FREQUENCY SOURCE FOR THE ISAC RFQ

The ISAC RFQ cavity nominally operates at a frequency of 35 MHz. It can operate in either self-excited or driven mode. In the self-excited mode, the RFQ cavity itself is one of the frequency-determining elements. In the driven mode, the frequency is slaved to an external stabilized, digitally synthesized frequency source. In order to achieve glitchless transitions between the two modes, the RFQ frequency source is buffered by a phase-locked loop. When operating in the driven mode, RF switches in the loop configure the loop to lock to the external direct digital synthesizer. Reconfiguration of the RF switches in the self-excited mode turns the phase-locked loop into an oscillator whose frequency is determined by the phase shift of the RFQ cavity. The overdamping characteristic of the phase-locked loop prevents sudden transitions in either frequency or phase during switching between operating modes.