... | ... | @@ -29,4 +29,27 @@ Then there are a few register stages to get off the FPGA, IQDAC, Mixer + RF anal |
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Therefore we observe two glitches in the output RF:
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* the first at around 120 ns is due to ad9910 re-syncing
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* the second at around 950 ns is due to the IF RF signal re-syncing |
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* the second at around 950 ns is due to the IF RF signal re-syncing
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## RF phase after injection when nco_reset has been delayed
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To avoid glitches in the RF, we must drive the sync signal into the first trigger unit after the RF has become stable. In addition, we have multiple injection frequencies with fixed target protons and LHC protons having 300 kHz difference. We need to measure the phase differences between these two signals and determine if there's one set of parameters, used to delay the trigger units sync signal, that could operate for all proton injections.
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To investigate this problem, I simulated two RFtrains at the separate injection frequencies.
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In the upper display from the trace below, you can observe a delayed version of nco_reset (blue) triggering after the RF has become stable (200 ns earlier). The display is in persistent mode because we're swapping between the RFtrains and we want to observe their differences. Some observations from the lower display that shows the zoomed in region around nco_reset_delayed:
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* the relationship between nco_reset_delayed and either RFtrain program is reproducible
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* the zero crossing of the two RF signals is separated by ~400 ps or 8 % of the RF
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* this is < 30 degrees of phase difference
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![delayed_nco_reset_injection_sim](uploads/30bb69ce1c59378e6e9df67753087f0c/delayed_nco_reset_injection_sim.png)
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The nco_reset_delayed signal was delayed some 70 WR clock cycles (16 ns) after nco_reset was received and sent to the RFNCO. This phase difference between fixed target and lhc protons after 1.2 us was pre-calculated to be around 140 degrees, not the 30 degrees that we observe.
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Its because we only start accumulating differences when rfnco and iqmod start outputting after 744 ns. See an RFNCO and IQMod simulation trace below:
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![rfnco_latency_sim](uploads/aa7d5e802704c3d2232adc8f86e422ca/rfnco_latency_sim.png)
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The first single cycle pulse that starts the process is nco_reset and the second is coefs_vld signal out of the RFNCO to drive the IQMod block.
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The RF signal is still unusable for at least 1 us after nco reset but we’re only accumulating phase differences for part of this period not all it.
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The conclusion from this investigation is that it should be possible to find a single set of parameters to define the delay for nco_reset for all proton injections. |
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