Grounding and Shielding of CMS Endcap Muon on-chamber electronics.

N. Bondar, B. Bylsma, S. Lusin, A. Madorsky, P. Robl, V. Sedov.

This document describes the current implementation of the grounding and shielding on the existing chamber prototype. Ideally, all the electronics should be enclosed into one giant Faraday cage, as described in [1]. In practice, however, this approach is sometimes problematic, and some violations are unavoidable. In case of the ME234/2 prototype, which is working in Lab 7, the following guidelines were used in implementation of grounding:

1. Cooling plate (thick aluminum plate with embedded water pipe) is used as a solid ground for all on-chamber electronics.

2. Cooling plate is connected in many points to the chamber frame forming common ground with chamber cage.

3. All electronics is connected to cooling plate in the carefully selected points to avoid ground loops.

4. All boards are under metal covers forming Faraday cages (to some extent). 5. Al cables (including power) are shielded, and the shields are connected to ground.

Fig. 1 shows the physical layout of the on-chamber electronics. Only wide end of the chamber is shown. The cooling plate is attached to the chamber near the wide end, and its form repeats the form of the chamber. Five Cathode Front-End boards (CFEB) are situated along the chamber’s wide end to minimize the length of the cables carrying analog signals from the cathode connectors. 24 Anode Front-End boards (AFEB) are connected right to the anode connectors along the “Anode” side of the chamber. The output of these boards in LVDS format is sent to Anode Local Charged Track (ALCT) board. As one can see, all analog signals are digitized right on the chamber, and only digital signals are transmitted away. This significantly reduces the risk of parasitic pickup introduced into chamber’s output. All the digital signals from the chamber are transmitted into peripheral crate in LVDS standard. AFEB – Anode Front-End Board. Grounding of the AFEB is provided by the aluminum cover attached to the board from the component side. This cover consists of two parts, one of which is right-angled and attached to the chamber side aluminum cover with two screws. The solder side of AFEB has grounded copper pour, which with the component side cover creates the structure similar to Faraday cage but open on the edges. The input connector of AFEB is connected directly to the anode connectors, eliminating the cable carrying analog signal.

Fig. 1. Chamber integration drawing showing all the boards and connections. See text below for details and abbreviations.

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Fig. 2. On-chamber electronics grounding schematics1.

1 For types of cables, see document [3].

Output digital data in LVDS format are sent to ALCT thru the cable #7. Braid of this cable is connected to one of the screws holding the AFEB cover. AFEB has two grounds - "input ground" - connected to the "internal ground" of the chamber - and "output ground" (cooling plate, frame and the chamber side covers joined together with screws). "Internal ground" on the chamber is formed from the chamber solid cathodes, external copper screen and copper ground jumpers on the anode and cathode outlets sides.

Fig.3. AFEB grounding.

Fig. 4. Aluminum cover protecting AFEBs.

Board with copper pour on the bottom Cable #7 to

ALCT

Cable braid connected to the screw

AFEB cover

Standoff

Component Output/Power connector

Input connector (carries internal chamber ground)

Screw

Aluminum cover

Chamber side cover

AFEBs See fig. 3 for details

3 cables #7 to ALCT

Cable holder

Aluminum cover holder

Chamber side cover (output ground)

These two grounds are connected together in the signal outlet sides (the best position, determined experimentally). The output cables #7 transport digital signals, so this cable ground is connected to the "output ground" of the chamber (see Fig.3). There is an additional cover enclosing all AFEBs. The main purpose of this cover is to protect AFEBs from mechanical damage, but it also serves as an EMI shield improving the noise situation.

Fig. 5. View of 3 AFEBs, cables #7 to ALCT, aluminum cover and its holder.

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cover holder

AFEB

Cable #7 to ALCT

CFEB - Cathode Front-End Board. The CFEB board ground plane is connected to the chamber ground primarily through the low impedance braided shield of the CFEB input cables. One end of the shield is connected to a spade terminal soldered to the cathode ground plane. The other end is connected to a spade terminal soldered to the front-end (analog input end) of the CFEB. There are six input cables for each CFEB ranging in length from about 16 cm to about 30 cm. In addition, the board ground plane is connected to the Faraday shield through two mounting holes screwed to the cooling plate, which serves as the bottom of the Faraday cage. The two grounded mounting holes are located at the analog front end of the CFEB. There are other mounting holes on the CFEB but they are not grounded in order to avoid additional current paths to the analog front end. It should be noted that the cooling plate does not provide a low impedance path to the chamber ground and it should not be used for this purpose.

