Ideal anode voltage waveform for operating magnetron
■A forming waveform of the modulator is very important for the stable oscillation of the magnetron. Our magnetrons have been designed to suppress the unwanted mode in the cavity and stably oscillate under the specified operating condition, but the oscillation stability and life time are affected by the modulator’s pulse waveform especially in the case of close to the life end of the magnetron. A complex structure of the magnetron cavity has some resonance modes originally, and these unwanted modes interfere with the stable oscillation at the fundamental pi mode by the happening jitter and mode shift if the waveform is not suitable for pi mode operation. Therefore the unsuitable waveform causes the missing pulse and arcing.
●Relationship between waveform and oscillation stability
■A magnetron can stably operate after oscillating on the pi mode at the pulse flat top. So the critically effective point for the stable oscillation is mainly at the pulse rising edge, and sometimes at the falling edge. The magnetron performance chart of the relationship between anode voltage and current is shown in Fig. 1. Before starting oscillation at the no output power, an impedance of the magnetron is high so only the reactive small level anode current calling a dark current flows. After continue to rise up the voltage, the anode current starts to flow effectively to the starting oscillation. In this rising up period, the magnetron can cause the jittering and the moding if the rrv (rate of rising of voltage) is not matched to the magnetron transient characteristics.
●Ideal anode voltage waveform for getting stable oscillation
■Straightly rise up the current without happens the jitter even though transiently and dynamically changed the magnetron impedance. The matching between this magnetron’s transient impedance and modulator’s output impedance is necessary for keeping the stable pi mode oscillation.
■Keep the rrv inside of specification of each magnetron (see “4. rrv”)
■Suppressed overshooting less than the maximum rate of anode current Exceeded overshoot causes the oscillation mode shifting and reduces the lifetime by the overcurrent.
■Keep the flatness at the pulse top without the rising and sagging The less pulse flat top of anode current expands the oscillation frequency spectrum bandwidth by the pushing characteristic. The effective microwave power is reduced by this changing frequency if the high resonance Q RF load of the application is connected by our customers.
■Straightly fall down without hunching
■Too much high undershooting sometimes make the noise from the magnetron output.
●rrv (rate of rising of voltage)
■The rrv (kV/μs) is defined as the rate of the steepest slope of the rising edge of voltage pulse above 80% amplitude. The rrv is specified each type of magnetron specification and the measuring method is as follows.
◆Store the anode voltage plot at the magnified pulse rising edge.
◆Measure the peak anode voltage and calculate the value 80 % of the peak.
◆Line the 80 % point of the peak voltage.
◆Find the steepest section on the slope in the over the 80 % voltage area.
◆Extend the steepest line in the plot.
◆Read the time and voltage at the 2 points are on the line.
◆Calculate the rising ratio with the read times and voltages.
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Datasheet |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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2021/12/10 |
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Rev. 1 |
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103 KB |
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