Difference between revisions of "CDC 06 02 2008"

Revision as of 20:01, 24 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 4 was used for this test.