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difx:ivs2016extras [2016/03/10 03:05]
walterbrisken
difx:ivs2016extras [2016/03/11 20:43] (current)
walterbrisken
Line 7: Line 7:
 The utility m5spec can read baseband data and generate spectra. ​ The program and output text file containing the spectrum is documented in {{DiFXUserGuide.pdf|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: The utility m5spec can read baseband data and generate spectra. ​ The program and output text file containing the spectrum is documented in {{DiFXUserGuide.pdf|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''​+''​m5spec /home/avntrainee/​difx_data/​n6043/​NRAO+301_0540_N6043_PT_No0008 ​ Mark5B-2048-16-2 ​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. 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.
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 ''​printVDIFheader NRAO+338_0039_TS036H_NL_No0001 5032 long | less''​ ''​printVDIFheader NRAO+338_0039_TS036H_NL_No0001 5032 long | less''​
 +
 +===== Detect the 80 Hz noise diode =====
 +
 +A program called m5fold "​folds"​ the power at a given frequency and produces a power curve as a function of phase in that period. ​ At the VLBA (and several other VLBI antennas) noise power is injected with a square wave and decoded by downstream electronics to determine the System Temperature.
 +
 +To try this, look at data from n6043:
 +
 +''​cd /​home/​avntrainee/​difx_data/​n6043''​
 +
 +''​m5fold NRAO+301_0540_N6043_PT_No0008 ​ Mark5B-2048-16-2 50 20000 80 fold80''​
 +
 +and then plot the output file "​fold80"​ with gnuplot. ​ The first column is the phase (expressed as time since the first phase bin in seconds), and columns 2 to 17 correspond to the 16 baseband channels. ​ The number "​80"​ in the above command is the frequency to fold.
 +
 +''​gnuplot''​
 +
 +''​p '​fold80'​ u 1:​5''​
 +
 +Try a couple other frequencies,​ such as one that is not commensurate with 80 Hz (such as 79 Hz).  What do you expect if you try 40 Hz?
 +
 +You can increase the number of samples in the folded profile by increasing the number "​50"​ in the command. ​ You can increase or decrease the amount of data to process by changing "​20000"​. ​ The bigger, the higher the output signal to noise ratio.
 +
  
 ===== Snoop on other people'​s correlation ===== ===== Snoop on other people'​s correlation =====
difx/ivs2016extras.1457539533.txt.gz · Last modified: 2016/03/10 03:05 by walterbrisken