When talking about high-speed circuits, we need to take proper care of signal terminations. Yes, I'm talking about transmission lines. We all know that, even at the printed-circuit level, improper termination of a high-speed signal can lead to all kinds of nasty realities in RF, analog signaling, or digital circuitry.
In RF circuitry, it's often called the voltage standing wave ratio, or (V)SWR, which represents the amount of reflected energy seen by the output amplifier. In an ideal system, regardless of type, the SWR is 1:1. That is, the power sourced by the driver is the same as the power sunk by the receiver.
Again, regardless of system type, if the (V)SWR is too bad, the reflected power not only can cause distortions, but also can burn out the driver or even the receiver. What am I talking about? Well, the video below explains what I did in a recent project. I have a digital signal with a programmable frequency being output on a pin. At low frequencies, you can see that the output looks pretty good, but as the frequency increases, you can see quite a lot of oddball behavior.
What causes reflections? I won't go into great detail, because many books have been written on this subject. For now, I'll say it's a result of transmission line theory. It's caused by an impedance mismatch between different parts of the circuit. An antenna is actually matching the impedance of free space to the impedance of your transmitter's output driver or receiver's input stage. I think (and correct me if you know better, but please do it in nice, easy terms that I can understand) that this has to do with the propagation velocity of a signal through the medium. Energy travels through the medium at a certain speed but is not absorbed at the same rate as it is being received. This causes a reflection (echo) to go back through the medium. Reflections will occur at any point where there is an impedance mismatch.
When a wire, cable, or PCB trace becomes a transmission line is a function of frequency and distance. A rule of thumb is that, when the length of the transmission line is more than about 2x the wavelength of the highest frequency being carried, you have a transmission line. That is, if my frequency is 1MHz (wavelength = 300m), then I have a transmission line when my cable becomes somewhere in the neighborhood of 600m long. This is because there are now multiple values on my line at any given time, and the reflection of one value can interfere with the value at any given point on the line, distorting the signal.
Each time the signal echoes, some energy is lost (heat), but as the signal bounces back and forth between the various discontinuities, the signals can stack up because of superposition.
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