Difference between revisions of "CDC 19 12 2007"
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[[Image:CDC_signal_afterpreamp.jpg|thumb|none|400px|figure 9: Signal after the preAmp (pink).]] | [[Image:CDC_signal_afterpreamp.jpg|thumb|none|400px|figure 9: Signal after the preAmp (pink).]] | ||
The signal is barely visible. | The signal is barely visible. | ||
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+ | How are the cables connected between the CDC and the HV-distribution board. The ground of these coax cables is floating (not connected to anything), the inner part of the cable is connected to the HV distribution board and a straw-wire. The HV board layout can be found here: [[ ]] (go to the end of the page) |
Revision as of 21:09, 12 December 2007
Interposer, preAmp, and signal cable
This is the continuation of CDC_12_12_2007.
Thanks to Fernando we have a new reference measurement; he measured the noise of the interposer+preAmp+signal cable: his measurement is shown in figure 1.
The differential signal was measured with the cable between the CDC-straw-wire and the HV-distribution board disconnected on the CDC side. The measured signal is shown in figure 2.
No oscillations are observed! If one compares to figure 1 the comparison is reasonable, but I think Fernando has a better oscilloscope!
For the sake of completeness, the cable between the CDC and the HV distribution board was also disconnected on the side of the HV-board, this signal can be seen in figure 3.
Again, no oscillations.
Conclusion: the oscillations are caused by the input signals on the preAmp. What follows are several measurements performed to acquire a better understanding of the situation.
Now all previous measurements were taken with HV power supply switched, off let's repeat the measurement showed in figure 3 but now with HV switched ON (the supply was always connected to the HV distribution board). The result is shown in figure 4.
The shape of the noise looks different but the amplitude does not change.
How do the noise/signals look like directly from the CDC-wire? Let's start with the noise, we have to points where we can measure: on the cable going from the CDC-straw-wire to the HV distribution board and directly behind the capacitor that decouples the signal from the HV. In figure 5 the noise is shown measured directly on the cable, notice that the HV has to switched off in order to perform this measurement.
This looks OK to me. Let's see how it looks after the capacitor on the HV distribution board: this is shown in figure 6.
One can see that the amplitude increases - also there is a baseline offset. What can cause the offset? Well in figure 6 the preAmp is switched on, in figure 7 the same is measured but now with the preAmp switched off (LV set to zero).
Well, the base line moved closer to zero, mark that the vertical scale in figure 7 is different than figure 6.
Let's look at the signals. After putting 1800 V on the CDC and placing my probe after the capacitor that decouples the signal from the HV I was not able to distinguish a signal in the noise, i had to increase the HV till 2130 V till I was able to see the signal showed in figure 8.
How could it be that the previous pre-Amp could see nice signals at 1800 V -> did it filter out the 100 MHz noise somehow? I am planning to try a suggestion made by Gerard to double the capacitance of the capacitor that decouples the signal from the HV, see how much it improves the situation. Anyway, figure 9 shows how this signal (showed in figure 8) looks like after the (new) preAmp.
The signal is barely visible.
How are the cables connected between the CDC and the HV-distribution board. The ground of these coax cables is floating (not connected to anything), the inner part of the cable is connected to the HV distribution board and a straw-wire. The HV board layout can be found here: [[ ]] (go to the end of the page)