Difference between revisions of "Noise studies"
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| + | After attaching the thermocouples to the small CDC prototype we observed that the post-shaper pedestal width was largest in the straws nearest the top of the prototype - it showed a steady increase moving up the detector whereas previously the widths had all been similar. At this point it went from approx 11ch (lowest row of straws) to 18 ch (highest row) (run 30765). The thermocouple wires were attached to the gas plenums at the top. After moving the thermocouple wires around and observing the signals on the oscilloscope we realised that the thermocouple wires were picking up e-m noise at 4-5MHz and passing this on to the nearest straws. | ||
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Noise studies - scope traces for ch16, which shows the largest amount of 4MHz pickup. The left hand side plots also show some cosmics. | Noise studies - scope traces for ch16, which shows the largest amount of 4MHz pickup. The left hand side plots also show some cosmics. | ||
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|width="400pt"|[[Image:outside_cage_no_grounds_4MHz.jpg|thumb|none|400px|Noise - zoom on 4MHz - 1950V, 90/10 premix, 10mV/div, 200ns/div]] | |width="400pt"|[[Image:outside_cage_no_grounds_4MHz.jpg|thumb|none|400px|Noise - zoom on 4MHz - 1950V, 90/10 premix, 10mV/div, 200ns/div]] | ||
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| + | This pickup was hardly noticeable in the lowest row of straws (xx cm below the top row). By moving another thermocouple wire around above the prototype we found that it had to be xxx cm away from the top row of straws for the extra pickup from the thermocouple to be unnoticeable. | ||
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| + | As well as picking up ambient e-m noise mostly 4-5Mhz the thermocouple control unit also contributed some noise of its own (20 Mhz). When taking Fe55 data the ADCs are triggered on signals in ch15 and this internal threshold was increased from xxx to xxx in order to eliminate the noise-only events. | ||
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| + | We moved the detector into a small mesh box to act as a Faraday cage. | ||
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After connecting the HV ground to the preamp end of the cable shielding | After connecting the HV ground to the preamp end of the cable shielding | ||
Revision as of 12:10, 13 August 2010
After attaching the thermocouples to the small CDC prototype we observed that the post-shaper pedestal width was largest in the straws nearest the top of the prototype - it showed a steady increase moving up the detector whereas previously the widths had all been similar. At this point it went from approx 11ch (lowest row of straws) to 18 ch (highest row) (run 30765). The thermocouple wires were attached to the gas plenums at the top. After moving the thermocouple wires around and observing the signals on the oscilloscope we realised that the thermocouple wires were picking up e-m noise at 4-5MHz and passing this on to the nearest straws.
Noise studies - scope traces for ch16, which shows the largest amount of 4MHz pickup. The left hand side plots also show some cosmics.
This pickup was hardly noticeable in the lowest row of straws (xx cm below the top row). By moving another thermocouple wire around above the prototype we found that it had to be xxx cm away from the top row of straws for the extra pickup from the thermocouple to be unnoticeable.
As well as picking up ambient e-m noise mostly 4-5Mhz the thermocouple control unit also contributed some noise of its own (20 Mhz). When taking Fe55 data the ADCs are triggered on signals in ch15 and this internal threshold was increased from xxx to xxx in order to eliminate the noise-only events.
We moved the detector into a small mesh box to act as a Faraday cage.
After connecting the HV ground to the preamp end of the cable shielding
The traces below also show ch16 at a much smaller timescale, 20 μs/div, with the HV supply to the CDC prototype on and off.
HV on (preamp ground & cable shielding connected) The envelope oscillates at about 27kHz.
HV off (HV ground & cable shielding connected)
