Reflection Current Studies with AGS CNI Chamber 7/1/02
A box was added to shield the preamp electronics from RF noise in the AGS. Another function of the shielding box is to provide a solid ground connection to the chamber.
Pictures of the shielding box can be found here.
1 2 3 4 5 6 7
note: The low voltage power supply cable for the preamp board is split into five wires. The wires are fed through LC filters in the side of the box. These wires are isolated from the box. Inside the box the cable is shaped into a torroid to filter low frequency noise. The connectors for the signal cables and bias voltage are not isolated from the box.
Signals with Shielding Box
The addition of the shielding box greatly reduced (by more than a factor of 10) the amplitude of the reflection signal from the current pulse. For the figures below, one side of the box was removed. Removing the side from the box had no effect on the size or shape of the reflection signal. This suggests that it is not the RF shielding that helps reduce noise levels, but rather it is the better ground connection provided by the box.
The noise levels in the figures below can be compared with the noise seen in the last test on 6/12/02 without the box (see Figs 5 and 6 here).
Figure 1. Ch. 2 with box
Figure 2. Ch. 6 with box
Figure 3. Ch. 7 with box
Figure 4. Ch. 11 with box
The reflection signals are smaller on channels 7-12 than they are on channels 1-6 (compare Figs 1 & 2 with Figs 3 & 4).
Also, the signals from pc board 1 (chan. 1-6) change when power is supplied to board 2 (chan. 7-12). The signals get much worse when board 2 is also powered.
Figure 5. Ch. 6 with boards 1 & 2 powered. Compare with Fig. 2 where only board 1 is powered.
However, the signals from board 2 do not change when board 1 is also powered.
Figure 6. Ch. 11 with both boards powered. Compare with Fig. 4 where only board 2 is powered.
Touching the connector on the pc board with your finger also effects the signal. It's thought that touching the connector either changes the grounding or the person's body acts as an antenna.
Figure 7. Ch. 2 while touching connector. Compare with Fig. 1.
Signals with Shielding Box Removed
The shielding box was removed in order to confirm that it was the presence of of the box that reduced the noise levels. The signals do get bigger when the box is removed, yet they're not as big as those seen on 6/12/02.
Figure 8. Ch. 2 without box. Notice the scale here is 20mV rather than 10mV.
Figure 9. Ch. 6 without box
Figure 10. Ch. 7 without box. The signals from channels 7-12 are not much worse when the box is removed.
Figure 11. Ch. 11 without box
The low voltage supply cable is shaped in a torroidal coil (see picture #6 above) to help reduce noise. The coil was removed from the cable to see what effect this had on the signal. There signal amplitude is slightly increased when the coil is removed.
Figure 12. Ch. 7 without torroidal coil in power cable (also with out shielding box)
As noted above, the low voltage power cable, which is fed through the side of the box, is thought to have some additional noise filter in place. Figs 8-12 were all taken using an unfiltered power cable. When this cable was replaced with the filtered cable, the signal decreased significantly.
Figure 13. Ch. 6 with filtered power cable. Compare with Fig. 9.
The shielding box was replaced, and the reflection signals are decreased again.
Figure 14. Ch. 6 after replacing shielding box. Here the filtered power cable was used. Conditions are the same as those in Fig. 2.
The reflection signals have a DC offset of ~40-50mV. There is a jitter in the signal on the level of ~10mV. When the box was initially replaced the jitter was larger (~20-30mV). After tightening the screws that hold the pc board in place and re-mounting the box, the jitter returned to the 10mV level. We suspect that the fluctuations in the jitter may be due to a problem with the grounding of the preamp pc board.
maintained by: Jeff Wood, wood@physics.ucla.edu
last updated: 8/22/02