Superfast BPM Processor Scheme and First Results
Stephen Molloy, QMUL2nd Mini-Workshop for Nano-Project at
ATF
FONT at ATF
• Micron-level stabilisation of the ATF extraction line beam.
• Measure position of start of train, and correct the end.– To be accomplished within 56ns train.– Latency must be kept low.
• BPM processor should work in <5ns.
Processor Design
• Obtain difference from hybrid and mix with 714MHz from ATF control system.
• Low-pass filter with 200MHz cutoff– 5-pole Chebyshev chosen due to low latency, and
strong out of band fall-off.
Issues• Non-zero hybrid isolation causes zero-offset.
– BPM centre can be moved with variable attenuation if necessary.
• Stripline -> hybrid cables must be matched in time to better than ~50ps.– This is possible, and has been achieved.
• 714MHz LO input to the mixer should be very phase stable with respect to the beam.
• A lot of power at the beam bunching frequency.– Low-pass filter must limit this to a very small value
before signal reaches feedback amplifier.
Phase Stability of 714MHz
Simulated Output - 100μm
Comments• Positive points
– Simulated output is linear with beam position.– Hybrid common-mode residual shifts BPM centre in a
predictable way.– Imperfect cable lengths (within achievable limits)
merely shift BPM centre.– Mixer leakage easily reduced by low-pass filter.
• Potential problems– Relatively large amount of power at beam bunching
frequency remains after low-pass filter.– Predicted latency of entire system is ~6ns.
• Remember latency should be <5ns for feedback experiment.
Alternative Filtering
• 3 or 4-pole Bessel band-pass filter before mixer.– Centred at 714MHz, bandwidth ~400MHz.– Reduces 357MHz entering mixer.– Reduces out of band power entering mixer, thus increasing dynamic
range of BPM.• Less 357MHz means low-pass filter requirements are relaxed.
– Less poles results in faster filter.– 3-pole Chebyshev with ~170MHz cut-off.– Total latency should be equal to or less than the previous scheme as the
band-pass poles have a larger bandwidth than the low-pass poles
Simulated Response of 3-pole Bessel BPF
Simulated Output - 100μm
Simulated Output - 1μm
Comments
• 357MHz beam bunching is not observed.• Power at 714MHz
– Due to mixer leakage.– Level equivalent to DC output when beam has
1μm displacement.• Predicted latency ~4.5ns
– Longer by ~0.5ns when 4-pole band-pass filter is used.
Recent Beam Tests• Single bunch tests
– Verified cable lengths were correct.– Stepped through each component in turn and
verified signal.– Found correct LO phase.– Verified output was correlated with beam
position.– Measured latency.
Recent Beam Tests
• Multi-bunch tests– Calibrated each of three processors using
corrector magnets.– Recorded many extraction pulses to measure
resolution.
Reminder!
Single Bunch - Raw Signals
Single Bunch - After hybrid
Single Bunch - After Band-Pass Filter
Single Bunch – After Mixer
Single Bunch – Final Difference Output
Single Bunch - Corrector Sweep
Latency Measurement
• Triggered the scope with a sum signal.• Recorded raw stripline signal, and final
output of processor.• Results
– 3-pole band pass scheme – 4.2+-0.2ns– 4-pole band pass scheme – 7.3+-0.2ns– No band pass, 5-pole low pass – 6.1+-0.2ns
Multibunch – 3-pole band-pass filter
Multibunch – 4-pole band-pass filter
Multibunch – No bandpass, 5-pole lowpass
Calibration Run – BPM13
Bunch charge during Dec 9th shift