Friday, November 12, 2010

Cable Calibration

In order to form the best possible result out of the LOFAR system, it is essential that we can calibrate for the signal travel-times. This means not only being able to know the direction from whence the signal came, but also how long it takes to travel through every cable and every circuit.

Each cable within the system is carefully manufactured and labelled. The serial number on the cable contains information about its performance and role. For example, the one in the photograph (CHY-0853425-1305090755) is an "C"able from the "H"igh-Band Array, "Y"-polarisation. It is "085" metres long and has a delay of "3425" tenths of a nanosecond. This means that, in laboratory conditions, it takes 342.5 nanoseconds for a signal to travel down this cable.

However, that is in the lab.

Although this is a pretty good measurement, to really get the best performance we need to know the signal delay in the field, now that the cable is in place. Things such as connectors, thermal variations and other deployment factors can contribute to subtle variations in the effective cable length. Calibrating these sorts of things is part of the vital commissioning work that is currently being carried out on the LOFAR Chilbolton station.

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