1 Design of a Mixed-Signal Feedback Damper System Michael J. Schulte * Some slides are provided by Craig Deibele (Oak Ridge National Laboratory) and Anil Polisetti (UW-Madison)

2 Project Group University of Wisconsin, Madison –Michael Schulte –Anil Polisetti –Suman Mamidi –Zaipeng Xie Oak Ridge National Laboratory –Craig Deibele –Saeed Assadi –Jeffrey Patterson Los Alamos National Laboratory –Rob McCrady –Bob Macek Hardware components from Sundance DSP Inc.

3 Outline Project Goals and Motivation System Overview System Components –Analog-to-digital and digital-to-analog converters –Field programmable gate array (FPGA) modules System Features System Status Future Work and Conclusions

4 Project Goals and Motivation Goal: Develop a mixed-signal feedback damper system for use at Oakridge and Los Alamos Provides –Flexibility in setting system parameters –Programmability to meet the needs of different experiments –Improved data collection and analysis –Ability to correct for dispersion

5 Instability at SNS Frequency spectrum of beam at Spallation Neutron Source

6 kickerpickup Analog Feedback Damper System Replace portions of the analog system by digital hardware Add new functionality and capabilities

7 Mixed-Signal Damper System The new system combines analog and digital components The digital components are clocked at a multiple of the ring frequency (≈ 1 MHz)

8 System Operation Voltages from pickup are sent to analog hybrid Analog hybrid subtracts voltages and sends results to low-pass filter Low-pass filter band limits the signal and sends outputs to ADC ADC provides digital signals to the FPGA subsystem for storage, and processing FPGA outputs are fed to a dual-channel DAC and are also stored for further diagnostics DAC converts outputs to analog signals which are sent to the power amplifiers Power amplifiers control the kicker electrodes

9 Digital Subsystem Implementation The digital subsystem includes synchronized ADCs, FPGAs, and DACs Desired operating frequency of roughly 400 MHz Multiply ring frequency by a factor of about 400 to obtain the system clock

10 Analog-to-Digital Converter SMT384 ADC Module –Quad-channel, 125 MSPS, 14-bit ADC –Data interleaving on FPGA provides a 500 MSPS, 14- bit data stream * Figure from Sundance DSP, Inc.

11 Data Interleaving and Storage FPGA SMT 398 FPGA Modules –Contains Xilinx Virtex-II Pro FPGA –Interleaves data from ADCs for further processing –Stores up to 8 million 16-bit samples * Figure from Sundance DSP, Inc.

12 Data Processing FPGA SMT 368 FPGA Module –Contains Virtex-4 FPGA –Stores up to 4 million 16-bit samples –Processes digital data and sends outputs to DAC * Figure from Sundance DSP, Inc.

13 Digital-to-Analog Converter SMT 350 DAC Module –SMT350 Dual-Channel 500 MSPS, 16-bit DAC –Accepts data at 125 MSPS and interpolates by 4 –Later upgrade to a true 500 MSPS, 14-bit DAC –Outputs sent to power amplifiers * Figure from Sundance DSP, Inc.

14 Data Processing FPGA The FPGA contains high-speed DSP modules Each module is programmable and can be bypassed Input from the ADC and outputs to the DACs are stored in high-speed memory

15 Programmable Delay Module Phase between the pickup and kicker must be maintained between -90 and +90 degrees Programmable delay module controls the overall system delay: FIFO length can be varied Additional fine-tuning of the delay is required

16 Comb Filters The comb filters dampen the ring frequency harmonics to save power –Comb filter output: –Comb filter frequency response: –t n is set as a multiple of the ring frequency (≈ 1  sec)

17 Comb Filters The ring frequency harmonics occur at multiples of roughly 1 MHz To dampen these harmonics, the clock for the comb filter must be synchronized to the ring frequency

18 FIR Filters The FIR filters compute: Serve as equalizers that correct for dispersion in analog components –Cables have non-uniform magnitude and phase versus frequency –Amplifiers have phase dispersion –Analog hybrids and low-pass filters have magnitude and phase dispersion

19 Cable Magnitude Dispersion The cables have magnitude and phase dispersion due to copper and dielectric losses characteristics of the measurement cable (in dB) vs. frequency

20 Cable Phase Dispersion The cables have magnitude and phase dispersion due to copper and dielectric losses Phase response of Ideal cable vs. Actual cable

