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Grant F. Sheffer (sheffer@triumf.ca)
TRIUMF - Scientific Instruments Manager
Tel (604) 222-1047 ext. 6575
Fax (604) 222-1074

Noise at Output of VTX Pre-amps

As you all may recall, we've had a "ring-back" problem at the output of the VTX. This occurs
when we have the postamp discriminators on and the ECL cables connected to the Fastbus
crate with the power on. It was suspected that this might have something to do with the 
Fastbus switching supply but I believe I've shown that this not the problem. When I attach
the ECL cables to some old PCOS modules in a powered CAMAC crate with a linear power
supply, I see the same effect. This noise level is in the order of 10 - 12 mV and is clearly a
serious problem. I have found that this noise level can be dramatically reduced by shielding
the ECL cables with a wrapping of Al foil.

In the following results, we are pulsing the central cathode of a U-V pair and looking at the 
output of a typical VTX channel. The output pulse height is in the order of 60 mV and the
discriminator threshold is about 12 mV at the VTX output. The ECL cables are attached to the 
Fastbus TDCs and the Fastbus power is on. Figure 1 shows the output waveform with the
discriminators turned off. The noise level is in the order of 1 mV and is essentially 
the "white" noise of the system.

Figure 1


In Figure 2, we see the output waveform with the dicriminators on and the ECL cable bundle
wrapped with Al foil but not grounded to any point in the system. The noise level is in the 
order of 2 - 3 mV.

Figure 2


In Figure 3, we have the same conditions as Figure 2 but with the Al foil shield grounded to the 
Fastbus crate power supply. The noise level is in the order of 1.5 mV.


Figure 3


It was also determined that grounding the Al foil shield to the CAMAC crate containing the 
discriminators had little or no effect. These are only very preliminary results and further tests will 
be required.



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 Noise at output of VTX Pre-amps / "Grant F. Sheffer"  


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