[evlatests] Results from T304 Attenuator and Requantizer Tests
Keith Morris
kmorris at nrao.edu
Mon Oct 24 10:35:43 EDT 2011
Antenna 5 did not participate in all four tests, and is therefore not a
good basis for comparison. I am working on finding patterns in the
other non-closing (or poorly-closing) baselines.
Rick Perley wrote:
> Michael and Keith ran some tests Friday afternoon, searching for an
> explanation for the closure problems noted on Cygnus A at L-band, and
> for the compression of the calibration (switched power) signal when
> observing Cygnus A.
>
> The working theory is that we are overdriving the digital system at
> L and S bands, in part due to setting the input powers at too high a
> level -- this is due to not taking into account that at L and S bands,
> the input spectrum does not fully occupy the 5 GHz BW. By setting the
> input power level at a value appropriate for full band occupancy, the
> spectral power density within the 1 GHz of actual FE bandwidth is a
> factor of 5 too high at L-band (7 dB). This is no problem to the analog
> circuitry in the wideband parts of the T304, but may be a problem when
> the 1 GHz of desired bandwidth at L-band is selected -- now the power
> level (not just the spectral power density) will be 7dB higher than
> expected. This is the reason why the output T304 attenuators are set to
> quite high levels (typically 25 to 31 dB -- the maximum).
> This might not be a problem since the output attenuators have the
> range to reduce the power level to the desired level -- for cold sky.
> But Cygnus A adds another 6 dB of power, which will exceed the output
> attenuator's capabilities, and may overdrive the station board
> requantizer.
> The same argument applies to S-band, but the effect is now at the 3
> to 4 dB level.
>
> The tests consisted of four parts:
>
> 1) Observe as normal: Set the attenuators on cold sky, and observe
> Cygnus and a calibrator alternately.
>
> 2) Set the atttenuators on Cygnus A, and observe both sources
> alternately, (with no change in attenuator level).
>
> 3) Set the attenuators on cold sky, but at a target level -7dB below
> the standard (to account for the actual spectral occupancy of the input
> signal). (I think this should cause the input T304 attenuator to be 7
> dB higher, but leave the output attenuator unchanged?)
>
> 4) Set the attenuators on cold sky, at the usual level, but adjust
> the stationboard requantizers to prevent overflow when on Cygnus A.
> This level would be used for both calibrator and Cygnus A.
>
> Results:
>
> A) The requantizers are most certainly overflowing when the
> observations are made with the standard setup (experiment 1). The
> requantizer state count distribution shows huge peaks at each end. (The
> sampler state counts looked nicely gaussian).
>
> B) The 'PDif Compression' is reduced in experiments 2, 3, and 4, but
> is far from eliminated. At L-band, when observing Cygnus A, the typical
> PDif Compression is 10 to 20 percent -- nearly all antennas show this.
> (Some antenna-IFs are as high as 50%, a few show very little
> compression). The compression reduction is modest -- the typical
> compression is probably 5 to 15% now.
>
> C) The non-closing effect was determined in the following manner:
> The data -- after basic calibration (but not corrected by the switched
> power -- this doesn't affect closure, and has other issues -- see
> below), I self-calibrated the 1445 MHz data using a 'golden' high
> resolution model made with VLA data in 'ancient times'. I then plotted
> Stokes 'V' -- this turns out to be quite sensitive to correlator based
> problems (provided they are different on the two polarizations). I found:
>
> Expt. 1: Large Stokes V visibilities, with 6 x 17 and
> 14 x 17 having values nearly equal to the total flux. (This is caused
> by the LCP apparently overflowing, giving visibilities near zero).
> Expt. 2: All Stokes V visibilities are less than 10 Jy
> (which is less than 1% of the Stokes I).
> Expt. 3: Large Stokes V found on 17 x 23 and 6 x 17.
> Smaller, but still significant values are found on many other correlators.
> Expt 4: A single antenna -- ea27 -- had modest (10 to
> 30 Jy) values of 'V' on its baselines to 2, 3, 4, 12, 18, 25, and 26 --
> a behavior quite different than any of the other experiments. All other
> baselines showed no 'V' at all.
>
> But, sadly, the situation is not as simple as it seems. The 'V'
> test only is sensitive to closure errors which are different between the
> polarizations. Another test is to subtract the 'golden' model from the
> current data (following self-cal), and looking at the residuals. This
> exercise paints a much darker picture:
>
> Large, non-closing, residuals of up to 100 Jy (about 7% of the
> total flux) are seen on *some* baselines -- only a few, but their
> locations are very odd. The following table lists the antennas and
> baselines, and approximate residual:
> Antennas Baseline Residual
> ---------------------------------------------------
> 1 x 5 w7 x w8 110 Jy
> 1 x 15 w7 x w9 110 Jy
> 5 x 15 w8 x w9 50 Jy
> 3 x 12 e8 x e9 50 Jy
>
> The following baselines have residuals between 30 and 50 Jy:
> 15 x 20 w9 x n9
> 1 x 26 w6 x w7
> 3 x 26 w9 x w6
> 1 x 4 w5 x w7
> 3 x 4 e9 x w5
> 3 x 25 w4 x e9
> 20 x 28 n8 x n9
> 3 x 6 e9 x n3
> --------------------------------------------------------------------------------------
> The distribution of the afflicted antennas is very odd, with a great
> preponderance of antennas on the west arm.
> This is not RFI -- the calibrator scans -- for all setups -- gave
> lovely data.
> This is not due to delay errors -- the analyzed data are from a
> single 2 Mhz channel at the middle of the band. (SNR is not an issue
> for this source!)
> This is not due to an error in the 'golden' image -- a 2nd
> self-calibration, using the image made from the data itself, produced no
> changes to the antenna gains.
>
> *** Summary ***
>
> While we were overdriving the requantizers when observing Cygnus A
> -- this is not the cause of the two central problems:
> a) The compression in the switched power when observing a strong
> source,
> b) The apparent introduction of non-closing errors when
> observing a strong source.
>
>
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--
Keith Morris
National Radio Astronomy Observatory
1003 Lopezville Rd.
Socorro, NM 87801
575-835-7060 (phone)
575-835-7027 (fax)
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