1 Timing and RF Distribution NLC -> ILC Josef Frisch.

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Timing and RF DistributionNLC -> ILC

Josef Frisch

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History

• RF phase and timing distribution concept developed for NLC

• Prototype of phase stabilized long fiber links tested• Redundant high reliability design concept developed• Many design concepts transfer to ILC• Note: Presentation is a slightly updated version of the

NLC system (without much reference of the current ILC timing / phase distribution designs).

• Discussion to focus on Availability / Reliability.

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Requirements (for purposes of discussion – NOT a specification)

• RF phase distribution with stability to ~ 1 picosecond peak to peak. – The compressor has tighter requirements which may

require a special system.

• Trigger timing distribution with stability to a fraction of a cycle of L-band: 100ps peak – peak (~30ps RMS).– This is to allow resynchronization circuits to reliably

select a single cycle of L-band.

• Single point failure resistant.

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Components: Fiducial Generator

• The machine is assumed synchronized to a 5 (or possibly 10) Hz fiducial. The fiducial generator would be enabled under software control, synchronized to the 60Hz power line, and to the 1.3GHz RF.

• This is a single point failure, but there is only one unit in the accelerator, so the failure rate is expected to be low.

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Components: Master Oscillator

• This must be low noise 1.3Ghz oscillator. • A variety of technologies are available, for example

Sapphire disciplined by Rubidium or GPS. • PSI specifies a sapphire based oscillator at

– -120dBc/Hz at 100Hz– -152dBc/Hz at 1KHz– -160dBc/Hz at 10KHz and above.

• This should meet ILC phase noise requirements.

• The Master Oscillator is a single point failure, but there is only one unit, so failure rate is expected to be low.

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Fiber Links

• Point to Point links using standard telecom fiber. • 1550nm laser diode source, modulated at RF

– 357MHz for NLC test, 1300MHz OK for ILC.

• Fiber spool in oven for fiber length compensation.

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Phase shift for 10 degree C fiber change, 1 month (note 1 degree X-band = 250 fsec).

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Components: Fiber Transmitter (1)

• Use conventional Telecom laser diode at 1550nm, directly modulated with RF.

– NLC tests done at 357MHz– Modern diodes OK for direct 1.3GHz modulation, and have (20dB) lower noise– Best to pulse diode so that reflected power measured with transmitter off.

• Note, must limit transmitter power to ~ 1mW, or get nonlinear effects in fiber which degrade performance.

• Fiber length compensation using ~5Km spool of fiber in oven– Requires few X 100 Watts:– Continuously cool, heat with fan and wire grid.

• Get ~10 second time delay from fiber. (with integration term).• Easy to close feedback loop

• Reflected phase measurement same as for receiver. Use downmix and digitizer system.

• Fiber transmitter is broad band, so fiducial can be applied as a bipolar phase shift to the RF.

• System cost low – all conventional components (except oven!). • Requires ~6 rack units per transmitter.

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Components – Fiber transmitter (2)

• Ovens are “unpopular”: large and consume power• Alternate scheme: Use wavelength tunable fiber working

against the fiber dispersion.– Need approximately 4nm/C tuning range, with 0.5pm wavelength

resolution.– Available commercially 100nm range, without mode hops, .02pm

resolution.• Cost ~$25K. (From New Focus). • Expanded use of DWDM telecom systems may substantially reduce

the price of tunable laser systems.

• Scheme was briefly tested for NLC and worked, but at that time wide band, hop free tuning was not available.

• Probably this is the technology of choice as the laser costs decrease.

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Fiber Transmitter Reliability

• Transmitters are redundant. Auto fail over is performed by the phase comparison unit.

• Transmitters can detect broken fibers from reflected signal– In principal can automatically TDR fiber to

help quickly find break or reflection.

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Components: Fiber

• Long haul fibers can use standard SMF-28 telecom fiber.

• Need low reflections – want fusion splices, not connectors except at transmit and receive chassis.

• Note that standard SMF-28 fiber is about as radiation sensitive as a human: a few hundred Rads can degrade its performance. – This varies dramatically with the exact fiber

composition. • Need to test transmission system with real

installed fiber.

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Components: Fiber Receiver

• Simple

• Converts optical to electrical signal

• Re-generates fiducial

• Error checking on optical signals

• Redundant, fail over in phase comparison unit.

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Phase Comparison Unit

• Located at the “crate” level– System design should allow accelerator operation

with a failed “crate”.• Local narrow band Phase Locked Loop

– Lock to either fiber system– Standby system has phase shifted to match active

system– Prevents sudden global phase shifts (MPS issue)

• Diagnostics to determine which fiber system is bad

• Must be relatively low cost – high multiplicity item.

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Phase Compare Unit – detecting failed channel

• If channel signal level drops, or if fiducials are not detected.

• Phase noise relative to narrow band VCO

• If slow drift is detected, use head / tail monitors, or beam phase measurement to decide

• Phase shifter on standby channel allows smooth changeover

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Phase Control Unit: Low Noise VCO

• Need good narrow band noise to allow phase memory between pulses

• Need low cost since this is a high multiplicity unit.

• Commercial multiplied VCXOs– Integrated phase noise few ps at 1Hz– Slew rate limit PLL feedback for MPS to prevent

sudden beam phase shifts– Can detect noise in fibers at frequencies above

~30Hz

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Head / Tail Monitor

• Phase detection to compare neighboring sectors.

• Used in conjunction with Phase Comparison units to detect failed fiber transmission systems.

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Beam Phase Monitor

• Can use Monopole HOM modes. – (have a hammer, everything looks like a nail!)

• Data taken at TTF as part of HOM alignment / BPM experiments– Experiment was primarily looking at Dipole modes – Monopole

modes were only used for testing system• Directly digitize cavity HOM signals with fast (5Gs/s)

scope• Look at phase of HOM Monopole modes relative to

1.3GHz phase reference• HOM modes are a good detector – high Q and

mechanical stability (in helium) give accurate measurement.

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HOM phase results / comments

• 1.4 ps RMS for each mode. • Mode difference 1ps RMS -> mode noise of 700fs RMS• Can probably do much better with an optimized system. • HOM couplers also see 1.3GHz signal in cavity.

– Provides a direct comparison of 1.3GHz vs beam time.

• Electronics specific to monopole modes would be low cost (standard down-mix / digitize system)

• Effect of Lorentz detuning not known – could be a major problem for this type of measurement.– If so, can always use conventional phase cavities.

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Triggers

• Assume triggers derived from 1.3GHz countdowns, reset by Fiducial.– 1.3GHz too fast for present day programmable logic –

limit few hundred MHz– Can work at a divided down frequency– Expect faster programmable devices by time ILC is

constructed

• Countdowns similar to SLAC PDUs (now running at 476Mhz).

• Due to need to reset frequency dividers running at 1.3GHz, need trigger stability <100ps!

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Issues / Conclusions

• Base technologies for a redundant phase and timing distribution system for the ILC have been demonstrated– Compressor phase is the exception! Needs R+D.

• Much engineering required to build a complete system

• Various alternate technologies available– Example is fiber laser based phase / timing

distribution system developed at MIT and DESY. – Need to do detailed engineering to evaluate trade-

offs.-