Friday, 28 September 2012

Using LOFAR LBA and HBA arrays simultaneously

Most LOFAR stations have two antenna fields. These are the Low-Band Antenna (LBA) array and the High-Band Antenna (HBA) array. The LBA and HBA antennas are capable of receiving a range of 10-90 and 110-270 MHz respectively. However, the signal processing is arranged slightly differently. Each "channel" in the signal processing system has three inputs into which the antennas are connected. These receiver units (RCUs), then have several signal paths that can be used, each of which switch in different filters. Thus, the different antennas and frequency bands can be selected and sampled.

Because each individual receiver unit has both an LBA and an HBA polarisation connected to it, it is not possible to observe with both of these simultaneously. Furthermore, because of the filters used in the RCUs, it is not possible to observe across the entire frequency range of the HBA simultaneously either. You must select one of these "RCU modes" for each given channel.

  •  RCU mode 3 = LBA input, 10-90 MHz filters
  •  RCU mode 5 = HBA input, 110-190 MHz filters
  •  RCU mode 7 = HBA input, 210-270 MHz filters

There are other modes as well, such as RCU mode 6, which uses not just different filters, but also a different clock rate (160, rather than 200 MHz) in order to sample the frequencies around 200 MHz (which would normally be aliased in the other modes). The point is that each RCU is limited to a single mode. This has led some to believe that LOFAR stations cannot observe with the LBA and HBA simultaneously.

Actually... they can.

Here at KAIRA, we have been using combinations of modes. In what we refer to as "RCU mode 357", we have been observing with RCU modes 3, 5 and 7 simultaneously. The modes are interleaved, so that there is still a distribution of antennas for each; this allows beam-forming to take place.

Of course, this is not for the faint-hearted, and one must be careful in mapping the channels and powering-up the HBA tiles in such a way so as not to cause power supply failures. However, when done correctly, multiple beams across all bands can be formed giving frequency coverage over nearly the entire VHF band. Here is one of our first results:

A small sample of data from KAIRA, ranging from ~15 to ~275 MHz. Each frequency channel has been divided by the median to bring out the radio features. The bright arcs are ionospheric scintillation during observations of Cas A. (D. McKay-Bukowski, J. Vierinen, and R. Fallows.)
Already we have seen some interesting things and we will be reporting these over the next few days.

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