Design of a direct conversion 200 MHz non-IQ scheme using the Dimtel LLRF4 card Bob Anderson

Outline Potential applications on ISIS 200 MHz direct conversion concepts Finding the best sampling/synthesis schemes 200 MHz band pass filter design Putting it all together

Applications on ISIS ISIS could have several systems operating around 200 MHz 70 MeV Hˉ linear accelerator Muon Ionisation Cooling Experiment Potential for the rapid prototyping of future systems such as ISIS injector diagnostics

The ISIS MHz injector RF system A 6-stage 70 MeV Hˉ linear accelerator 1 RFQ stage 4 Alvarez drift-tube linacs 1 de-buncher cavity All require phase, field level and cavity tuning control Existing analogue control system

ISIS showing the 70 MeV injector and the MICE experiment

The Muon Ionisation Cooling Experiment RF systems currently under construction Two separate 201 MHz RF cavities and power amplifier chains Require phase and field level control Digital low level RF control system planned from the start

LLRF scheme key features Direct synthesis of a 200 MHz carrier with defined phase and amplitude Non I/Q sampling of several 200 MHz carrier derived plant signals Carrier and sampling frequencies are both integer multiples of a shared fundamental frequency Thus reference and sampling frequencies remain phase locked with each other

Direct synthesis from clock frequency Dividing down a fast clock to generate a frequency comb

Compound pulse shapes Will eliminate most of the unwanted harmonics

Don’t forget image frequencies

Larger increments push the clock frequency higher … The low frequency peak also moves It is also possible to run the clock at a frequency BELOW the synthesised frequency?

Conventional I/Q sampling problems Conventional I/Q processing is compromised by ADC differential non-linearity This creates harmonics of the carrier, odd harmonics alias back to the carrier frequency The errors introduced can impact calibration, repeatability and stability Doolittle, THP004, LINAC 2006, Knoxville, TN

Non I/Q sampling High number of samples to maximise measurement accuracy More samples means higher processing overhead and delay due to the number of RF cycles the measurement needs Need to share the same clock as used for synthesis of the carrier Individual samples are weighted by the same table used to synthesise the carrier

I/Q and non I/Q sampling

Raw sampled output Filtered output

Theta Table cut and paste from Excel Spreadsheet

200 MHz band pass filter Previous success using a 3 layer FR408HR circuit as a 200 MHz 90 degree 4-port hybrid Use of CST Microwave Studio Manufacture in 200 off quantities for ~ £15 each Design complete Currently selecting the manufacturer

200 MHz filter response

200 MHz filter dimensions: 84 x 170mm

Putting it all together (MICE) Questions remain about the operating frequency – Carrier frequency may have to follow cavity temperature? – Cavity tuning could control the LLRF clock? – Implement fast phase rotation to track cavity detuning?

Putting it all together (MICE) LLRF controls the cavity amplitudes and phases The absolute phase between cavity 1 and 2 fields is critical If 2 different frequencies are needed for a “cavity warm up mode” then separate clocks and LLRF4 cards will be needed Potentially LLRF could also control the cavity mechanical tuners

Putting it all together (ISIS)

Band-pass filters and connections for tuner control not shown in this overview

Putting it all together (ISIS) Preliminary functional block diagram of the LLRF4 firmware for ISIS

Putting it all together (ISIS) Bench test of the ISIS reference design using a RPi EPICS server

Putting it all together (ISIS) EPICS client screen display of the ISIS reference design bench test

The work presented here builds on and was inspired by related projects at the STFC Accelerator Science and Technology Centre. Special thanks are due to: Andy Moss Lili Ma Graham Cox James Wilson