive pathways or interconnected routes that allow the transmission of signals between them. When we talk about a net, we need to consider two elements: a signal path and a return path. There is no net if one of these is missing.
This also makes sense, because if we think in terms of current, we know that current always flows in loops. This means that we need to have one path, where the signal current flows, and the counterpart, where the return current flows (instantaneously) back to the source.
The Impedance, Z, is defined as the ratio between voltage and current, so Z = V/I.
In mathematical terms, this ratio Z includes a real part, R, which we call resistance, and an imaginary part, X, which we call reactance.
Now, let's talk about something called round trip time of flight. This is basically the time it takes for something to go from one point to another and then come back. In our context, it's the time it takes for our measurement signal to travel down a transmission line, and then return.
Now that we have defined these fundamentals, let's explain what we mean when we talk about impedance. We often hear the term impedance, and most of the time we don't really specify what type of impedance we are talking about.
For instance, when talking about fabricating a printed circuit board with controlled impedance, which impedance do we mean? Is this impedance dependent on the length of the net, or not? Let's see what the three types of impedance that we need to distinguish are.
Instantaneous Impedance
Firstly, we need to define the instantaneous impedance:
🔓 Instantaneous impedance is the impedance of a transmission line at a specific location and time.
As we have previously defined, this is the ratio between voltage and current at a specific location and time. This also means that this value could differ in other parts of the same transmission line. Let's take a net and suppose we want to send a signal from the source to the load, which is connected at the end of the transmission line. As the signal propagates, it's important to clarify that the propagating signal has no idea what is connected at the end of the line yet.
Before the signal reaches the end of the transmission line, it doesn't know if the line is shorted, if there is a load connected to it, or if the line is open. The only thing the signal "knows" at each moment in time, as it propagates through the transmission line, is its ratio between the signal voltage, and its current; in other words, its instantaneous impedance.
Characteristic Impedance
Now, let's discuss what we mean when we say we want to produce a PCB with controlled impedance.
🔓 When we say that a net has a characteristic impedance, we mean that its instantaneous impedance is consistent along the entire length of the net.
This means that if I were to measure the instantaneous impedance of this net, I would find that the value is the same regardless of where I measure it. This is crucial because it means that the signal will find a clear path throughout its journey from source to destination.