Let's take a look at the concept of harmonic generation with SRD step recovery diodes.
In this video a bandpass microstrip filter is used to select an output harmonic of a fast diode.
The diode is a PIN diode, commonly used for RF attenuators, but the also presents a very fast step recovery action.
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https://www.youtube.com/watch?v=t9bO-UvuIyQ