Design Approaches for the X band LLRF Systems Uros Mavric, XB-10, 2 nd of December 2010, Cockcroft Institute, UK

XB-10, Cockcroft Institute 4 First World War Front on the Soča River

XB-10, Cockcroft Institute 5 LLRF in General Tasks 1.Drive high power RF with pulses ( amp&pha modulated, different lengths etc.) or CW signal at desired frequency – feedforward 2.Perform regulation ( variations of the RF drive signal or signals driving other actuators (e.g. tuners) ) based on the signals that are fed back - feedback (classical/ pulse-by-pulse feedback) Limitations 1.RF loop parameters: loop delay, open loop bandwidth, static/dynamic cavity detuning, saturation curve (nonlinearities) of the RF loop, drifts of components due to temperature/humidity variations etc. 2.Electronics limitations: limited sampling speed, limited bandwidth of components, processing speed, data transfer, limited dynamic range (noise and nonlinearities) etc. In any case, the measurement can’t be better than what is defined by the receiver

XB-10, Cockcroft Institute 6 Dynamic Range –Nonlinearities define the upper limit of operation. If possible, try to keep at 80% of full scale of the component that is the closest to saturation. –Noise defines the lower limit of operation: In feedforward operation, integrated noise (phase, amplitude) in the cavity is usually mainly defined by the drive section (LLRF + HLRF) For intra pulse (~ ms) feedback operation, the integrated noise depends on the loop gain In slow feedback operation the correction applies to slow disturbances (the correction is based on 1 or n RF pulses). F. Ludwig, M. Hoffmann, H. Schlarb, S. Simrock : Phase stability of the next generation RF field control for VUV and X-Ray FELs, EPAC 2006

XB-10, Cockcroft Institute 7 Some of the Puzzles in the Field of LLRF –Digital vs. Analog –FPGA vs. DSP –Feedback vs. Feedforward –Small form factor vs. large form factor –What is the needed resolution of the ADCs (bits)? –Amplitude/Phase vs. I/Q control –Simple PI vs. complex controllers (e.g. MIMO) –… More information about this in proceedings of LLRF05,07,09 workshops It depends! requirements for stability parameters of the RF pulse RF frequency application characteristics of the high power RF components available resources (money, time, people,…) number of units needed …

XB-10, Cockcroft Institute 8www.i-tech.si Libera LLRF Digital LLRF system 2U form factor Drives 1 klystron 32 RF inputs (up to 12 GHz) µTCA based EPICS ready Provided from HW only to all-in-one (HW, SW and FPGA)

XB-10, Cockcroft Institute 9www.i-tech.si EMMA EMMA – Electron Machine For Many Applications NS FFAG – Non Scaling Fixed Field Alternating Gradient proof of concept accelerator Master-Slave configuration 19 NC cavities, 59 RF input signals Commissioned August, 2010

XB-10, Cockcroft Institute 10 L-band System ACC 4,5,6 Measurements 24 SCRF cavities, 72 RF signals acquired and processed Driving at nominal power, nominal pulse length with 30 bunches at 1MHz 8pi/9 passband mode on ACC6 Regulated Vector Sum of 24 Probe Signals

XB-10, Cockcroft Institute 11 S-band System (on the bench) RangePhase [deg] Over 30 min with 10 s Sampling / 24fs 10 Hz – 10 MHz / 12fs FFT of 1 M samples (decimated by 13). Hann windowing. The plot shows the result after averaging 20 1M buffers. The sampling frequency is 94 MSPS (undecimated). 6e-3 % [400 Hz - 3MHz] 5.8e-3 deg (6.5 fs) [400 Hz - 3MHz]

XB-10, Cockcroft Institute 12 S-band System (on the machine) (S0/A, S0/B) –10 Hz rep rate –2 us long RF pulses

XB-10, Cockcroft Institute 13 Design of the X band LLRF System (General Remarks) Analog Electronics (Front-end, back-end, LO generation/distribution) –High-frequency radio receivers/transmitters design techniques –Good engineering practice : Take care of matching of each components and losses (especially in X band) Do the frequency translation to lower frequencies early in the processing chain Standard pitfalls (power supplies, bad shielding/grounding etc.) –Several approaches : Compact design on printed circuit boards or connectorized solutions –For PCBs choose best substrate (mechanical properties, homogeneous dielectric constant, low temperature coefficient, price etc.) Digital Electronics –Not treated in details here

XB-10, Cockcroft Institute 14 Some Critical Architectural Choices Single IF vs. double IF stage Superheterodyne principle with one/two IF frequencies Single IF requires filtering of the other sideband (RF-/+IF) on the LO that can not be integrated on a PCB (undersampling ?, image rejection filters ?) Double IF architecture needs generation/distribution of two LOs Large form factor vs. small form factor Large : good heat dissipation, good isolation between channels, easier to layout Compact : low temperature gradient over the board, lower distribution losses LO generation PLL Multiplications, division both LO distribution resistive splitters couplers

XB-10, Cockcroft Institute 15 RF Front-end & Back-end Span 1-30 GHz RO3003 or RO5887 Components –Mixers (of-the-shelf) –Amplifiers (of-the-shelf) –Thermally dependent pads –Filters (not many available) –… Mixer LO Power [dBm] P1dB [dBm] Isolation [dB] Return loss LO/RF/IF [dB] Insertion Loss [dB] HMC (not +13)6.5L-R 14, L-I / -6/-14-7 HMC L-R 36, L-I 26-12/-8/-12-8 HMC L-R 36, L-I 36-6/-16/-13-9 SIM-24MH+1310L-R 36, L-I 22-6/-9/-7-7 Amplifier Gain [dB] P1dB [dBm] S11 [dB] S22 [dB] NF [dB] Vcc/Idc [V/mA] Isolation [dB] AVA /13438 HMC /9038 HMC / RO3003 or RO …

XB-10, Cockcroft Institute 16 LO Generation/Distribution PLL based generation Phase noise at the output will be a composite of VCO and reference phase noise profiles. The final profile can be further optimized by properly designing the low-pass (LP) filter. Multiplication and division This method usually adds less phase noise in the process of LO generation. LO distribution Resistive splitters (made out of 0402 resistors) Reactive couplers (Wilkinson, coupled lines couplers etc.)

XB-10, Cockcroft Institute 17 Signal Processing Depends on type of feedback used For short RF pulses (~us range) classical feedbacks are not effective since the closed loop bandwidth is in the ms range. Pulse-by-pulse feedback The speed of switching is defined mainly by the bandwidth of the back-end (~10 MHz, tau~30 ns). However tau can be decreased down to ~5 ns by rearranging the filters. The bandwidth of the spline generation depends on the complexity of the function we want to implement. Amp Pha Amplitude and phase measurements at the output of the LLRF system. Phase follows an arctan function defined in the spline tables. Amplitude is not affected.

XB-10, Cockcroft Institute 18 Thank you for your attention

XB-10, Cockcroft Institute 19 Mechanical Solution –N-type input connectors omit need for transitions, extra cables/patch panels etc. –Connect to