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1. CERN, LLRF05 Workshop 12/10/05 ‚Precision low-noise field detectors‘ F. Ludwig, M. Hoffmann, G. Möller, S. Simrock / DESY T. Filipek, R. Romaniuk / Warsaw University Content : 1 Stability requirements for phase and amplitude for the XFEL 2 Next LLRF system for optimized detector operation 3 Limitations from noise and non-linearity 4 Down-converter prototype for CW-modulation scheme 5 LLRF phase noise budget 6 Summary and Outlook

2. Stability requirements on phase and amplitude for the XFEL • Actual LLRF control system using a switched LO-signal : Amplitude stability : - Rotation of the LO-signal in four 90o steps using a squared LO-Signal. Phase stability : • Bandwidth for transforming the • squared LO-signal : voltage 0.5V/div voltage 2mV/div • Stability requirements of the cavity field vector sum : voltage 0.5V/div time 500us/div time 100ns/div peak-to-peak comparison (normalized to A=1V) time 5us/div ACC5, Probe DCW, AN-36 - 86dB dynamic range of signal-to-noise.

3. Next LLRF system for optimized detector operation Where comes the noise from and what do we measure after the down-converter ? Microphonics . . . We measure all noise sources of the loop for a finite gain, but not the residual jitter between beam and reference! How can we improve this ? • - Conceptional improvements using noise reduction methods, e.g. filtering and averaging. • Sort the priorities: low noise, low drift, high linearity, absolute accuracy. • Improve each component. • - Minimize residual jitter by increasing or decreasing the loop gain.

4. Next LLRF system for optimized detector operation Measuring bandwidth : • Proposed LLRF control system operating with a CW LO-signal : Master-Oscillator high-power cavity klystron 1300-81 Jitter transformation : RF-input 1300 down-converter LO-input 1300-81 81=9 x 9 FPGA DAC-clock ADC-clock + Higher harmonics and disturbancies using bandpass filters can be suppressed. + Narrowband filtering the IF-signal reduces distortions from mixer non-linearities. + Averaging reduces ADC-noise and no aliasing effects. + No noise from LO-driver. - Precise synchronization system.

5. Down-converter limitations from noise and non-linearity • Compromise between noise and linearity : • Active Gilbert-mixers: • Passive Mixer + FET: + High conversion gain + Low LO drive needed + Low LO/RF crosstalk - Normal NF - Additional 1/f-noise • + High linearity • + Low NF • Large LO drive needed • (additional phase noise) • - High LO/RF crosstalk IP3 IP2 Non-linearity Noise floor Dynamic Range (DRout) Spurious Free Dynamic Range (SFDRout) Noise • Filtering of distortions : Filter Signal distortions : - intermodulation effects (IP3) - higher harmonics (IP2) Signalinformation Multi-channel detector board : - Filtering of distortions - Linearization during beam pauses - Gilbert cell mixer

6. Actual down-converter • Noise from actual down-converter : LLRF05: G.Möller, [43] Multichannel down-converter board for cavity field detection at the TTF. First mixer stage determines the noise performance. Actual down-converter performance: - - (Switched LO-Signal) (CW-LO-Signal) (Cavity filtered)

7. Down-converter prototype for CW-modulation scheme ADC clock 80 MHz 80 MHz Oversampling 1291 MHz Stripline Filter BPF LO-input -5dBm 1300-9 MHz 9MHz SMD-Filter Evaluation Board Active Mixer LT5522 Low-Noise-Amplifier LO Input Matching Circuit 1300 MHz Stripline Filter Attenuator AD6645 14 Bit, 80 MSPS, 100fs jitter BPF Output Matching Circuit Input Matching Circuit BPF IF Output Matching Circuit RF-input LNA 14 1300 MHz

8. Down-converter prototype for CW-modulation scheme • Undersampling : • Oversampling : Signal bandwidth: Bandlimited noise from Mixer Noise from ADC 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 ADC-noise, oversampling, clock phase noise requirements Oversampling promises better SNR than undersampling. LLRF05: T.Filipek, Frequency Conversion in Field Stabilization System for Application in SC cavity of linear accelerator.

