[Halld-tracking-hw] ADC125 as a TDC...

[Beni Zihlmann]

Hi Gerard,
thanks for the idea. I took only the fastest trip for each preamp card.
had here the result. This cleans up the spectra dramatically. Also you
can see that for a given card the timing seems to drift for groups of
strips. In particular strip 1 ot 24 you see a distinctive wiggle.

cheers,
Beni

> Hi Beni,
> 	I'm not sure what to make of that tail either. One question that might be
> related: are you putting a point on the histogram for every strip? What would it
> look like if you just used the earliest strip hit? Certainly the side strips
> which see smaller signal will not cross the timing threshold as soon as the
> central strip(s) do. That effect must produce a tail to the left in this plot,
> but whether it would be such an extreme tail, I don't have a clue.
> 	Have you had an opportunity to begin to optimize the LE threshold? (I'm still
> guessing this would mainly be done with a goal of overall drift coordinate
> resolution, so maybe that has to wait a bit. But on the other hand also would
> worry that all results are threshold dependent and is it set roughly where we
> will want it?)
> 	Sincerely,
>
> 		Gerard
>
> On 5/17/2011 8:20 AM, Beni Zihlmann wrote:
>> Hi All,
>> attached are 3 plots. The first is a 2d plot for FDC wire 11 on cosmic
>> data. The vertical axis is the strip number and the horizontal axis
>> is the time difference between the strip and wire 11. The times are
>> determined using Gerard's algorithm with interpolation on the FDC
>> data for both the wire and the strip. Note that the time difference
>> is calculated as "t_wire - t_strip" so the long tail to the negative times
>> is due to the strip while we have a rather sharp edge on the right side.
>> this can be seen more clearly when looking at the projection of the 2d
>> plot to the horizontal axis. This is shown in the next to plots individually
>> for the channel 0-23 and 24-47 which represents the two preamp cards
>> for the top and bottom cathode planes. I try to extract a width by fitting
>> mostly only the upper edge. As you can see the width is between 1 and
>> 1.5 ns. So the individual resolution may be about sqrt(2) better. You can
>> also see that the timing resolution depends on the average signal amplitude
>> which is smaller for the second preamp (channel 24-47). I do not know what
>> causes the shift in time between the two preamp card. This shift is certainly
>> much larger than any cable length miss match etc. But Lubomir observed
>> this behavior already earlier.
>> What causes the long tail to negative time difference is not completely clear
>> to me. One reason might be that there are considerable gain variations between
>> strip channels and I plot all hits not just the one with the largest amplitude
>> in a hit.
>> Let me know if you have additional ideas what else one could tests.
>>
>> cheers,
>> Beni
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