[Halld-cal] Input to the next iteration on electronics

[Elton Smith]

Hi Andrei,

In order to keep of record of these discussions as well as make sure no 
one is excluded from the conversation, I am copying this message to the 
halld-cal listserver.

I believe Zisis has answered the issue of bias distribution. It seems 
you are proposing to do what we intended all along for the production.

As far as TDCs, there is no problem in using saturated pulses into the 
discriminator. (Of course the pulse to the fADC should not saturate). 
These pulses effectively give you smaller rise times. The timing 
resolution from leading edge discriminators is proportional to the 
(pulse height/threshold), so the higher gain allows setting the 
threshold to a higher value for the same time resolution. That gives 
more flexibility.

Thanks, Elton.

Elton Smith
Jefferson Lab MS 12H5
12000 Jefferson Ave
Suite #16
Newport News, VA 23606
(757) 269-7625
(757) 269-6331 fax


On 5/22/12 5:15 PM, Zisis Papandreou wrote:
> Hi Andrei:
>
> Thanks for the discussion.  Let me make some comments on the bias 
> distribution.
>
> The plot that you attached indeed shows how the distribution was set 
> up for the beam test.  This is not how we will have it for the 
> production run.  (Elton and I discussed it at length this afternoon).
>
> 1. The bias distribution was designed with a several things in mind.  
> Load on the line was a key one, ability to power SiPMs in sums 
> individually, space restrictions in getting cables into the SiPM 
> enclosure, and of course cost.  With 4 biases, each line powers 10 
> units, and these were distributed as in the picture Fernando drew 
> (attached to your slide 9 and also attached below).  Bias 1 load is 
> driven mostly by cells 1 and 2, with the remaining two rows adding a 
> smaller amount.  Bias 2 is symmetric in load to Bias 1.  Bias 3 and 4 
> loads should be less.  One could argue that in the outer rows (see 
> picture) arrangement should be 1234 (from top to bottom) instead of 
> 4321 as is now.  But probably this makes little difference.  Fernando, 
> please correct me if I am wrong.
>
> 2. I am not sure I understand the muon/cosmics requirement for 
> powering in layers.  If we want the whole inner layer on, we power 
> both lines 1 and 2.  We can then leave 3 and 4 off.  Bias 1 powers 
> cells 1 and 2, and the second row in the double sum (9-12).  Bias 2 
> powers cells 3 and 4, and the first row in the triple sum (13-16). So 
> there is no conflict in operating individual rows in sums. For muons I 
> would expect nominally all units to be on, with the exception of runs 
> were we want to sample the individual ones.  Layer biasing looks nicer 
> but I don't see what we lose as it is.  Obviously, the two 
> arrangements lead to different patterns (e.g the current system cannot 
> power the inner layer and the 1st row in the double sum at the same 
> time), but the current one respects load on line Fernando can probably 
> provide numbers on load. Am I missing something that cannot be done?
>
> 3. I agree with you that what we want is to have both SiPMs on any 
> given cell be on at the same time.  The bias distribution has the 
> flexibility to do this as it is.  What we want to do effectively is a 
> "label matching", so that U1-D1 power the opposite ends of cells 
> (instead of U1-D2 as it was for the beam test).  This would be less 
> confusing to remember than the beam test system.  For the beam test, 
> not all combinations of biases on both sides were taken (ie there 
> probably was no U1-D2 combination to illuminate both ends of the same 
> cells).  The ones taken were a minimal set (if I can call it so) to 
> show that each SiPM individually in a sum was operating ok.  The 
> production assignment can be accomplished without redesigning the 
> board.  It is a cabling and channel assignment issue with the 
> distribution system.
>
> Cheers, Zisis...
>
>> semenov at jlab.org <mailto:semenov at jlab.org>
>> 22 May, 2012 1:12 PM
>> Elton and Fernando:
>>
>> I do not agree with Elton's plan, and I did try to explain my ideas 
>> in the
>> last slide of my yesterday's talk (but it looks like nobody was 
>> willing to
>> spend 5 more minutes to discuss very serious issues :)
>>
>> OK, one more time.
>>
>> The last paragraph in my talk was: "Gains in the timing channels 
>> should be
>> big enough to provide enough efficiency on the low edge of the dynamic
>> range with minimal stable thresholds of discriminators, and should be
>> small enough to not saturate the signal shape and not to burn
>> discriminators on the high edge of the dynamic range. The test of the
>> chosen final timing settings on cosmics is required."
>>
>> To be specific, I propose to set the threshold of 50 mV as a conservative
>> estimation of "minimal stable thresholds of discriminators"; with this
>> threshold, the gain in timing channel should be the same as in FADC
>> channel to cover the low edge of our dynamic range. Because we want not
>> just triggering of discriminator (that is OK if the threshold is just
>> below of the peak amplitude) but good timing (that requires the threshold
>> to be at least in the middle of the pulse front), I recommend the gain in
>> the timing channel 2 times bigger than the one in the FADC channel. 
>> Taking
>> into account 2-Volt dynamic range of FADC, we will have 2x2V = 4V pulses
>> for the events in the high edge of the dynamic range, that is not very
>> small but (probably) manageable amplitude for the discriminator.
