VEGAS Testing August 27 2014 Part 2 - Richard and Adam


  • Attempt to better understand the noise spikes in VEGAS data
  • Data in TGBT14A _912_24

Brief Background

Numerous spikes can be seen in the VEGAS data. These spikes occupy a single channel or pair of channels, and look a lot like RFI, but in bands not common for RFI. The results here utilize Q-band (Rcvr40_52), where RFI should be minimal. The spikes can more easily be seen by convolving the data with a high pass filter. In GBTIDL:

result=convol(getdata(),digital_filter( 0.1, 1, 200, 64))

It was originally thought that these spikes were limited to modes 2/3, but were recently found in mode 4. (here, the strong spikes are at 8192 and 24576, which are multiples of 512, but there are also spikes at 13474 and 25121, which are not.) Digitally filtering the data showed significantly more spikes than initially obvious as can be seen here. To see how consistent these spikes were across frequency space, we used three banks looking at three different frequencies (no overlapping wavelengths), all in mode 4. This plot shows the locations of the largest spikes (defined as >1.e8) in bin-space, with spikes from each bank getting their own value (1-3). This shows that the spikes more or less appear in the same bins in every bank of the spectrometer. The lack of a 1-1 correlation is likely caused by the very rudimentary thresholding used to determine the locations of the spikes, causing some of the spikes to not be detected (as most are suppressed when comparing to this).

Preliminary Analysis

We observed in modes 1,2,3, total power with cal, 10 minute tracks, repeated twice for each mode. Input signals were the external noise source (Bank A), and Q-band looking at blank sky, for the other banks. A typical mode 2 total-power (with cal) scan is show here:

As can be seen, no spikes are obviously visible in the ten-minute integration. However, following Ron Maddalena's suggestion, we then used GBTIDL "gettp", and convolved the data with a digital filter. The filter response is shown here:

The results for modes 1,2,3, with the two successive scans overlaid, are shown below:

As can be seen, no spikes can be seen in mode 1, but a rail of spikes can be seen in modes 2 and 3. The spikes overlay exactly in channel between successive scans in the same mode, but do no overlay in frequency space between modes. The spikes are quite repeatable in height from scan to scan, but not exactly so. The spikes are not a perfect rail (i.e. not evenly spaced in frequency). Text files containing the location of mode 2 and 3 spikes are here:

Previous data show similar spikes in mode 4, but these seem more intermittent, and were not visible in this dataset. But, spikes were seen in higher numbered modes (data analysis still underway). Here is an example of spikes in a previous Mode 4 dataset:

To see if the spikes could be due to noise calibration, we looked for spikes in data without noise calibration:

It appears that the spikes are not dependent on the calibration switching.

Time Series Analysis

We took a 20 min scan in mode 2 with VEGAS (scan 79 in TGBT14A_912_26). At the same time, we used the archivist to record the ADCPWR1 values, (the manager grabs 16k samplers in a chunk). Adam took an FFT of each chunk separately, and then averaged the 1200 rows together, see below:

  • Time series processing; raw bandpass spectrum:
    RawADC spect.jpg

  • Time series processing; filtered bandpass spectrum:
    FilteredADC spect.jpg

We took the contemporaneous VEGAS FITS file, get GETTP, binned the data by a factor of two to get the same number (8K) of channels, and plotted the results, see below:

Here are some plots in channel space:

  • In both Modes 2 and 3, spikes appear at ... 5120, 5632, 6144 ... so appears to be every 512 channels.

  • The spikes have been present since at least 27 February 2014 (TGBT13B_502_37)

Varying fftshift

Joe has done some tests with the SRBS, see


  • Using SpikeTest to look for spikes in different modes:
vegasConfig = """
receiver = 'Rcvr40_52'
beam = 'B12'
obstype = 'Spectroscopy'
backend = 'VEGAS'
vegas.subband = 1
deltafreq = 0
bandwidth = 1500
swmode = 'tp'
swtype = 'none'
swper = 1.0
swfreq = 0.0
tint = 1.0
vframe = 'topo' 
vdef = 'Radio'
noisecal = 'lo'
pol = 'Circular'
restfreq = [{"restfreq":42000, "bank":"A","beam":"1","bandwidth":1500},                                 # noise source, mode 1
                    {"restfreq":42000,"bank":"B","beam":"1","bandwidth":1500},                                 # IF, mode 1
                    {"restfreq":42000,"bank":"C","beam":"1","bandwidth":1500,"nchan":"high"},        # mode 2
                    {"restfreq":42000,"bank":"D","beam":"1","bandwidth":1080,"nchan":"low"},         # mode 3
                    {"restfreq":42000,"bank":"E","beam":"2","bandwidth":187.5,"nchan":"low"},        # mode 4
                    {"restfreq":42000,"bank":"F","beam":"2","bandwidth":187.5,"nchan":"medium"}, # mode 5
                    {"restfreq":42000,"bank":"G","beam":"2","bandwidth":187.5,"nchan":"high"},       # mode 6
                    {"restfreq":42000,"bank":"H","beam":"2","bandwidth":100,"nchan":"low"}]            # mode 7
off = Offset("J2000", 0.0, 0.0, cosv=False)
Break("Check Balance")
#SetValues("IFRack.IFNoise", {'noise_bandwidth':'broadband', 'state':'Ready'})
#Break("get prepared to switch in the noise source")
Track(source, None, 600.0)

