Re DAC clock: presumably, the plan is to distribute a 10MHz (or 100MHz, or whatever) clock with really low close-in phase noise to each MCH, and then from each MCH to the AMCs via the backplane. Why not send this from the AMC to the FMC boards via the FMC connectors? Then, have the FMC board generate the DAC clock using an integer-N PLL. If done properly, an int-N PLL at a couple of GHz is cheap, has a small footprint and will give as good phase noise/phase stability as using an external synth plus coax. The loop filter bandwidth can be low enough to take out any cross-talk from the FMC/backplane. Plus, this way the DAC clock has a well-defined phase relationship to the main system clock (the 10MHz or whatever), which may come in handy for keeping everything synchronised... Basically, it is not possible to generate a sufficiently high performance DAC clock for many applications from something that comes over the backplane, due to crosstalk/signal integrity issues. Joe Britton has some data on clock noise over uTCA backplanes, I believe. The idea would be to have a clock selection crossbar switch on the FMC card, which allows the user either to use an external clock brought in on an SMA connector, or to use a clock generated by a PLL on the AMC card referenced to a backplane clock (and there will be backplane clocks for digital timing purposes). This way people who really care about phase noise can have it as good as they want, while people who don't care as much can dispense with the extra wiring and clutter involved in feeding external clocks to each FMC module. The issue is just that for the most demanding signal generation, using backplane clocks and an on-chip PLL is considerably worse than a dedicated clock that would come in on the front panel from a dedicated low-noise clock source. Mike Biercuk, his student, and Will Oliver have made a nice writeup looking at the importance of low phase noise master clocks for gate fidelity, that describes a number of the important issues: http://arxiv.org/abs/1602.04551 Best, Daniel
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