Development of a new concept automatic frequency controller for an ultrasmall C-band linear accelerator guide
The frequency dependence of the phase difference between the drive microwave and the reflected microwave of the standing wave type accelerator guide. In this figure, .
The mechanization of the existing analog AFC for S-band medical linear accelerators. The microwaves that are supplied to and reflected from the accelerator guide are picked up by the BHC. These microwaves are mixed by a hybrid coupler and converted into video signals by crystal detectors. The video signals are processed into output by the difference integrator. The output controls the frequency of the voltage controlled oscillator via the servo amp.
The mechanization of our AFC. The microwaves that are supplied to and reflected from the accelerator guide are picked up by the BHC. These microwaves are mixed by a hybrid coupler and converted into video signals by crystal detectors. The video signals are compared in the comparator and the binary quantized output (i.e., positive sign or negative sign of the output) is generated. The binary quantized output counts up or down the frequency command to the digitally controlled signal generator by one least-significant bit at every pulse. If the binary quantized output is positive, the frequency command is counted up, and if the binary quantized output is negative, the frequency command is counted down.
The microwave block diagram and operational time chart for our accelerator system. MSB=most significant bit. LSB=least significant bit (corresponding to the resolution of the frequency command).
Details of the new AFC. The drive and reflected microwave are picked up by the BHC. These microwaves are conditioned through low pass filters (LPF), adjustable phase shifter, and adjustable attenuators and are input into the hybrid coupler. The outputs of the 3 dB hybrid coupler are converted into video signals. The video signals are conditioned through the buffer and trim amps and sampled by the sample and hold amps. These sampled video signals are compared at the comparator. The positive or negative sign of the comparator output adds or subtracts the frequency command register by one least-significant bit.
The time history of the value of the frequency command register at the rated x-ray output. The pulse repetition frequency is . This register value corresponds to the resonance frequency of the accelerator guide.
The calculated capture frequency of the existing AFC when applied to our C-band accelerator guide.
The calculated capture frequency of our new AFC when applied to our C-band accelerator guide.
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