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LLRF for the SPS 800 MHz cavities

LLRF for the SPS 800 MHz cavities. G. Hagmann, P. Baudrenghien . Block diagram. 800 MHz TX-cavity chain.  3.145 MHz. Cavity:. 800 MHz TX-cavity chain (cont’d). TX (IOT) :  1 dB @  3 MHz -4 dB @  5 MHz. Measurement reproduced from Eric’s presentation.

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LLRF for the SPS 800 MHz cavities

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  1. LIU meeting LLRF for the SPS 800 MHz cavities G. Hagmann, P. Baudrenghien

  2. LIU meeting Block diagram

  3. LIU meeting 800 MHz TX-cavity chain 3.145 MHz Cavity:

  4. LIU meeting 800 MHz TX-cavity chain (cont’d) TX (IOT) : 1 dB @ 3 MHz -4 dB @ 5 MHz Measurement reproduced from Eric’s presentation Conclusion: We aim at minimum ±6 MHz Closed-Loop bandwidth

  5. LIU meeting Vector sum

  6. LIU meeting New 800 MHz System (VME) LL Cavity 1 LL Cavity 2 LL Cavity 1 LL Cavity 2 LL Common

  7. LIU meeting Clock Distributor New 800 MHz System (VME) VME bus (A24D16) RF LowLevel Backplane Cavity Loops Linux FrontEnd 200MHZ Quadrupler CTRV CMM RFFG Switch&Limit

  8. LIU meeting VME Cards RF design & FPGA (Controls, Acquisitions,…) on the same board Clock Distributor Switch & Limit J.Lollierou, J.Noirejan, G.Hagmann G.Hagmann

  9. LIU meeting VME Cards 200MHZ Quadrupler RF Function generator G.Hagmann M.Naon, T.Levens, G.Hagmann

  10. LIU meeting Cavity Loops v1 G. Hagmann BE-RF-FB designer SRAM 2x8 Mbyte for diagnostics 2 x Duplex Optical Serial Links 2 in & 2 out 2Gbits/s (≤3.2Gbits/s) Xilinx Virtex-5 SXT VME P1 backplane for slow controls/readout SSB Modulator IF (I & Q) ≈25MHz Dual TxDac 16 bits • Dedicated backplane (P2): • Power Supply • Clocks • Interlocks • … 4x RF Demodulator RF&LO mixing IF ≈25MHz 14 bits ADC Fs=4·IF ≈100MHz LO Distribution 3rd HiLumi LHC-LARP

  11. LIU meeting Status

  12. LIU meeting Cavity Loop : IQ demodulation TWC 800 MHz Frequencies : LO = 31/8 * Frf200 ≈ 775 MHz Fs = Frf200 / 2 ≈ 100 MHz Fif= Fs/4 ≈ 25 MHz The 800 MHz RF signals (waveguide coupler or cavity sum) are mixed down to a 25 MHz RF using a 775 MHz LO. The IF is then sampled at 100 MSPS Acquisition 14Bits, 100MSPS => ENOB ≈ 11bits (12MHz) => Channels crosstalk < 11bits Fs

  13. LIU meeting Fine delay (1 Turn) Longitudinal Damper (From Beam Control, Optical Gbit link) RF Function Receiver (Voltage/Phase offset Setpoint) Polar Loop In-situ Observation & BBNA Feed forward (Beam synch clock) Ref Phase:  200MHz Cavity Σ +  setPoint Ref Phase from 200MHz Cavity Σ Noise (Blow-up) Comb filter => Gain @ ± n∙frev ± m∙fs FiFo 1T-delay Comb filter (Beam synchronous clock) Cavity filter => Sign Inversion@ cavity zeros Cavity filter (Absolute Cavity response) NCOup&down Modulation RF Modulator (Single side band Transmitter) RF Modulator

  14. LIU meeting TWC 200 MHz Phase Σ φsetPoint TWC800 4x TWC200 Σ TWC200 Σ Bunchshortening Bunchlengthening

  15. LIU meeting Filters (implementation) Down/up modulation Beat frequency computation (from pre-defined function)

  16. LIU meeting Mid-1980s Comb filter • The OTFB must have large gain on the exact revolution frequency sidebands (fRF k·frev) to fully compensate the transient beam loading. The result will be a precise amplitude/phase ratio of 200-800 MHz RF for all bunches • The OTFB must also have gain on the synchrotron sidebands (fRF k·frev m·fs) to reduce the real part of the effective cavity impedance. The result will be an increased threshold for longitudinal coupled-bunch instability. We aim at covering dipole mode (m=1) with full gain, and some gain for the quadrupole mode (m=2) • The synchrotron frequency range is • LHC 25ns, Q20 fs < 1 kHz • Fixed target, Q26fs < 1.4 kHz • LHC ion, 12inj, Q20:fs < 2.2 kHz • With the classic simple IIR filter, the maximum gain G is inversely proportional to the bandwidth (around the revolution frequency lines) • The required minimum 2 kHz -3 dB BW, results in a maximum gain of 2 linear. Not very impressive! Conclusion: The simple IIR filter cannot be used

