[evlatests] Sensitivity of 3-bit vs. 8-bit

[Rick Perley]

    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).