5 Frequency tuning
The approach of applying a monochromatic source is the most popular one in microwave power modelling and has been followed so far. However, it does not reflect all phenomena that occur in real life applications. The most widespread microwave power source, a magnetron, is an imperfect device gradually changing its frequency during the heating, and in fact, it may even “jump” from one frequency to another. Let us quote here the two effects of frequency “pulling” by the load and “pushing” by the power supply. Although both have been deeply investigated, the question about the actual operating frequency (or frequency spectrum) of the magnetron as a function time, and for a particular load, is still puzzling for many designers, especially those dealing with small low-loss loads.
To enhance the adequacy of QW-BHM for microwave power engineers, QWED has started implementing its new regimes, which will mimic the physical behaviour of various real power sources under various conditions. First of them is frequency tuning. It is available in two regimes: automatic and manual. The automatic frequency tuning assumes that the source tunes automatically to the deepest resonance in the considered frequency band. We recognise that this may be an oversimplified approach to such unpredictable devices as magnetrons. However, we treat it as a starting point for further more elaborate regimes, for example, based on the digitised Rikke diagrams. Moreover, this assumption perfectly suits microwave power applications with solid state power sources, where the return loss versus frequency can be monitored in time, and the source frequency can be tuned to the deepest resonance by the controller, so as to maximise the matching.
In the manual mode for frequency tuning, new frequency value to which the source should be tuned is given by the user. In this case the user may choose an arbitrary frequency from the considered frequency band, not necessarily the one corresponding to the deepest resonance.
This chapter contains the following topics: