Digital low level rf control system with four different intermediate frequencies for the International Linear Collider

A field programmable gate array–based digital low level rf (LLRF) control system will be used in the International Linear Collider (ILC) in order to satisfy the rf stability requirements. The digital LLRF control system with four different intermediate frequencies has been developed to decrease the required number of analog-to-digital converters in this system. The proof of concept of this technique was demonstrated at the Superconducting RF Test Facility in the High Energy Accelerator Research Organization, Japan. The amplitude and phase stability has fulfilled the ILC requirements.

Figure 1

Typical rf forward pulse, reflection, and cavity field signal in the case of no beam. The pulse signal is composed of filling time and beam acceleration time.

Figure 2

Simplified diagram of digital LLRF feedback loop controlling eight superconducting cavities.

Figure 3

Two-tone measurement setup. Test point 1 is the output of the combiner and is measured by a spectrum analyzer. Test point 2 is the output of the $I/Q$ converter and is estimated using Eqs. (2) and (3). The magnitude of the $I/Q$ converter output is changed into dBFS.

Figure 4

Layout of the STF. It is composed of a normal conducting photocathode rf gun, two superconducting nine-cell cavities in the capture cryomodule, eight superconducting nine-cell cavities (cavity number 1–8) in the cryomodule CM-1, and four superconducting nine-cell cavities in the cryomodules CM-2a (cavity number 9–12).

Figure 5

Front-end of digital LLRF control system with four different IFs. Each cavity output is mixed with different LO frequencies and is then filtered by LPF. The four IFs are combined by a passive power combiner and its output is connected to one ADC.

Figure 6

LO and ADC sampling clock generation for digital LLRF control system with four different IFs. AD9510 from analog device is used for the frequency divider.

Figure 7

Frequency response of the BPF for LO generation. The center frequencies are normalized and the bandwidth of all filters is 4 MHz.

Figure 8

MO and LO phase noise versus offset frequency. The rms jitter for each oscillator within the interval of offset frequency from 10 Hz to 1 MHz is as follows: $\mathrm{\Delta }{T}_{\mathrm{rms}}(\mathrm{MO})=56\mathrm{fs}$, $\mathrm{\Delta }{T}_{\mathrm{rms}}(\mathrm{LO}1)=57\mathrm{fs}$, $\mathrm{\Delta }{T}_{\mathrm{rms}}(\mathrm{LO}3)=57\mathrm{fs}$, $\mathrm{\Delta }{T}_{\mathrm{rms}}(\mathrm{LO}5)=57\mathrm{fs}$, and $\mathrm{\Delta }{T}_{\mathrm{rms}}(\mathrm{LO}7)=58\mathrm{fs}$.

Figure 9

The amplitude and phase of eight cavities with four IFs technique.

Figure 10

Flattop of vector sum with four IFs technique. The amplitude and phase stability are 0.006% (rms) and 0.46° (rms), respectively.