Incorporating the above requirements into a realistic set-up for pulsar observations
leads to the following block level diagram for pulsar observations (see Fig 17.1).
In a modern radio telescope, most of the processing of the signals is carried out in
the digital domain, after down conversion to a baseband signal (of bandwidth ).
Hence the first block is an analog to digital convertor (ADC), which is run
on an accurate and controlled sampling clock.
For multi-element or array telescopes, the signals from the different elements need
to be phased. This involves proper adjustments of amplitude, delay and phase of
the signals (see chapter 6).
The output of this block is the phased array signal which goes to the `Spectral
Resolution Block'. For a single dish telescope, the signal comes directly from the
sampler to this block. This block produces the multiple narrow-band channels from
the single broad-band data. This can be achieved using a filter bank or a FFT
spectrometer or an auto-correlation spectrometer. The output is a baseband voltage
signal for each of
frequency channels, sampled at the Nyquist rate. For a
multi-element telescope, the location of this block and the Phased Array block can
be interchanged, in part or in whole. For example, at the GMRT, the integer sample
delay correction is done before the FFT; the fractional sample delay correction and
the phase correction is done in the last stage of the FFT and the addition of the
signals is done in a separate block located after the FFT. Note that for incoherent
phased array operation to be possible, the addition of the signals MUST be after the
spectral resolution block, because the square law detection has to be carried out
before the incoherent addition can be done.
The second orthogonal polarisation from the telescope is also processed similarly till
the output from the spectral resolution block. These outputs can then be given to
two different kinds of processors. The first is a total power adder that simply adds
the powers of the signals in the two polarisations to give a measure of the total
intensity from the telescope as a function of time and freqency. The second is a
polarimeter that takes the voltage signals from the two polarisations and produces
the four Stokes parameters, as a function of time and frequency. The data from the
incoherent phased array, for example, can only be put through the total power path.
The outputs from these two processors are then put through an adder that integrates
the data to the required time constant, .
The final output going to the recorder then is either one (total intensity mode) or
four (polarimetry mode) signals each containing
frequency channels coming at
the rate of
samples per second. The net data rate into the recorder is
then
samples per second for the total intensity mode and four times
as much for the polarimetry mode. As an example, if data from 256 spectral channels
is being acquires with a time constant of 0.25 millisec, the data rate is 1 mega
samples per second for the total intensity mode. If one sample is stored as a two
byte word, we can see that a storage space of 1 gigabyte would get filled with about
2 minutes of data!
In cases where the data rate going into the recorder in the above set-up is difficult
to handle for storage or off-line processing, special purpose hardware to do some of
the processing on-line can also be used. Typical examples of such processing would
be on-line dedispersion, on-line folding at the pulsar period and on-line gating of
the data (to pass on only some region of each pulsar period that is around the on-pulse
region). Each of these techniques reduces the net data rate so that it can be
comfortably acquired and further processed off-line. The choice of the processing
technique depends on the scientific goals of the observations.