To respond and add to WKetel's comments: Part of this experiment was aimed at understanding the behavior of switched amplifiers driving circuits that they may not have been designed to drive. The model we use at 434 MHz is one where the output tank circuit is hit with a 25% duty cycle (at RF) pulse train and it is well documented, but what happens if the tank circuit is tuned to twice the intended frequency and it is still hit with a 25% duty cycle pulse train?
Good question about why one should try this for his application. You are correct in saying that there are a lot of ISMRF IC's around that work at 868 MHz, and some of them are already simple and inexpensive, as opposed to the more sophisticated ones that have shaped modulation, low phase noise, and high current drain. I think the motivation for this article was part lab experiment and part creative thinking for developing a very simple very inexpensive part in the future. - Larry Burgess
This is an interesting analysis and a new(sort of) application of a technique that radio amateurs have been using since at least 1950, which is the oldest dated publication that I have that references it. Hams call it "frequency doubling" or "frequency multiplication", and they have used a similar technique to obtain multiples of 2x, 3x, and 5x the input frequency. One thing to note is that because the operation is quite nonlinear, any amplitude modulation of the signal is quite distorted. Now, to address the comments from titux, we should understand this in order to assure that we avoid unintentionally transmitting signals on frequencies other than the intended ones. This is a problem associated with all nonlinear amplification, which is usually cheaper, more efficient, and simpler than linear amplification. And most designers aim for cheaper, simpler, and more efficient.
ISM RF ICs are offered as 'use it as it is' parts, leveraging on their fulfillment of functional requirements. So, many users never even get to know the underlying RF basics. This article appreciably sheds a light on what is below the surface and may be a good track for some lab excercise.
From an engineering point of view, since there is plenty of ISM ICs designed for the 868/915 bands, the first question may be 'why should one try this for his application?'.