18. When Rc does not equal Rch or RL in a PFN.
a. Charging waveform when Rc is less than Rch.
Figure 32 shows the
charge waveform of a PFN in which the characteristic resistance is less than
the source resistance. Notice how the voltage charges up in steps. Each
step after the second step is a little bit less than the preceding step. If
you draw a line through the tips of each step, you see that the line is
exponential.
That's why this charge is called an exponential charge.
If
the ratio of Rc to Rch changes, the steps will change somewhat but it will
still be exponential. You will see more steps on the scope if the mismatch
becomes worse.
Figure 34.
Rc less than Rch.
b. Discharging waveform when Rc is less than RL. Figure 35 shows the
exponential discharge across a load (magnetron) that has a greater
resistance than the characteristic resistance of its PFN. The oscilloscope
shows how the voltage across the resistor is highly negative at the start of
the discharge, and decreases in steps until it is completely discharged. As
before, the ratio of Rc to RL determines the amount of discharge in each
step. The greater the mismatch, the more steps there will be.
42
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