21 Cable Dispersion Cable magnitude and phase dispersion is seen in its time domain response Time-domain response of Ideal cable vs. Actual cable

22 Reducing Cable Dispersion Find the frequency (S 21 ) characteristics of the cable using a vector network analyzer Determine the equalizer characteristics that compensate for the cables magnitude and phase dispersion Phase response of idea cable, actual cable and the equalizer versus frequency

23 Reducing Cable Dispersion Determine the number of taps and tap values for the equalizer Run the equalizer in series with the cable to reduce dispersion Comparison of the time-domain responses of an ideal, actual, and de-embedded cable

24 Reducing Overall Dispersion The impact of other sources of dispersion in the system can be reduced using similar techniques FIR filters can be designed to compensate for multiple dispersion sources: Phase spectrum of anti-aliasing LPF with cut-off frequency of 180 MHz

25 System Status Digital subsystem (ADCs, FPGAs, DACs) currently under construction by Sundance DSP Preliminary FPGA subsystem design –Modeled, synthesized, and initial testing –Correct operation of digital components –Operating frequency of roughly 400 MHZ –Simulation model developed using Matlab and DSP System Generator Technique for compensating for dispersion –Developed for cables –In-progress for other analog components

26 Future Work Design and test the FPGA control to set system parameters and filter coefficients Develop a GUI to enter parameters and filter coefficient Use the Matlab simulation model and experimental data to refine the design Test complete digital subsystem (first without and then with the ADCs and DACs) Test complete system at ORNL Improve the design based on experimental results Automate the system for adaptive processing –Hardware/software adjustment of parameter values

27 Conclusions The mixed-signal feedback damper system has important benefits –Added flexibility and programmability –Improved data collection and analysis –Reduce dispersion from analog components It also has significant challenges –Achieving high clock rate on ADCs, FPGAs, and DACs –Generating clocks synchronized to the ring frequency –System integration and testing Preliminary results look promising and the system has the potential for future upgrades

28 Backup Slides

29 Mixed-Signal Damper System The new system combines analog and digital components The digital components are clocked at a multiple of the ring frequency

30 Digital Subsystem Implementation The digital subsystem includes synchronized ADCs, FPGAs, and DACs

31 Offset and Gain Multipliers Offset multipliers correct the closed orbit offset –Set M 1 and M 2, such that is close to zero when the beam is stable Subtractor provides scaled voltage difference Gain multiplier controls overall system gain Implemented using high-speed DSP48 modules

32 Data Processing FPGA The FPGA contains high-speed DSP modules Each module is programmable and can be bypassed Input from the ADCs and outputs to the DACs are stored in high-speed memory

33 Alternative Mixed Signal System The offset multipliers and subtraction can be implemented using analog circuits + Reduces system complexity + May improve accuracy of voltage difference –Introduces additional distortion from analog components –Reduces available diagnostic information

34 Digital Subsystem Implementation Implementing the offset multipliers and subtraction using analog circuits + Eliminates one of the ADCs and one of the FPGAs modules –Reduces available diagnostic information, since individual voltages are no longer available

35 Digital Subsystem The digital subsystem includes synchronized ADC, FPGA, and DAC modules

36 External Clock Source The external clock source is synchronized to a multiple of ring frequency. For SNS, this is obtained by a multiple of 450. –Ring frequency of SNS 1MHz (approx) –Required clock is 450 MHz (approx)

37 External Clock Source Locking the clock is crucial for correct operation –Very important to maintain phase difference between pickup and kicker –Range of phase difference for proper operation of system from -90 to +90 degrees –Phase difference controlled through programmable delay module

38 External Clock Source Frequency response of a Comb Filter properly locked to 1MHz

39 External Clock Source Frequency response of the same comb filter with an error of 50KHz in the original clock –Leads to an error of 22.5 MHz in the clock to the FPGA.

40 External Clock Source contd. Comparison of comb filter outputs.

41 Programmable delay Phase difference between Pickup and Kicker between -90 and 90 to maintain negative feedback. Any change in this phase difference can lead to driving the instabilities instead of damping them. This phase difference is obtained by varying the delay values.

42 Comb Filters Structure of a Comb Filter,t d is a constant, here 1µsec Used to save power for stable fixed beam offsets

43 Comb Filter contd. Need for the Comb Filter Fourier Spectrum of the beam Major part of spectral power due to ring harmonics. Notching out ring harmonics will not affect the instabilities.

44 Comb Filter contd. Transmission characteristics