9. LLRF phase noise budget – Residual jitter Beam jitter (simplified) 1st order LP 1st order HP 1st order LP - MO and klystron contributions decreases with gain. The effective noise bandwidth for the down-converter is given by : - Down-converter contributions increases with gain.

10. Phase noise budget (Switched LO, single cavity) • Phase noise spectra : • Contributions to residual jitter : - Noise is filtered by the cavity. TTF2 (new supply) Measured down-converter noise is larger than residual noise and beam jitter. • The down-converter is not a good • indicator for the residual jitter!

11. Phase noise budget (Switched LO, single cavity) • Phase noise spectra : • Contributions to residual jitter : - Noise is filtered by the cavity. TTF2 (new supply) Measured down-converter noise is larger than residual noise and beam jitter. • The down-converter is not a good • indicator for the residual jitter!

12. Summary and Outlook • Summary : • The CW-modulation scheme combines many advantages, for example : • - No aliasing effects and ADC-noise reduction. • - Filtering of distortions, which allows a linearization with improved SNR. • For multi-channel systems Gilbert mixers are recommended. • Oversampling promises better SNR than undersampling. • The down-converters noise contribution to the beam jitter • is reduced by the cavity transfer function. - Design a multi-channel board and test within accelerator environment. - Beam jitter caused by LLRF should be measured with fs-resolution. • Outlook : • Decrease mixers noise : • Increase mixer output : • Linearize the down-converters • characteristic within the beam pause. • - Passive front end structures. • Parallel structures of detectors (VLSI prefered). • pHEMT Gilbert mixers (promise higher gain and lower noise). • Additional „Zero-Phase“ detectors.

13. Summary and Outlook Thanks for your attention!

14. Summary and Outlook Backup Slides

15. Actual down-converter (Designed by G.Möller/DESY/MHF-p) + High LO/RF isolation - Mixing into baseband causes additional noise 8-channels to ADC-Board : 8-channels from cavity probe : LO-Input : LLRF05: G.Möller, [43] Multichannel down-converter board for cavity field detection at the TTF.

16. Choice of LLRF system for optimized detector operation • Actual LLRF control system using a switched LO-signal : • Phase and amplitude detection of the cavity field vector : Rotation of the LO-signal in four 90o steps, using a 250kHz squared LO-Signal. (-I,+Q) (+I,+Q) (-I,-Q) (+I,-Q) • Down-converter output IF-signal : voltage 0.5V/div voltage 0.5V/div Bandwidth for transforming 250kHz squared pulses : but required regulation bandwidth is only : time 500us/div time 5us/div

17. Down-converter prototype for CW-modulation scheme • SNR gain from ADC oversampling : I samples Digital I,Q-Detection Averaging or Filtering BPF BPF RF-input LNA ADC Q samples LO-input ADC clock x N : M SNR gain from averaging within the measuring time : Master-Oscillator Sample frequency Measuring bandwidth Clockjitter Internal jitter Equiv. Input noise of ADC Quant. noise • Number of samples : Oversampling - The signal within the bandbass filter, respectively noise from mixer stage will not be averaged. Undersampling Optimal IF frequency, clock phase noise requirements LLRF05: T.Filipek, Frequency Conversion in Field Stabilization System for Application in SC cavity of linear accelerator. Measuring bandwidth

18. IP3 IP2 Non-linearity Noise floor Spurious Free Dynamic Range (SFDRout) Noise Filter Signalinformation Down-converter limitations from noise and non-linearity • Compromise between noise and linearity : • Active Gilbert-mixers: • Passive Mixer + FET: + High conversion gain + Low LO drive needed + Low LO/RF crosstalk - Normal NF - Additional 1/f-noise • + High linearity • + Low NF • Large LO drive needed • (additional phase noise) • - High LO/RF crosstalk Dynamic Range (DRout) • Filtering of distortions : Signal distortions : - intermodulation effects (IP3) - higher harmonics (IP2) Multi-channel detector board : - Filtering of distortions - Linearization during beam pauses - Gilbert cell mixer