Difference between revisions of "CDC 06 02 2008"

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                 (before shielding = 200 channels, see figure 4.1)
 
                 (before shielding = 200 channels, see figure 4.1)
 
*  new preAmp + shaper = 5.5 channels
 
*  new preAmp + shaper = 5.5 channels
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[[Image:pednoshield.jpg|thumb|none|400px|figure 4.1: Pedestal before shielding the upstream end plenum.]]
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[[Image:pedshield.jpg|thumb|none|400px|figure 4.2: Pedestal after shielding the upstream end plenum.]]

Revision as of 12:30, 29 January 2008

1 News on the noise issue

Inspired by Gerard's visit and equipped with his differential probe I had a first look at a non-oscillating preAmp. I put 1900 V on the straws and checked all connected straws and they are all looking good (= I see signals). Time to do some additional noise studies.

Setup: 

*) 16 armed straws
*) HVDB (HV distribution board) that Gerard brought with him
*) new preAmp, also the one the Gerard brought with him
*) 1.5 meter cable + differential probe (which attenuates the signal 0.366x) (NO shaper) + scope

The VME crate was switched off, the only thing on (within a radius of 10 m) was a PC and a NIM crate.

I took a first look at the noise on a random straw (shown in figure 1.1):

figure 1.1: The noise on a random straw (average).

Pickup with a frequency of 67.80 kHz can be observed. It was coming from a CRT computer screen, so that was switched off.

For further studies i selected 4 typical channels/straws: i will call them straw 1,2,3, and 4. The scope traces of these channels (respectively) are shown in figure 1.2-1.5:

figure 1.2: The noise on straw 1 (average).
figure 1.3: The noise on straw 2 (average).
figure 1.4: The noise on straw 3 (average).
figure 1.5: The noise on straw 4 (average).

The noise is what it is, this is our starting point. After trying out some things I put Al-foil over the upstream plenum + HVDB. I had most success when the foil was touching the cooling strip of the new preAmp. When the foil was touching the grounding of the cable (between preAmp and differential probe) I saw the preAmp oscillating??? again (forgot to take traces of that one, sorry). The scope traces of the noise are shown in figures 1.6-1.9 (for straw 1-4 respectively).

figure 1.6: The noise on straw 1 (average).
figure 1.7: The noise on straw 2 (average).
figure 1.8: The noise on straw 3 (average).
figure 1.9: The noise on straw 4 (average).

These traces are averages, the sample traces are shown in figures 1.10-1.13

figure 1.10: The noise on straw 1 (sample).
figure 1.11: The noise on straw 2 (sample).
figure 1.12: The noise on straw 3 (sample).
figure 1.13: The noise on straw 4 (sample).

The improvement is vast! I also looked at the signals with this new setting, they are shown for each straw in figures 1.14-1.17.

figure 1.14: A signal on straw 1.
figure 1.15: A signal on straw 2.
figure 1.16: A signal on straw 3.
figure 1.17: A signal on straw 4.

They look pretty good, I think.

After this test point some straws were wrapped with Al-foil. The effect was that the shape of the noise stayed the same but the amplitude of the noise went down from 1.38 to 1.08 mV (average); straw 1 was used for this test.


2 Some studies concerning noise and shielding - without shaper

Setup:

  • HV = 1800 V
  • NO shaper


Three straws are observed. We'll call them straw 1,2, and 3 (those are the same straws as in previous section).

Test 1:

  • Shielding over the upstream end plenum + preAmp: yes
  • Extra shielding over the straws: no
  • Shielding over the downstream end plenum: no


The oscilloscope traces of straw 1,2, and 3 are shown in figure 2.1, 2.2, and 2.3.

figure 2.1: Noise on straw 1.
figure 2.2: Noise on straw 2.
figure 2.3: Noise on straw 3.


Test 2:

  • Shielding over the upstream end plenum + preAmp: yes
  • Extra shielding over the straws: no
  • Shielding over the downstream end plenum: yes


The oscilloscope traces of straw 1,2, and 3 are shown in figure 2.4, 2.5, and 2.6.

figure 2.4: Noise on straw 1.
figure 2.5: Noise on straw 2.
figure 2.6: Noise on straw 3.


Test 3:

  • Shielding over the upstream end plenum + preAmp: yes
  • Extra shielding over the straws: yes
  • Shielding over the downstream end plenum: no


The oscilloscope traces of straw 1,2, and 3 are shown in figure 2.7, 2.8, and 2.9.

figure 2.7: Noise on straw 1.
figure 2.8: Noise on straw 2.
figure 2.9: Noise on straw 3.


And why not? Test 4:

  • Shielding over the upstream end plenum + preAmp: yes
  • Extra shielding over the straws: yes
  • Shielding over the downstream end plenum: yes


The oscilloscope traces of straw 1,2, and 3 are shown in figure 2.10, 2.11, and 2.12.

figure 2.10: Noise on straw 1.
figure 2.11: Noise on straw 2.
figure 2.12: Noise on straw 3.

3 Signals with shaper

I just had a look at the signal after the shaper board. I took 3 different straws this time (again called straw 1,2, and 3). The scope traces of the noise are shown in figure 3.1, 3.2, and 3.3, and the ones of the signal are in figure 3.4, 3.5, and 3.6.

figure 3.1: Noise of straw 1.
figure 3.2: Noise of straw 2.
figure 3.3: Noise of straw 3.
figure 3.4: Signal of straw 1.
figure 3.5: Signal of straw 2.
figure 3.6: Signal of straw 3.

Notice the different vertical scale of straw 2, this is channel 3 on the shaper. Did Gerard changed the gain of this channel? No he did not. After reconnecting the cable this "effect" disappeared.

4 Pedestals

The pedestal width of:

  • CDC + HVDB + CLAS preAmp + old postAmp = 29 channels
  • CDC + HVDB + new preAmp + shaper = 29 channels (but the gain of this setup is much higher) (see figure 4.2)
               (before shielding = 200 channels, see figure 4.1)
  • new preAmp + shaper = 5.5 channels


figure 4.1: Pedestal before shielding the upstream end plenum.
figure 4.2: Pedestal after shielding the upstream end plenum.
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