Chapter 5

Pulsar observations place higher demands on data acquisition systems than most other astronomical observations. High time resolution is necessary to search for millisecond and rapidly rotating pulsars (Chapter 6), to resolve microsecond and nanosecond scale structure in individual pulses (Chapter 7) and to obtain high-precision arrival times (Chapter 8). Since pulsars are generally weak sources, observations covering a wide bandwidth are desirable to maximise sensitivity. As discussed in Chapter 4, the frequency-dependent refractive index of the interstellar medium means that pulses emitted at higher radio frequencies travel faster and arrive earlier than those emitted at lower frequencies. For a finite receiver bandwidth, this dispersion process broadens the pulse so that its signal to noise ratio (S/N) is reduced. Frequency resolution therefore is vital to combat the effects of dispersion. We distinguish between two main approaches to de-dispersion: (a) incoherent devices (filterbanks and correlators) where the phase content of the signal is not recorded; (b) coherent devices (baseband recorders and coherent de-dispersers) which make full use of the incoming signal to properly recover the true pulse shape.