Re: FEU84-3 / FADC Timing

[Matthew Shepherd <mashephe@indiana.edu>]

Hi Fernando,

Mihajlo has been making nice progress with a single FADC card.  We do  
this by dividing the pulse and send one copy into the card.  The other  
part we bounce off the end of the long cable and also send it into the  
card.  This leaves then two copies of the same pulse in a single  
readout buffer of the FADC.  We can shift the clock phase between the  
samples by varying the length of the cable used to generate the  
reflection.  We apply the time measurement algorithm to both pulses  
and look at the RMS of the difference between the pulses over many  
events (the mean of the difference gives the delay).  For large pulses  
(near full scale on the 8 bit FADC) we are see typical RMS differences  
of order 150 ps or single channel timing resolutions around 100 ps.   
It is impressive (suprising?) that the timing resolution is this  
good.  We are also investigating dependence on pulse height.

One conclusion is already evident:  the synchronized clock for all  
readout channels is absolutely essential.  If we would had some +- 4  
ns noise on top of this it would completely wash out the sharp timing  
resolution we can get with the PMT FADC system.

I think we also need to explore absolute time stability of a single  
tube.  The cable lengths will certainly stay fixed, but changing the  
voltage may shift the absolute transit time within in a tube at the  
level that we are sensitive to in our calibrations.

We look forward to hearing your input and plan to present some results  
at the collaboration meeting next week.

-Matt


On Apr 15, 2008, at 7:45 AM, Fernando J. Barbosa wrote:
> Hi Matt,
>
> We have clock distribution boards that output multiple (five) NIM  
> replicas and were designed for providing synchronized clocks to  
> multiple fADCs on a small setup. These were recently designed by  
> Gerard and Kim and are available with 250 MHz clocks. These sources  
> include the power supply.
>
> If these are of interest to you, we can ship you one now.
>
> Regards,
> Fernando