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Metrology System to Beam Combining Facility ICD INT-406-VEN-0012 v0.4.doc Page 8 of 16

is supplied via the measurement board to which they connect however the measurement board itself does not

provide this power, it is sourced externally and a suggested arrangement is outlined below.

A specially configured front panel ribbon cable is required to carry the reference signal from the first

measurement board to each of the other measurement boards in the system. It is not recommended to carry

the +15V supply to other measurement boards using this cable because the laser head power supply can only

supply two measurement modules at most when a 20m laser head cable is used. Therefore a different power

supply arrangement is required for supplying power to receivers via their measurement boards.

The measurement board does not supply the +15V however but all reference and signal input connectors (as

well as the front panel ribbon cable link which carries the reference signal to other measurement boards)

have this power connection bussed. Provided one of the two front panel reference signal connectors (dual

measurement board) is supplied with +15V then a receiver module plugged into either or both channels will

be powered. The first two receivers get power from the first measurement board via the reference signal

cable from the laser head. Further receivers are powered from their respective measurement boards provided

one of the other reference input sockets is supplied with power from the local +15V power supply in the

rack.

It is suggested that this additional +15V power supply provides power to each additional measurement board

via an individual cable which is plugged into one of the reference channel cables.

The power supply should be sized to provide 2.5A at +15V ±1V (for up to 8 receivers which each take

<270mA).

10.2.2 Shear Camera

No allocations are made but it should be noted that the shear camera fire-wire cable length is limited to 20m

for superior cable. If standard cable is used the limitation becomes 10m. Use of a repeater allows an

additional 4.5m of cable to the standard 10m length. The repeater draws power from the fire-wire cable and

therefore it would be undesirable to use more than one repeater per cable. Cable trays are approximately

2.5m above floor level and therefore cable length from the table surface to a tray directly above (if one is

available) is approximately 1.25m. The distance of the furthest shear camera from the south wall is

nominally 7.6m. Allowing the cable to rise and travel through the wall by the shortest route and then fall to

the same level implies a further 3m length making the total minimum length for this run 10.6m.

10.2.3 Mirror mount control

There is no cabled arrangement envisaged for controlling the motorised mirror mounts which are

incorporated into the metrology beam expander blocks. A handset may be plugged into the mirror mount to

be adjusted. If remote operation from outside the BCA is desired then further cabling and interfacing would

be necessary.

10.2.4 Grounding

In ‘Chapter 3 System Design Considerations’ of the Agilent Laser User Manual on laser measurement

systems, a subsection entitled ‘System Grounding’ describes the grounding practise that should be adopted.

This section is reproduced in Figure 8. The body of the laser will be connected to the metrology table while

the receiver modules will be mounted in the rack containing all the other metrology electronics. This

arrangement should be sufficient to comply with the grounding advice but if local regulations require that the

metrology table be grounded then an earth loop will be created which may affect the signal to noise in the

system. If explicit grounding of the metrology table is required then the best method would be to route that

ground with the laser head cable and connect it to an earthing point in the metrology rack. In this way the

loop area is minimised and it is unlikely that any other currents can flow through this earth link.

Alternatively the laser head mounting plate could be electrically isolated from the table using very thin

insulating film.

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