7 PuMa

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The WSRT Pulsar Machine (PuMa) is a fully digital and highly flexible machine. It has been used now for observations concerning many different aspects of radio pulsars, ranging from surveys, to timing, high-resolution polarimetry, and interstellar scattering studies. Note that PuMa is made up of two completely independent halves each of which can take up to 40 MHz of bandwidth to give a total of 80 MHz. This 80 MHz of bandwidth is taken from the same adding box used for VLBI (see earlier in this manual).

PuMa can operate in two different observing modes.

In the first mode (Mode-0), baseband voltages of X and Y linear polarisation states are Nyquiest sampled. The maximum coherent bandwidth is 10 MHz, with a total maximum bandwidth of 20 MHz (10 MHz in each PuMa half). Using a FIR (finite impulse response) filter, bandwidths of (10/F) MHz (F=1,2,4,...32) can be defined. To suit the required dynamic range of various possible observations, each sample can be represented by 1, 2, 4 or 8 bits. This comes at the cost of bandwidth as each PuMa half has a maximum data throughput of 10 Mbytes/s. Some examples of Mode-0 observing possibilities are given in Table 6. The exact range of mapping of the bit levels can be specified a priori by the user. With this mode, one can de-disperse the recorded voltage series ``coherently'' with offline software achieving a maximum time resolution of 100 nanosec (reciprocal of 10 MHz band width), for further analyses.

 

 

 

PuMa 1 PuMa 2
Bands Bandwidth (MHz) Bits Bands Bandwidth (MHz) Bits
0 10 2 4 10 2
0 5 4 4 5 4
0 5 4 4 10 2
0,1 5 2 4,5 5 2
0,1,2,3 2.5 2 4,5 2.5 4

 

Table 6: Some possible PuMa configurations in baseband recording mode. The two PuMa halves are shown as PuMa 1 and PuMa 2 respectively and the WSRT bands are shown (0-3 for PuMa 1 and 4-7 for PuMa 2). Note that the bandwidth shown is for each band, thus in example 4 there is a total of 4x5 MHz recorded.

 


In its Mode-1 PuMa is analogous to a traditional filter-bank type pulsar machine except in this case the frequency channels are defined digitally. PuMa can synthesise (by Fourier transforming the Nyquist-sampled base band signal) anywhere between 16 and 4096 spectral channels across each of the eight 10 MHz bands supplied by the WSRT. Naturally this spectral resolution comes at a price in terms of sampling time. It is also possible to reduce the sampling time within PuMa to either reduce the data rate or allow more bits to be recorded and/or all Stokes parameters (noting again the maximum throughput of each PuMa half is 10 MBytes/s). The sampling time can be defined as,
t samp= 100 Nch Ns2a NS2A n s (1)

where Nch  is the number of spectral channels in 10 MHz, Ns2a and NS2A are possible sample adding combinations whose ranges are given in Table 7 (see the PuManual for more info).

 

 

  Parameter Range
General Observation Duration Depends on data rate
  WSRT bands to be used [0...7]
  Number of bits 1,2,4,8
Mode Dependent    
Mode-0 Bandwidth reduction Factor (FIR) 1,2,4,...,32
  Polarisations to be stored X,Y or both
Mode-1 Number of Channels (Nch per 10 MHz) 16,32,...,4096
  Number of Samples to Add (Ns2a) 1,2,4,...,4096/Nch
  Number of Sharcs to Add (NS2A) 1,2,3,6
  Stokes parameters to be stored [I,Q,U,V]

Table 7: The ranges of values for some of the important PuMa parameters. Note that [...] defines any combination of values.  


In Mode-1, PuMa is also capable of recording all the Stokes parameters, I, Q, U and V (or any combination of these), with a user-supplied number of bits per sample with possible values of 1, 2, 4 and 8. The range of mapping, as in Mode-0, can be specified by the user.

A subset of the possible Mode-1 observations is given in Table 8.

 

 

NBands Nch Ns2a NS2A t samp Nbits Polarisations Data Rate (MB/s)  
8 64 1 1 6.4 ms 2 I 10  
8 64 64 6 2.4576 ms 2 I 0.06  
8 64 16 1 0.1024 ms 4 IQUV 5.43  
2 4096 1 1 0.4096 ms 2 I 2.54  

 

Table 8: Four of the many possible PuMa Mode-1 combinations are shown. Note that the number of channels is across each of the 10 MHz bands indicated in column one and that the data rate is given for each PuMa half.

Simultaneous multi-frequency observations are also possible with PuMa and WSRT. These involve the splitting of the array into combinations of dishes and/or bandwidth. While this does reduce the sensitivity due a decrease in the number of telescopes and bandwidth it does provide truly simultaneous observations using the same instrument. An example set-up would be to have 5 dishes and 40 MHz of bandwidth at 21cm, 5 dishes and 30 MHz of bandwidth at 840 MHz and 4 dishes and 10 MHz of bandwidth at 328 MHz. If you are interested in this exciting possibility please do contact the PuMa representatives at ASTRON.

For further information see the PuMa Manual.

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