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difx:ivs2016extras

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Optional exercises

If you have interest in exploring the baseband data in detail, the following things can be tried:

Make high resolution spectra of baseband channels

The utility m5spec can read baseband data and generate spectra. The program and output text file containing the spectrum is documented in Reference Manual sections 5.50. This program works on many varieties of VLBA baseband data (including the Mark5B data in the first dataset), but does not work on VDIF data with multiple threads (e.g., from the second dataset). The dataformat parameter appropriate for dataset 1 data is Mark5B-2048-16-2, and a complete command to generate spectra with 500 spectral points per baseband channel is:

m5spec /XXX/YYY.ZZZ 500 2000 spec.out

Here the number 2000 is the number of FFT frames to process. Increasing that number will lead to more averaging and a spectrum with higher signal to noise. It might be instructive to change the values of the two numbers (number of spectral points and number of FFTs to process) to get a feel for how these parameters change the output.

The gnuplot program can be used to plot the data. First start gnuplot in a terminal:

gnuplot

Then at the prompt you can issue plotting commands such as:

p 'spec.out' u 1:2 w l

Column 1 (identified in the 1:2 parameter) is the spectral point frequency offset (starting at 0 and extending to the bandwidth of the channel) and column 2 is amplitude data for the first baseband channel. There should be three features of note:

  • The bandpass rolls off toward the edge of the band but does not drop to zero. What does this imply?
  • There are spikes in the data every 1 MHz. These are pulse cal tones. How does the relative strength of pulse cal tones change as the number of spectral points is increased? Why?
  • RFI: signal on top of the bandpass that is not related to pulse cal tones is probably RFI. The S-band channels especially have RFI.

It might be instructive to step through the 16 baseband channels to see what real data looks like.

Snoop on other people's correlation

The correlation status messages are sent to a multicast address that everyone on the local network can access. Thus you can see what other machines are doing without interfering. To do this, start a 2 more terminals and in each run the setup_difx setup script. Then run

errormon

in one and

statemon

in the other. These two terminals will show the same output that your dedicated monitor terminals have, but will include information from all jobs running in the room.

difx/ivs2016extras.1457037509.txt.gz · Last modified: 2016/03/04 07:38 by walterbrisken