Q&A on Reference Timing Study

(based on email thread between X, Brad S., and Carlos Y.)

Q1: In Carlos' analysis note he made a plot of a high rate elastic coincidence run and pointed out that the good events all have multiplicity=3. Was it because the RefTime still had all 3: 3/4, ELclean, ELreal, at that time? So for runs like this (high electron rate and clean due to coin trigger), each electron would trigger all three?

For our single arm elastic runs, both HMS and SHMS are dominated by multipility=1 events. (These runs had Elclean as trigger). Why?

Carlos: For the initial setup of SHMS triggers, the reftime was formed by 3/4 OR EL-REAL OR EL-CLEAN. Therefore most of the reference time signals had all 3 pre-triggers, so the corresponding multiplicity is 3. Since EL-CLEAN automatically requires ¾, EL-CLEAN was removed from reftime signal soon after SHMS was commissioned.

Brad: The trigger setup in Carlos' experiment was considerably more complicated than what we used for A1n/d2n. In our case, all of our DAQ triggers (3/4, EL_clean, EL_real) required the presence of the 3/4 pre-trigger in their formation, so only the 3/4 is present on the reftime circuit. (Much better.)

The multiplicity histos on the left in the PDF you mention show a linear drop vs multiplicity on the log scale -- that is consistent with a single, Poissonian high-rate signal incident on the TDC window. That is what we would expect from the single 3/4-driven reftime in a high rate run. (You can actually fit the exponential and extract a 3/4 rate from the multiplicity plot since you know the TDC window width from the plot on the right. Good challenge for a student!)

Q2: The most important question is: which multiplicity should we use to help us decide on where to put the RefTime cut?

Carlos: The multiplicity cut is helpful if you want to clean your spectrum so you can see which events are underneath the main peak, and then you can select an appropriate cut, as Brad said, that it is not tootight. Your cuts will never be perfect, there will always be some bad events that sneak into your cut.I would suggest putting a multiplicity cut on the highest multiplicity. Then you should get a clean spectrum. From your slides 4 and 5, you can see that the multiplicity 1 cut gives you the cleanest spectrum, whereas a multiplicity of 2, gives you an additional broad spectrum which corresponds to the reference times that are NOT correlated with your physics events and must be cut out. Otherwise, the analyzer will pick the first reference time it finds in the window.

A multiplicty==1 cut indeed selects only those events for which there was ONLY one reference time. But this does not necessarily mean that this reference time came from a physics event at the target.  These reference times that you still see at the tail came at a different time. A possible explanation is that radiation from the Hall still enters the heavily shielded hut, although is very rare, and forms these 3/4 triggers that have nothing to do with the events at the target.

***Something to try out would be to put a cut on these reference time events at the tails and look at some target variable.

Brad: You do not ever want to put a multiplicity cut on a ref time in the actual analysis. The 'good' hit will land inside the appropriate window that is identified with a multiplicity==1 cut (which just serves to make the 'good' timing really obvious). The extra random hits are safely ignored.

The good reftime has a fixed relationship to the DAQ trigger since it is really a copy of the 3/4 trigger component, fanned out an delayed by a fixed amount in the hardware and cabling. Because of the slow (40 MHz) baseline clock that the TDCs use, that results in the smeared peak structure you see in bins 3500--4400 for the correct reference time. The other hits are randoms that arrive in the TDC windows, but are not copies of the pulse that generated the DAQ readout trigger. (XZ: hence you do not want to include that “red shoulder to the left of the mainpeak” in the cut. However, any shoulder or other interesting feature that appears under high load should be looked at and understood before cutting it out).

You may note that (4400-3500)*0.1 ns/bin = 90 ns which is much larger than the naive 25ns wide peak you might expect from the above paragraph. What I think is happening here is that the fixed timing between the [DAQ trigger] and [3/4 ref-time copy] is being split across two 25ns wide clock cycles in the TDC. Sometimes the TDC sees 14 coarse clock pulses between the DAQ trigger and the 3/4 reftime, and sometimes 15. There is also a 4ns period clock in play which accounts for some of the roll-off to the right and left of the main peaks and the gap between them. This is one of the reasons why we don't want to use too tight cuts on the ref times, and why we need to verify that the cuts are still good for a representative sample of runs (particularly if there could be timing shifts).