  • Scan 1: Spiketest 1 sec integration times, Modes 1,1,2,3,4,5,6,7
  • Scan 2: SpikeTest 30 sec integration times, modes as above
  • Scan 3: Spiketest2 30 sec integration times, modes 1,8,9,10,11,12,13,15 (SpikeTest2)
  • Scan 4: as above
  • Scan 5: SpikeTest3 30 sec integration times, modes 1,16,17,18,20,21,22,23
  • Scan 6: as above
  • Scan 7: Spiketest4 30 sec integration times, modes 1,24,25,26,27,28,29
  • Scan 8: as above
  • Scan 9: SpikeTestMode2_3TpNoCal
  • Scan 10: as above
  • Scan 11: SpikeTestMode2EightBanks
  • Scan 12: as above
  • Scan 13: SpikeTestMode3EightBanks
  • Scan 14: as above
  • Scan 15: SpikeTestMode4EightBanks
  • Scan 16: as above
Topic attachments
I Attachment Action Size Date Who Comment
FilteredADC_spect.jpgjpg FilteredADC_spect.jpg manage 64 K 2014-08-28 - 16:37 RichardPrestage Time series processing; filtered bandpass spectrum
RawADC_spect.jpgjpg RawADC_spect.jpg manage 87 K 2014-08-28 - 16:36 RichardPrestage Time series processing; raw bandpass spectrum
digital_filter.pdfpdf digital_filter.pdf manage 6 K 2014-08-27 - 20:01 RichardPrestage the digital filter
mode1_spikes_2scans.pdfpdf mode1_spikes_2scans.pdf manage 33 K 2014-08-27 - 20:01 RichardPrestage mode 1, digital filtered, two scans overlaid
mode2_spikes_2scans.pdfpdf mode2_spikes_2scans.pdf manage 182 K 2014-08-27 - 20:00 RichardPrestage mode 2, digital filtered, two scans overlaid
mode2_tp_no_visible_spikes.pdfpdf mode2_tp_no_visible_spikes.pdf manage 82 K 2014-08-27 - 20:52 RichardPrestage Mode 2 Total Power
mode2_tpnocal_filt.pdfpdf mode2_tpnocal_filt.pdf manage 75 K 2014-08-28 - 11:52 AdamKobelski Spikes in data without noise calibration
mode2channel.pdfpdf mode2channel.pdf manage 91 K 2014-08-29 - 12:53 RichardPrestage mode 2 channel space
mode2spikes.txttxt mode2spikes.txt manage 1 K 2014-08-27 - 20:49 RichardPrestage location of mode 2 spikes above a threshold.
mode3_spikes_2scans.pdfpdf mode3_spikes_2scans.pdf manage 160 K 2014-08-27 - 19:58 RichardPrestage mode 3, digital filtered, two scans overlaid
mode3channel.pdfpdf mode3channel.pdf manage 96 K 2014-08-29 - 12:57 RichardPrestage mode 3 channel space
mode3spikes.txttxt mode3spikes.txt manage 2 K 2014-08-27 - 20:48 RichardPrestage location of mode 3 spikes above a threshold.
mode4_filt_zoom.pdfpdf mode4_filt_zoom.pdf manage 183 K 2014-08-28 - 11:23 AdamKobelski mode 4, digital filtered, zoomed in for detail
mode4_raw_with_spikes.pdfpdf mode4_raw_with_spikes.pdf manage 174 K 2014-08-28 - 11:22 AdamKobelski Mode 4 data showing spikes in the raw spectra
new_rfi_quant.pngpng new_rfi_quant.png manage 120 K 2014-08-28 - 11:44 AdamKobelski Spikes show up in the same place in different banks
vegasidl.pdfpdf vegasidl.pdf manage 92 K 2014-08-28 - 16:42 RichardPrestage VEGAS spectrum of corresponding data
vegasidlraw.pdfpdf vegasidlraw.pdf manage 110 K 2014-08-28 - 16:42 RichardPrestage VEGAS raw spectrum of corresponding data
Topic revision: r10 - 2015-01-27, RichardPrestage
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