  17. LIU meeting A Modern Comb filter 6 kHz 2-sided BW around the frev lines 36 dB stopband attenuation • Step1: Decimation by R=5 (?) -> 20 MSPS • Step2: FIR structure at 20 MSPS • Step3: Interpolating LPF FIFO, N=462 8 MHz single-sided -3 dB BW The gain and BW can now be defined independently, at the expense of filter complexity. For example G=17 (25 dB), ±3 kHz BW in the above design (15 taps)

  18. LIU meeting Cavity filter Frf flat top ~801.6MHz ∆f=+0.71MHz Frf flat botom ~799.8MHz ∆f=-1.12MHz Freq ~3.2MHz Fcav 800.888MHz • The cavity impedance is real-valued. Its sign flips at multiples of 3.2 MHz frequency • We will compensate this with an all-pass filter with zeros at these frequencies • This filter will be implemented in the FPGA, after the RF synchronous demodulation, but its response must not follow the ramping RF frequency (almost 2 MHz drift at 800 MHz for FT beams)

  19. LIU meeting Cavity filter (in baseband) • The Filter is implemented with digital circuitry operated with a beam synchronous clock • We compensate this by • down modulating the antenna signal by the beat frequency (∆f=Frf – Fcav) • filtering with beam synchronous clock • Up modulating down modulation with ∆f Up modulation with ∆f Before Filtering Filtering After Filtering ∆f ∆f F F F • ∆fRF200≈ 450 KHz, ∆fRF800 = 4∙∆fRF800 ≈ 1.8MHz = ∆f • 57% wrt to 1st zeros (3.14MHz) !! • But the zeros still move (negligible): • ~3.14∙0.45/200 ≈ 7kHz ( = 0.23%) A candidate sign-flipping filter

  20. LIU meeting RF ON-OFF Modulation • The generation of harmonically related clocks for demodulation (LO) and sampling require a stable, 200 MHz reference locked to the varying revolution frequency during the cycle: Frequency Program output? • Modulation of the 800 MHz can be applied at the set-point level • The ON-OFF modulation of the 200 MHz will require some additional treatment to extract 200 MHz phase in CavLoop module. Manageable

  21. LIU meeting Operation with Ions (preliminary) • We could use the 200 MHz RF-AVG (= 4620 frev) as reference for generation of the clocks • Compensation of transient beam loading would work correctly with fixed frequency acceleration as it depends on the revolution period only • The tracking of the 200 MHz phase must be studied Frf200 Frfavg (ions) Sawtooth

  22. LIU meeting Planning • The two cavities are equipped with antenna on all cells • 1 antenna per cell • The Antenna Summing network is being designed • Help from D. Valuch • Prototype beginning of 2014 • Electronics pre-series & prototype are available or being produced • Tests beginning 2014 • We are developing the following firmware functionalities for start-up • 1T-delay feedback • 200MHz cavity sum phase extraction • Polar loop (if time allows) • The design will be adapted according to these first results • We would benefit from a period with IOT-Cavity but without beam, during SPS hardware commissioning, at start-up

  23. LIU meeting Thank you for your attention…

  24. LIU meeting References [1] D. Boussard et al., Controls of Strong Beam Loading, IEEE Transaction on Nuclear Sciences., 1985 [2] P. Baudrenghienet al., Control of strong beam loading. Results with beam, Chamonix 2001 [3] T. Mastoridis et al., RF system models for the CERN Large Hadron Collider with application to longitudinal dynamics, Phys. Rev. Sp. Topics. AB, 13, 102801, 2010 [4] P. Baudrenghien et al., The LHC RF System. Is it working well enough ? Chamonix 2011 [5] D. Boussard, Travelling-Wave structures, Joint US-Cern-Japan Intl School, Tsukuba, 1996 [6] P. Baudrenghien, CAS RF 2000 and 2010

  25. LIU meeting Back-Up slides

  26. LIU meeting CavityLoop : Up-modulation RF Modulation, SSB 16Bits DACs: Span : 20MHz SFDR > 100dB phase Gain ∆ : +/- 1° ∆G : +/- 0.5dB Low phase noise: (LO from Agilent E8663B still)

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