1. Future FONT BPM Processors C.Perry 25 June 2009

2. Types of Processor Two types of processor: a) present mixer type b) baseband type Both will be made because: - baseband type should be simpler to use and adjust - but it will have marginal resolution at low bunch charge - and machine may have to operate at low Q to help get 37nm spot size

3. General Details Both types will have a number of features in common: - physical form: '6U VME format' module to mount in a subrack - channels: each module will have two channels, for X and Y of one BPM - normally, the X sum output will be unused - monitoring: sum and diff signals will be duplicated on isolated outputs - stripline phasing: all stripline signals will be taken via mechanical phase adjusters

4. Mixer Processor(1) This will be similar to the present, but: - high-level mixer to improve linearity - improved filters (giving some reduction in delay) - low-noise amplifiers on outputs - LO phasing to each mixer separately adjustable on the module using IQ modulators controlled by DACs - low-level LO input, with all necessary amplifiers on the module - provision for monitoring and tracking the LO phase drifts - provision for monitoring and tracking the digital processor sampling clock drifts

5. Mixer Processor(2) Monitoring and tracking LO phase drifts: - objective: to allow drifts of LO phase relative to beam to be observed and corrected, without interference to normal operation - basic method: - the X-sum channel will be set in quadrature - output added into Y-sum output after a delay - read by Y-sum ADC 3 or 4 samples after peak - possible variant: - drive channel with split Y-sum signal as now - no advantages, and slightly messier note: this does not extend to relative drifts within BPM and processor, these phases are assumed to be set by a separate calibration procedure and to be stable thereafter - the IQ modulator/DAC should be stable indefintely to < 1 deg

6. Mixer Processor(3) Monitoring and tracking sampling clock drifts: - narrow system bandwidth to widen the output pulse slightly - tweak filters to improve symmetry of output pulse - look at difference between Y-sum output samples before and after main sample - this slightly increases delay (by about 1.5ns) - less than has been saved by tweaking filters - could keep full bandwidth, but this reduces sensitivity

7. Baseband Processor (1) Concept: - pass bipolar pulse output from BPM through a LPF - this converts the output to a slower bipolar pulse that can be measured directly - penalty is that the amplitude is very much reduced Advantages: - only one adjustment: to sample on the peak - no LO is required - it is inherently linear - it is simpler

8. Baseband Processor (2) Disadvantages: - bandwidth must be low enough that: - output can be accurately sampled on the peak - a high-quality 180 degree hybrid can be obtained - 120MHz (Bessel response) bandwidth has been demonstrated with good hybrid performance and looks safe for sampling - this means that even with a low-noise amplifier, sufficient s/n for 1um resolution needs a bunch charge ~6E9 or more - some increase in b/w and improved s/n may be possible: - going to 200MHz might be possible - this gains a factor of 2 in resolution, or in bunch charge - but it seems probable that this will start to compromise performance in other ways

9. Baseband Processor (3) Monitoring and tracking sampling clock drifts: - delay the X-sum signal, invert, and add into the Y-sum output - this adds a second bipolar sum pulse after the main one - with the correct delay, a sampling point will fall at the zero crossing - 3 samples after the peak sample - variants: - could split the Y-sum instead of using the X- sum - messier, and no benefit - could make the sum filter slower, and add-in without a delay - simpler, but less sensitive and less stable

10. Baseband Processor (4) Caution: - it is assumed that: - the X and Y sum and diff outputs are accurately aligned in time - the 3 ADCs in the group serving one BPM sample simultaneously - this should be assured by construction - can supplement by (permanent) trimming of the hardware in the lab - only the overall sampling phase for each group of 3 ADCs is adjustable - this also applies to the mixer processor, but sampling is less critical