Frank Ludwig

Balanced optical-microwave phase detectors for optoelectronic phase-locked loops

A balanced optical-microwave phase detector for the extraction of low-jitter, high-power, and drift-free microwave signals from optical pulse trains is presented. The phase detection is based on electro-optic sampling with a differentially biased Sagnac loop. Because the timing information is transferred in the optical domain, the regenerated microwave signal is robust against drifts and photodetector nonlinearities. In a first experimental implementation, 3 fs in-loop relative timing jitter (integrated from 1 Hz to 10 MHz) between a 44 MHz optical pulse train and a 10.225 GHz microwave signal is demonstrated.

Long-term stable microwave signal extraction from mode-locked lasers

Long-term synchronization [13-fs (10 Hz-10 MHz), <50 fs (for one hour)] between two 10.225-GHz microwave signals at +10 dBm referenced to a 44-MHz repetition rate mode-locked fiber laser is demonstrated using balanced optical-microwave phase detectors.

RF backplane for MTCA.4 based LLRF control system

Low-level RF (LLRF) control systems developed for linear accelerator-based free electron lasers (FELs) require real-time processing of thousands of RF signals with very challenging RF field detection precision. To provide a reliable, maintainable, and scalable system, a new generation of LLRF control based on MTCA.4 architecture was started at DESY for the FLASH and European-XFEL facilities. In contrast to previous RF control systems realized in 19-in modules, we could demonstrate field detection, RF generation, RF distribution, DAQ system, and the high-speed real-time processing entirely embedded in the MTCA.4 crate system. This unique scheme embeds ultra-high-precision analog electronics for detection on the rear transition module (RTM) with powerful digital processing units on the advanced mezzanine card (AMC). To increase system reliability and maintainability and to reduce performance limitations arising through RF cabling, we developed and embedded in the MTCA.4 crate, a unique RF backplane for RTM cards. This backplane is used for distribution of high-performance local oscillator (LO), RF, and low-jitter clock signals together with low-noise analog power supply to analog RTM cards in the system. In this paper, we present the design and architecture of the MTCA.4 crate with the RF backplane and successful test results of the LLRF control system.

Standardized Solution for Management Controller for MTCA.4

The Micro Telecommunications Computing Architecture (MTCA) standard is a modern platform, that is gaining popularity in the area of High Energy Physics (HEP) experiments. The standard provides extensive management, monitoring and diagnostics functionality. The hardware management is based on the Intelligent Platform Management Interface (IPMI), that was initially developed for management and monitoring of complex computers operation. The original IPMI specification was extended and new functions required for MTCA hardware management, were added. The Module Management Controller (MMC) is required on each Advanced Mezzanine Card installed in MTCA chassis. The Rear Transition Modules (RTMs) require Rear transition module Management Controller (RMC) that is specified in MTCA.4 extension specification. The commercially available implementations of MMC and RMC are expensive and do not provide the whole functionality that is required by specific HEP applications. Therefore, many research centres and commercial companies work on their own implementation of AMC or RTM controllers. The available implementations suffer because of lack of a standard and interoperability problems. The Authors developed a unified solution of management controller fully compliant to AMC and MTCA.4 standards. The MMC v1.00 solution is dedicated for management of AMC and RTM modules. The MMC v1.00 is based on Atmel ATxmega MCU and can be fully customized by user or used as a drop-in-module without any modifications. The paper discusses the functionality of the MMC v1.00 solution. The implementation was verified with developed evaluation kits for AMC and RTM cards.

Noise and drift characterization of direct laser to RF conversion scheme for the laser based synchronization system for FLASH at DESY

Next generation FELs (Free Electron Lasers) require a long and short term stable synchronization of RF reference signals with an accuracy of 10 fs. To overcome the limitations of a coaxial cable based system, an optical synchronization system is being developed for FLASH at DESY. It is based on the distribution of sub-ps optical pulses, where the timing information is encoded in the precise repetition rate. The optical pulse train has to be converted into an RF signal to provide a local reference for calibration and operation of RF based devices. The drift and jitter performance of the optical to RF converter influences directly the phase stability of the accelerator. Three different methods for optical to RF converters, namely the direct detection using a photodiode, injection locking of a DRO, and a sagnac-loop interferometer are currently under investigation. In this paper we concentrate on the jitter and drift performance of the direct photodiode conversion and show its limitations.