>>
>> If we go with Elton's plan, for the events in the high end of dynamic
>> range, we will have pulses of 5x2V = 10V amplitude in the input of
>> discriminator (sometimes even more), and I don't think this is a good 
>> idea
>> unless we intend to burn our discriminators.
>>
>> I do understand the origin of the desire to put bigger signals to the
>> timing channels: indeed for PMT operation, the bigger portion of charge
>> from PMT anoge to the timing channel means more stable pulse front (just
>> because of statistics) => better timing. Unfortunately, these
>> considerations don't work for our case: we discussing the 
>> amplification of
>> already existing pulse, and bigger gain just means bigger amplified
>> unstable pulse (viz., the same shape, just bigger amplitude). No win 
>> here,
>> just disadvantages (if we remember about pulse distortion because of
>> saturation effects).
>>
>> Now about the bias distribution: Fernando, I'm terribly sorry but your
>> statement on the meeting about the bias distribution was incorrect. 
>> If you
>> have a look on pages 1 and 2 of your document "BCAL Readout Tests -
>> Milestone" (GlueX-doc-1951), you will see that the bias distribution for
>> upstream and downstream boards in Hall B test were exactly the same =>
>> with U1,D1 bias (for example), the most inner biased cells look on
>> different cells of BCAL (because of the mirror flip). Moreover (just 
>> to be
>> sure that this is not some error in the paper but the real thing we 
>> had in
>> the tests), see the event display (in the attachment) for the run 587
>> (U1,D1). In the first (inner) layer, the biased upstream and downstream
>> readout cells look on different areas of BCAL. If we will use U1,D2 
>> biases
>> (that we did not in the test), the inner layers will be OK, but the rest
>> of the module will not be seen from both sides again.
>>
>> Again: I do propose change the biasing distribution to the layer-wise.
>> If we will make 2 types of boards (viz., upstream boards with a mirror
>> bias distribution in respect to the downstream boards), it will be better
>> than our present situation, but there are 2 disadvantages here:
>> 1. We will need to develop 2 types of boards;
>> 2. In the cosmics tests, it's very useful to have whole layer powered (to
>> select muon passage) but not just a half of the layer.
>>
>> Hope, you will agree with my arguments.
>>
>> Andrei
>>
>>
>>
>> Fernando J Barbosa <mailto:barbosa at jlab.org>
>> 22 May, 2012 6:17 AM
>> Hi Elton,
>>
>> Some updates/comments:
>>
>> 3. The fADC250s have signal amplitude dynamic ranges of 0.5V, 1V and 
>> 2V. We will have to set the gain of the readout to fit within one of 
>> these ranges (currently 2V) while allowing for Vover operation. I 
>> will need to know the new gain setting as a percentage of the present 
>> configuration.
>>
>> 4. The changes have already been  implemented (for the final 
>> implementation) so that the new heat spreader (we already have these 
>> on hand) will cover all the electronics; heat from the heat spreader 
>> to the outside cover (ambient) has been implemented via standoffs and 
>> modeled by Jim to provide efficient heat removal; the cooling 
>> plate/system has enough capacity to handle any residual heat. The new 
>> transition board is practically complete. All these changes will make 
>> assembly a snap, literally.
>>
>> Best regards,
>> Fernando
>>
>>
>>
>> Elton Smith <mailto:elton at jlab.org>
>> 21 May, 2012 7:06 PM
>> Hi all,
>>
>> We need to provide Fernando with input to the next (final) iteration 
>> on the electronic boards, so these can be prototyped and tested in 
>> preparation for production. The following changes are planned for the 
>> next set of boards:
>> 1. TDC gain (relative to fADC). Presently the boards have been 
>> adjusted with a x5 gain (down from x10 for the original boards)
>> 2. Operating point for the SiPMs (i.e. nominal overvoltage).
>> 3. Electronic amplification of the fADC (depends on the operating 
>> voltage).
>> 4. Mechanical changes to the assembly to efficiently extract heat 
>> from the electronics, including changes to the transition board.
>>
>> I think we need to provide this information soon, otherwise the next 
>> iteration cycle will start impacting the overall schedule. Here is a 
>> straw proposal for each of the above, which I offer for discussion 
>> and comments:
>> 1. I suggest we stay with the x5 amplification. We can take a look at 
>> the runs with different thresholds, but we still  have quite some 
>> flexibility in what thresholds to use, as we are presently using a 
>> nominal threshold of 100 mV.
>> 2. I suggest we use Vover=1.2 V. We want to make sure that we can 
>> operate the SiPM over a range of bias voltages and this would allow 
>> us to run between Vover=0,9 to 1.4 V spanning a fairly broad range. 
>> It also gives us higher PDE with a nominal increase in dark rate. If 
>> we run at 0.9 V, we loose PDE and I do not think we will want to run 
>> much lower than this, so we would be limited at the low end of 
>> adjustability. Yi and Serguei can comment on the dark rate/PDE 
>> trade-off.
>> 3. Pick the electronic amplification for the Vover=1.2 V setting that 
>> reasonably matches the dynamic range at the 5 deg setting.
>> 4. These changes will simplify assembly and likely make heat removal 
>> more efficient. They improve the overall design.
>>
>> Comments and feedback are welcome.
>>
>> Thanks, Elton.
>>
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