Effectiveness of Shielding Methods Using a Pockel s Cell Driver

Transcription

1 Effectiveness of Shielding Methods Using a Pockel s Cell Driver Tristan Dennen (UCLA) September 19, 2007 Abstract Pockel s Cell Drivers produce a 2.5 kv pulse used for simulating ANITA events. The noise produced by the drivers is on the order of 5 V in the near field, which is insignificant during the actual operation. However, in ground testing, the output pulses are attenuated to several tens of millivolts, making the signal to noise ratio unacceptably low. We have investigated several methods of noise shielding including copper tape, copper mesh, ferrite tiles, EM absorbing foam and a heavy-duty RF-shielded box. The results of these investigations are presented here. No Shielding With no shielding of the Pockel s Cell driver except the original box, the noise close to the device varied between 3 and 12 V, depending upon where on the box the measurement was taken. At six inches directly in front of the box, there was about 3 V of noise. All measurements were made with a crocodile clip attachment clipped to itself to form a loop antenna, and are very approximate, since they depend heavily on exactly where the antenna is placed. Copper Mesh After wrapping the box in copper mesh, the noisiest areas were attenuated by between -16 and -23 db (power). Less noisy areas saw more moderate attenuation. The data are shown in Table 1. Copper Tape Much of the noise from the driver comes from slits in the box. Copper tape would ideally take care of these trouble areas, but the original box is made of painted metal and non-conductive plastic panels, so the effectiveness of copper tape is limited. Even so, taping all seams, the power light and the AC input area provided around -20 db of attenuation close to the device. The data are shown in Table 2. EM Absorbing Foam The original box for the pulser has a lot of open space inside, allowing standing waves to form. To get rid of the standing waves, the box was filled with EM absorbing foam. This gave between -6 and -18 db of attenuation, with roughly comparable improvement for more and less noisy areas. The data are shown in Table 3 1

2 Ferrite Tiles Another method to reduce noise from inside the box is to place ferrite tiles directly on the electronics. Ferrite tiles were added to the box with foam, but gave no discernible improvement. The high voltages in the electronics caused one of the ferrite tiles to smoke and give off (presumably) toxic fumes. The data are shown in Table 4. As can be seen, several of the data points indicate that the noise was larger after the tiles were added. This is a result of the inaccuracy of the measurements, and gives a rough idea of the size of systematic errors. Ferrite tiles are supposed to give around -3 db attenuation. However, they work using magnetic materials so perhaps for the large fields used here they become saturated. Their effectiveness would probably be greater for smaller signals, such as from CPUs, which they are designed for. RF Shielded Box An RF shielded box was purchased from It was advertised to give -60 db of attenuation. The box was machined to allow signal input and output and AC input, which inevitably lowered its shielding effectiveness. The Pockel s Cell Driver was placed inside the RF shielded box and hooked up. To lower noise inside the box, the interior AC cable was wound with copper tape. In addition, a foot of the exterior AC cable was wound with copper tape, since it was seen that this was the noisiest part of the assembly (50 mv lowered to 15 mv with copper tape). The AC feed-through was filtered, but it is difficult to know if this helped. All told, the RF shielded box lowered the noise close to the device to between 6 and 10 mv, which is -60 db attenuation. At a distance, attenuation was somewhat lower, but still enough to raise the signal to noise ratio to acceptable levels. To measure the distant noise, the loop antenna was fixed at 1 meter and 3 meters from the front of the device. For each distance, 25 pulses were averaged to get rid of room noise. At 1 meter, the RF shielded box lowered the noise from 156 mv to 3.3 mv, giving db attenuation. At 3 meters, the shielded box lowered the noise from 73 mv to 640 µv, giving -41 db attenuation. The room noise averaged over 25 triggers may account for as much as 300 µv on all measurements. These distant noise levels are much lower than the signal level of 125 mv peak-to-peak after -86 db attenuation. It should also be noted that during actual use, the signal pulse is carried by cable to about 50 m from the pulser before being broadcast. Thus, the signal to noise ratio will depend only on noise levels 50 m away. Combined RF Shielded Box, Copper Tape and EM Absorbing Foam Combing the RF shielded box, the copper tape and the EM absorbing foam lowers the noise at 1 meter from 1.89 mv to 1.13 mv, which is -4.5 db. At 3 meters, the effect was smaller, lowering the noise -1.8 db from 1.42 mv to 1.15 mv. The noise close to the device dropped from 7 mv to 4 mv, roughly -5 db. At these low voltages, the attenuations from the RF box, copper tape and foam do not appear to add linearly. This could be an indication that the bulk of the noise at this point is coming from the cables rather than the pulser itself. If this is the case, lowering the noise further would require an unreasonable amount of effort. 2

3 Conclusions The RF shielded box is the most reliable solution, giving (with penetrations and cables) up to -60 db attenuation close to the device and -40 db at a distance. Copper tape is difficult to work with and easily gets messed up with use, but it does provide about -20 db attenuation, a good inexpensive solution for temporary shielding needs. Combined with EM absorbing foam, -30 db attenuation is not an unreasonable expectation. Copper mesh is also inexpensive and easy to work with for temporary needs. It can give a good -20 db if it is applied carefully. 3

4 Table 1: Shielding effectiveness of copper mesh. Position of Measurement Before Mesh (V) After Mesh (V) Attenuation (db) AC Socket AC Socket Power Light Input Input/Output Output Six Inches In Front BL Corner LB Corner Left Seam LF Corner RF Corner Right Seam RB Corner Table 2: Shielding effectiveness of copper tape on seams, the power light, and the AC input area. Position of Measurement Before Tape (V) After Tape (V) Attenuation (db) AC Socket AC Socket Power Light Input Input/Output Output Six Inches In Front BL Corner LB Corner Left Seam LF Corner RF Corner Right Seam RB Corner

5 Table 3: Shielding effectiveness of EM absorbing foam. Position of Measurement Before Foam (V) After Foam (V) Attenuation (db) AC Socket AC Socket Power Light Input Input/Output Output Six Inches In Front BL Corner LB Corner Left Seam LF Corner RF Corner Right Seam RB Corner Table 4: Shielding effectiveness of ferrite tiles. Position of Measurement Before Tiles (V) After Tiles (V) Attenuation (db) AC Socket AC Socket Power Light Input Input/Output Output Six Inches In Front BL Corner LB Corner Left Seam LF Corner RF Corner Right Seam RB Corner