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The signal will then be reflected back on the same wire, will be shifted again by l/4 on its way back, and will be merged with the original signal. Assume that there is a reflector at the end of this wire. Half of it continues to the output on the right, whereas the other half goes into a wire that introduces a l/4 delay. The signal is coming from the left, and is virtually split into two parts. Now look at Figure 2 for an easy way to do it. I’m sure you’ve followed me up to this point, right? So, you will also easily understand that, rather than delaying by l/2, we would get the same result if we delayed the signal twice-each time by l/4. What happens if we then sum the original signal and its copy delayed by l/2? Naturally, they will cancel out, and the result will be zero-a null signal. We get the same sine signal, but with an opposite phase. Now, imagine that we delay this signal by half the wavelength.

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On a wire, or more exactly on a so-called transmission line comprising the wire and the accompanying current return path, the propagation speed will be close to the speed of light in free space, c, which is 300,000,000m/s.įIGURE 1 – The sum of two sine signals cancel out if one is shifted by half a wavelength. This wavelength is the propagation speed of the signal along the wire, divided by the frequency of the signal. Such a signal has a given wavelength, usually noted as l (the Greek letter, “lambda”).

Assume that you have a sine-shaped signal propagating on a wire. Let’s start with a very simple mental experiment (Figure 1). I will provide some resources for further reading, if you want to dig more deeply into the subject. Anyway, I hope that you will catch the important points. The downside is that my explanations will approximate. So, the good news is that this article should be easy to read. This may jeopardize the performance of your project, at least if you are using high frequencies or fast digital circuits.įinally, and as usual, I will not use math to convey these principles, but rather words and small experiments. I will also show you that you may inadvertently make such a filter, for example with an improper printed-circuit design.

Qucs simulator how to#

More precisely, my goal is to explain what a stub filter is, and how to build one. If you don’t know why, then keep reading because this will be my topic this month. The wire becomes useless because one of its ends is open. If we then disconnect the wire from B, no current can flow.

Welcome back to “The Darker Side.” As electronics designers, we all know what a wire is and how it works, right? If we connect a wire from A to B, a current can flow between these two points.