Timing and RF Distribution NLC ILC

1
Timing and RF DistributionNLC -gt ILC

• Josef Frisch

2
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.

3
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.

4
(No Transcript)
5
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.

6
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.

7
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.

8
Phase shift for 10 degree C fiber change, 1 month
(note 1 degree X-band 250 fsec).
9
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.

10
(No Transcript)
11
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.

12
(No Transcript)
13
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.

14
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.

15
Components Fiber Receiver

• Simple
• Converts optical to electrical signal
• Re-generates fiducial
• Error checking on optical signals
• Redundant, fail over in phase comparison unit.

16
(No Transcript)
17
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.

18
(No Transcript)
19
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

20
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

21
Head / Tail Monitor

• Phase detection to compare neighboring sectors.
• Used in conjunction with Phase Comparison units
  to detect failed fiber transmission systems.

22
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.

23
(No Transcript)
24
HOM phase results / comments

• 1.4 ps RMS for each mode.
• Mode difference 1ps RMS -gt 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.

25
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 lt100ps!

26
Issues / Conclusions

• Base technologies for a redundant phase and
  timing distribution system for the ILC have been
  demonstrated
• Compressor phase is the exception! Needs RD.
• 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.-