Multichannel downconverter for the next generation RF field control for VUV- and X-ray free electron lasers

For pump-probe experiments at VUV- and X-ray free- electron lasers, in the injector and bunch compression section the electron beam and timing reference must be in phase within 0.01 degree (rms) and have an amplitude stability within 10-4 (rms). The performance of the field detection and regulation of the acceleration RF critically influences the phase and amplitude stability. For the RF field control, a multichannel RF downconverter is used to detect the field vectors and control the vector sum of 32 cavities. In this paper a new design of an 8 channel downconverter is presented. The downconverter front-end consists of a passive RF double balanced mixer input stage, intermediate filters, and an integrated 16 Bit analog-to-digital converter (ADC). The design includes a digital motherboard for data pre-processing and communication with the controller. In addition, we characterize the downconverter performance in amplitude and phase jitter, temperature drifts, and channel cross-talk in a laboratory environment.

RTM RF Backplane for MicroTCA.4 crates

We developed a new Rear Transition Module (RTM) Backplane for MicroTCA.4 crates that is compliant with the PICMG standard and is an optional crate extension. The RTM Backplane provides multiple links for high-precision clock and RF signals to analog μRTM cards. Usage of an RTM Backplane allows to significantly simplify the cable management, and therefore to increase the reliability of electronic controls when multiple analog RF front-ends are required. In addition, the RTM backplane allows also to add so called extended RTM (eRTM) and RTM Power Modules (RTM-PM) to a 12-slot MicroTCA crate. Up to four 6 HP wide eRTMs and two RTM-PMs can be installed behind the front PM and MCH modules. An eRTM attached to the MCH via Zone 3 connector is used for analog signal management on the RTM backplane. This eRTM allows also installing a powerful CPU to extend the processing capacity of the MTCA.4 crate. Remaining three eRTMs provide additional space for analog and digital electronics that may not fit on the standard RTM cards. The RTM-PMs deliver managed low-noise (separated from front crate PMs) analog bipolar power (+VV, -VV) for the μRTMs and an unipolar power for the eRTMs. This extends functionality of the MicroTCA.4 crate and offers unique performance improvement for analog front-end electronics. This paper covers a new concept of the RTM Backplane, a new implementation for the real-time LLRF control system and performance evaluation of designed prototype.

Multi-channel down-conversion for MicroTCA.4 based control systems

One of the major benefit of the MicroTCA.4 system is to combine high-speed digital data processing AMC boards with high precision analog signal conditioning RTM boards. We present a multi-channel down-converter based on the MicroTCA.4 system operating in the 1GHz to 4GHz range with an excellent short-term amplitude stability of 6.5E-5 and phase stability of 3.5E-3 degree within a 1MHz bandwidth at 1.3GHz. The down-converter consists of the entire receiver, packed as an RTM board, which down converts the high frequency signal into an intermediate frequency, which is non-IQ sampled by a multi-channel 16-bit digitizer realized as an AMC board. We present all relevant signals along the detection chain and discuss the signal integrity within the RTM and AMC boards for a spurious free operation of the best ADCs available today.

2pi low drift phase detector for high precision measurements

In modern particle accelerators RF phase detectors have to fulfill high demands on the stability and the accuracy to meet the goals of state-of-the-art synchronization systems. Especially challenges are those cases where the RF signal phase shift accuracy must be measured with fs-accuracy for several hours over the full 2π phase detection range. In these cases special measures on the components non-linearities and the RF channel isolation have to be taken. The long-term stability of the phase detector is mostly affected by temperature and humidity variations. To meet the synchronization goals we have built a phase detector that incorporates a high speed dual ADC with a special front-end for continuous phase drift calibration. In the front-end we successfully combined the RF signal with an RF calibration signal (second tone) to compensate common phase drift that occurs in the microstrip lines, the RF transformers and the ADC. The second-tone RF signals are directly converted to the digital domain by the fast ADC and based on signal processing in the FPGA used to calculate the RF signal phase shift correction caused by detector drifts. In this paper potential error sources of the analog and the digital part of the so-called two-tone calibration technique that limit the phase detector precision are discussed. Finally, the experiment results are presented showing a long-term phase stability better than 4 fs rms evaluated at an RF frequency of 1.3GHz.

Performance of the new master oscillator and phase reference system at FLASH

The master oscillator (MO) and phase reference system at FLASH must provide several RF reference frequencies to widely spread locations with low phase noise and small long term phase drifts. The phase noise requirement of the 1300 MHz reference is of the order of 0.1 deg. while the short and medium term phase stability are of of the order of 0.1 deg. and 1 deg. respectively. The frequency distribution system employs a temperature stabilized coaxial line for RF power distribution and a fiber optic system for the monitoring of phase drifts. Presented are the the concept, design and performance measured in the accelerator environment.