In the end (after proper ref-time subtraction) the detailed timing of the DAQ trigger at the TDC will drop out of all of the precision physics/detector signals, and you are left with a nice high-resolution time signal.

Followup: What I'm seeing in the 1190 raw ref time plots (pT1_*, pT2_*) seems generally reasonable (at this point).

In general, we have to be mindful about how we interpret the non- reference subtracted values that the TDC emits. Those values were never intended to be used for precision timing since, by design, proper use requires the subtraction of a reference time which effectively removes details and event-by-event offsets introduced by how the slow 40 MHz clock is handled internally.

Q2 followup: But we don’t see double-peak in HMS hT1, hT2. These two have a nice single peak at about 30ns in width, see plot below. If there is a clock cycle # jitter then shouldn’t we see double peak on HMS too?

Brad: If the pulse to be measured lands in the middle of a 25ns TDC slow clock cycle, then you'll just get a single ~25ns wide box structure in the TDC. If the pulse falls near the edge of a 25ns TDC slow clock cycle, then that it can land in one of two slow clock cycles.

Q3: What is causing the “cutoff” beyond the main peak, for the plot on p1 is beyond 4250?

Brad: For technical reasons, the reference time pulses are at the end of the TDC readout windows. The sharp cutoff at the right end of the timing spectrum (at 4250) on p1 is also connected to the width of the reftime pulse that is sent to the TDC input. That results in a 'dead' period after the leading edge of the first (true reftime) pulse arrives that is on the scale of 20-30ns. If the TDC windows was lengthened you would start to see some 'grass' following this gap to the right of the main peak.

Q4: What are pT1 and pT2? Two copies of the reference time sent to the DC?

Brad: In general each v1190 TDC module in use requires its own copy of the reference time. Historically 'pT1' is from ROC2, slot 20. 'pT2' is from ROC 2, slot 19. pT1 is used with most of the SHMS hodoscope signals and pT2 is used for trigger related signals and S2y+(B) signals.

They should have identical counts and basically identical structure. (Small differences may be present due to offsets or walk between the independent clocks in the two TDC modules.)

Q5: Are the TDC X-axis reverse of the actual time? Is that why the cut in the param file is always set at a negative value?

Brad: The negative value in the cut file is not because the timing spectrum is negative (or reversed). That is true for 'common stop' TDCs like the Fastbus 1877 modules that we used to use (and are still used in Hall A), but it is *not* true for CAEN 1190 modules.

Larger raw time values from our TDCs mean the hit came at a later time.

The reason for the negative cut parameter is documented in the header of the param files. For example, "/PARAM/SHMS/GEN/p_reftime_cut_elclean.param"  ; Cut to select the Reference time when multiple hits in reference time   ; The units in channels for the module (CAEN tdc or FADC)  ; negative value refcut means that the first reference time greater than the abs(refcut)  ;     is used as reftime. If no ref time is found  greater than the abs(refcut) then first  ;     reference time is used.  ; positive value refcut means that the the first reference time greater than the abs(refcut)  ;     is used as reftime. If no ref time is found  greater than the abs(refcut) then no  ;     reference time is used and warning message is produced.

So, the negative value allows for a 'fall-back' to use a reftime hit that would otherwise fail.  This would mean that all of the timing that uses that reftime is suspect for that event (and/or that the reftime cut is wrong).

** Honestly, I would recommend using positive values in our files, at least to start, so we get warnings on missing reference times and can determine what is going on.

Q5: And what is the 3-peak structure in REf for ROC2?

A: Still being investigated by Mark Jones, Simona M. et al.

Brad: The 'FADC_TREF_ROCx_adcPulseTimeRaw' leaves measure a copy of the reference time that is fed into an FADC module.  That provides a clean timing reference for the TDC data that the FADCs also provide.

Q6: In the analysis note, Table 1 shows hDCREF1-5 are used for reference time, but Table 3 shows only hDCREF1 and hDCREF5. Which one is correct?Carlos: Both tables are correct. Table 1 shows a comprehensive list of all the reference times, even if they are not used in the analysis software during the subtraction.

Table 3 actually shows the reference times which are currently being used in the ref.time subtraction in the software. You can actually select which ref. time you want to use in the subtraction (probably an expert should do this),

We have multiple reference times for redundancy, in case one of them fails. But for the SHMS DC, we do have a single reference time for each TDC, in which case, all of them are used. It has something to do with the type of ROC being used.