[evlatests] Sensitivity of 3-bit vs. 8-bit
Barry G. Clark
bclark at nrao.edu
Wed May 11 12:21:14 EDT 2011
As I've said before, I consider the phase noise in a single channel
as the gold standard. This method probably doesn't, but might, couple
in spectral anomalies through the calibration process. We are going
for 1% after all.
Anybody got any ideas about the 'noisy' 8 bit digitizers?
> I applied a 'fresh set of eyes' to 3 and 8-bit data taken at
> C-band
> on April 21.
>
> The observations were of 3C147, J0217+7349 (our favorite northern
> 'dot' source), and of a nearby 'noise' field, devoid of strong
> emission. The 3-bit observations were taken first, with all
> appropriate
> levels set correctly (says Vivek, who ran the test). The 8-bit
> observations followed about 45 minute later. I utilized only the
> northern data, so there will be no important change in elevation.
> The data, for both 3 and 8 bits, were taken with 8 subbands of 128
> MHz width each, with 2 MHz channel resolution, spanning 5488 through
> 6512 MHz. Unfortunately, subbands 4 and 5 both have contamination
> from
> the Mangas-Grayhill telecom link (centered at 6004 MHz, 30 MHz wide),
> so
> these subbands were not utilized in the analysis.
>
> The data are of excellent quality. Calibration followed the usual
> path, except that I did not attempt to transfer the gain calibration
> from 3C147. We are interested in ratios between 3 and 8 bits, so I
> merely used the approximately correct flux of 4.1 Jy for J0217+7349.
> Once the levels were set, the 3-bit data showed excellent
> amplitude
> and phase stability.
>
> Examination of the bandpass solutions showed very good flatness,
> except in subband 7, which has (for reasons unknown to me) a strong
> slope of 4 dB over 128 MHz in 15LCP and 22 LCP. Maximum (power)
> differentials within all other subbands are less than 2 dB.
> Curious sharp features, seen in antennas 12 and 22 in subbands 6,
> 7,
> and 8 were found to be due to an *enormous* internal birdie on those
> antennas, which I recall is due to the sampler intself (we had these
> features in the previous design also). Autocorrrelation spectra are
> available for two of the four antennas (12 and 22), so I could see the
> characteristics of this tone. The maximum power density (in radio
> astronomy units) is about 2000 Jy within the 2 MHz channel resolution.
> The tone is unresolved, but is not of perfectly constant frequency --
> it
> is seen to 'wobble' a bit in the 'waterfall' plots. The tone
> frequencies are different for each antenna and IF -- hence we don't
> expect a strong cross-correlation between any pair of antennas.
> But we do see faint imprints of these 'tones' in the
> cross-correlation data, enough to cause the sharp features in the
> bandpass spectra noted above. Some sort of coupling would seem to be
> happening.
>
> It would be good if autocorrelation spectra were made available
> for
> all four antennas, in future tests...
>
> The main purpose of this test was to measure the sensitivity
> degradation between 3 and 8 bit data. To do this, I formed histograms
> of the noise scatter in the imaginary part of the visibility on the
> 'noise' field. Checks with the real part showed excellent agreement
> in
> all cases. I also checked the parallel hands against the crossed
> hands,
> with again excellent agreement.
>
> Below are the 'degradation matrices' for four of the subbands: 2,
> 3, 6, and 8. Subband 1 was not utilized since we know the 8-bit path
> is
> severely degraded. Subbands 4 and 5 have RFI. Subband 7 has the
> strongest internal birdies, and a strong spectral slope, as noted
> above.
>
> For the four chosen subbands, I measured the noise histograms,
> using
> the central 45 channels. All showed gorgeous Gaussian shapes, with no
> outliers or sharp features. The matrices below give the percentage
> degradation of the noise in 3-bit to that in 8-bit modes. A value of
> 5%
> would be considered expected. A value of 10% is a little worrisome.
> RCP is in the upper right, LCP in the lower left.
>
> Subband 2 (5680 MHz,center)
>
> 12 15 22 28
> -------------------------------------
> 12 | X 5 8 3
> 15 | 3 X 7 3
> 22 | 12 12 X 7
> 28 | -2 -3 9 X
>
> Subband 3 (5808 MHz)
>
> 12 15 22 28
> -------------------------------------
> 12 | X 7 4 0
> 15 | 3 X 9 2
> 22 | 6 12 X 8
> 28 | -4 0 1 X
>
> Subband 6 (6192 MHz)
>
> 12 15 22 28
> -------------------------------------
> 12 | X 11 19 13
> 15 | 7 X 9 6
> 22 | 23 23 X 11
> 28 | 5 2 17 X
>
> Subband 8 (6448 MHz)
>
> 12 15 22 28
> -------------------------------------
> 12 | X 3 10 7
> 15 | -1 X 10 -2
> 22 | 18 12 X 4
> 28 | -2 -8 6 X
>
> The overall average degradation 6.5%, which is only slightly
> greater
> than that anticipated. The dispersion of this average is 6.5%.
> It's clear that antenna 22 on the RCP side has an anomalously high
> loss in 3-bit mode. It is also notable that antenna 28, on the RCP
> shows effectively no loss at all -- presumably this is telling us
> something about the 8-bit side.
> Subband 6 shows the highest loss ratios -- this is caused by a
> *decreased* rms noise from the 8-bit side, but I have no easy
> explanation for this. Perhaps something in the calibration.
>
> The strong spectral slope seen in subband 7 on some antennas gives
> an opportunity to see if the noise in the 'spectrally weak' channels
> is
> higher than on the spectrally strong ones. The slope is notably
> strongest for antennas 22 and 15 in LCP, where there is a 3 dB
> difference between channels 12 and 52. The expected trend in the
> noise
> is easily seen on this baseline:
>
> Channel 12: rms = 135 mJy
> Channel 32: rms = 116
> Channel 52: rms = 113.
>
> All other baselines to these two antennas show the effect as well,
> but not as strongly (as expected, since the slopes in antennas 12 and
> 28
> are not as steep).
>
>
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>
More information about the evlatests
mailing list