Phased Array Requirements

Like all radio sources, the sensitivity of pulsar observations benefits from the availability of a large collecting area. However, because of the compact nature of the source of radiation (typically a few hundred kilometers across), a pulsar is effectively a point source for the largest interferometer baselines on the Earth. Hence, there is not much to be learnt from making a map of a pulsar! This means that single dish observations are enough for pulsar work. However, since pulsars are relatively weaker sources (typical average flux densities $\le 100 \,$ mJy, large collecting areas are very useful and hence array telescopes are used for this advantage. These array telescopes are not used in the interferometer mode, but in the phased array mode (see chapter 6). This means that much of the complicated hardware of the correlator required for measuring the visibilities on all baselines is not needed. In phased array mode, pulsar observations can be carried out in two different ways : (i) incoherent phased array observations and (ii) coherent phased array observations. In the incoherent phased array mode, the signal from each antenna is put through a detector and the output from these is added to obtain the net signal. In coherent phased array mode, the voltage signal from each antenna is added and the summed output is put through a detector to obtain the final power signal. For an array of $N$ antennas, the incoherently phased array gives a sensitivity of $\sqrt{N}$ times that of a single antenna, while the coherent array gives a sensitivity of $N$ times that of a single antenna. The incoherent array has an effective beam that is same as that of a single antenna of the array, whereas the coherent array has a beam width that is much narrower than that of a single antenna, being $\sim \, \lambda/D$, where D is the largest spacing between antennas in the array. The coherent phased array mode is ideally suited for observations of known pulsars. The incoherent phased array mode is most useful for large scale pulsar search observations, where the aim is to cover a maximum area of the sky in a given time, at a given level of sensitivity. For a sparsely filled aperture array, incoherent phased array observations will certain be faster for such applications.