Resonant Circuits in a Magnetron Tube

THE COMPLETE MICROWAVE OVEN SERVICE HANDBOOK: Operation, Maintenance, Troubleshooting and Repair

Chapter Seven: The High-Voltage System (continued)

The whirling cloud of electrons, influenced by the high voltage and the strong magnetic field, form a rotating pattern that resembles spokes in a spinning wheel (Fig. 7-5). The interaction of this rotating space-charge wheel with the configuration of the surface of the anode produces an alternating current flow in the resonant cavities of the anode. This is explained as follows: as a "spoke" of electrons approaches an anode vane (or the segment between two cavities), it induces a positive charge in that segment. As the electrons pass, the positive charge diminishes in the first segment while another positive charge is being induced in the next segment.

Current is induced because the physical structure of the anode forms the equivalent of a series of high-Q resonant inductive-capacitive (LC) circuits. The effect of the strapping of alternate segments (mentioned earlier) is to connect the LC circuits in parallel (Fig. 7-6).

7.3.2 Resonant Circuits

A resonant circuit consists of a coil and capacitor connected in parallel and produces high-frequency current flow as next explained in Figure 7-7. In step 1, the battery (which represents the energy being supplied to the circuit by the passing "spoke" of electrons) will charge up capacitor Cl. In step 2, when S1 is switched to position "B" (which represents the electrons having passed by and therefore no longer inducing a charge) the capacitor discharges through coil L1. The current flow through the coil causes a magnetic field to develop around the coil, which is accompanied by an internally induced voltage of a polarity that opposes the capacitor discharge. In step 3, Cl has completely discharged and the energy is now stored in the magnetic field that surrounds the coil. In step 4, the magnetic field begins to collapse around the coil causing the voltage induced within it to change polarity. This tends to keep the current flowing in the original direction, which at step 5, charges the capacitor with a polarity opposite from its original charge. Consequently, at step 6 the capacitor again discharges through the coil, although in an opposite direction, starting the process all over.

This seesaw action can produce extremely high oscillations, the frequencies of which are determined by the values of the capacitance and the inductance.

In each resonant cavity of a magnetron tube, the walls act as an inductor (or a coil), and the parallel sides of the opening form the plates of a capacitor (refer back to Fig. 7-5). Since the amount of inductance and capacitance is very small, the frequency of the alternating current is (continued next page)

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