Fig. 6. CFEB grounding and connectors.

Each chamber has five CFEB’s. Each CFEB is enclosed in its own Faraday cage. The Faraday cages are formed by the cooling plate on the bottom, aluminum “C” bracket partitions on the sides, and a common cover over all five CFEB’s. The current prototype cover has openings to allow the CFEB connectors to extend outside the cover. Future covers will be extended and have small slots just large enough to allow entry of the cables.

Input cable #2 from the cathode connectors

Grounding points

Analog ground

Digital ground

InterCFEB cable #3 to next CFEB

Power cable #6 from LVDB

Output cable #5 to CLCT

Output cable #4 to DMB

Grounding contacts

Fig. 7. CFEB board installed on chamber. Cover is removed.

Partition

Partition

Input cables and connectors

Output cables and connectors

Power

Fig. 8. CFEB cover installed.

Wide end of the chamber

Fig. 9. Analog cable connection to the input connector of CFEB.

Fig. 10. Analog cables #2 attached to the connectors.

Cable braid attached to spade terminal

Cable braid attached to spade terminal

ALCT – Anode Local Charged Track board. ALCT also has ground plane split into analog and digital part. There is the Main Grounding point right in the splitting point, and theoretically, the board should be connected to the cooling plane only in this point. However, in practice the best result is achieved when all four corners of the board are grounded also. The digital LVDS cables #7 run from AFEB output connectors to 24 input connectors of the ALCT. Braids of these cables are grounded at AFEB (see AFEB section) and ALCT using spade terminals, providing good EMI shielding. Currently, only digital part of ALCT is covered with EMI shield. Later the shield will be modified to cover all ALCT.

Fig. 11. ALCT connections.

Power cable #9 from LVDB

2 JTAG cables #10 to Control Module

cable #11 to DAQMB

cable #12 to TMB

Ground splitter

Analog part

Digital part

Main grounding point

24 cables #7 from AFEBs

24 spade terminals for AFEB cable braid connection

6 cables #8 from test strips

testing connector

Fig. 12. ALCT with EMI shield installed.

Cables #7 from AFEBs

LVDB – Low Voltage Distribution Board LVDB converts the incoming low voltage power (+6.5 V analog and digital, -5.5V analog) into the voltages necessary for all other on-chamber boards. LVDB is grounded thru the output cable’s return wires (see Grounding Schematics Fig. 2) to avoid ground loops. Input voltages are supplied by Vicor DC-DC converters. The outputs of DC-DC converters are floating, so they are also grounded only thru the return wires. The power cables’ shields are connected to the return lines on the LVDB and they are not connected at the CFEB and ALCT end. In Chuck Rush and Ben Bylsma’s opinion this configuration is not the best way to connect the shields, and although it is working fine now at Fermilab, it may not in another environment. However, for future LVDB boards the shield should optionally be connected to the cooling plate through the mounting holes. There is an EMI shield cover for LVDB as well. It encloses all the board and attaches to the partitions on both sides of LVDB.

Fig. 13. LVDB connections.

Cable #15 from slow control system

Cable #13, ANALOG+ power input from DC-DC

Cable #14, ANALOG -power input from DC-DC

Cable #13, DIGITAL+ power input from DC-DC

Cable #6, power to CFEBs Cable #9, power to ALCT

Fig. 14. LVDB installed on chamber. Cover is removed to expose heatsink.

High-Voltage. HV power supply has floating output, and the return wire is connected to the chamber case, eliminating the ground loops. Chamber has 30 or 18 HV inputs, and the multiconductor cable(s) #1 will be used to deliver the HV power to chamber. See the document written by F. Catarsi and G. Passuello (CAEN) for complete set of HV grounding recommendations [2].

Heatsink is insulated from the board and grounded on both ends.

References:

1. Endcap Muon Grounding Strategy. T. Y. Ling, C. J. Rush, Ohio State university. 1999.

2. General guidelines for HV grounding. F. Catarsi, G. Passuello, CAEN, 2000. 3. CMS EMU CSC cable list.