[evlatests] Aliasing in narrow-BW observations

[Rick Perley]

    Miller Goss ran a spectral line test last Thursday which has 
revealed an heretofore unsuspected problem which has significant 
consequences for 'transition' observations. 

    In his test, Miller observed in a very narrow bandwidth -- 780 kHz, 
in mode '2AC', producing 256 spectral channels for each of the two 
correlations.  After generating the bandpasses (which looked entirely 
normal),  he soon found that all, and only, the EVLA-EVLA baselines had 
large offsets in their bandpass-corrected visibility amplitudes.  
'Large' here means of order 20% or more.  VLA to VLA baselines were 
fine, as were EVLA to VLA baselines. 

    There are only two ways that I could imagine this happening -- 
either there is RFI in EVLA antennas, which only correlates between 
them, or there is some lower level broadband signal which is present in 
EVLA antennas only. 

    The first hypothesis was easily dismissed, as all the spectra are 
clean.  There is no RFI. 

    For the 2nd hypothesis to be viable, it has to only affect narrow 
bandwidth observations, as numerous (and nearly innumerable) tests and 
observations have been made at wider bandwidths, without any hint of an 
EVLA-EVLA extraneous correlation.  From this consideration, it was 
suggested that perhaps we are seeing some aliasing around the baseband 
end, which is present only in EVLA antennas, and which is significant 
only in the bottom ~500 kHz of so of the passband. 

    A test for this was executed this afternoon, and has clearly shown 
that this is the origin of the large offsets seen in Miller's data.  In 
this test, I observed a strong calibrator in mode '1A', at BW = 3 MHz.  
(Other BW were observed also -- and some other problems were found -- to 
be reported on later).  There are then 256 channels across the spectrum, 
for  12 kHz resolution.  Good-looking bandpass solutions were found.   
Bandpass-calibrated correlator spectra were then generated, which 
clearly shows the extent of the problem:
       Extending up from baseband, for about 700 kHz, there is a large 
(up to a factor of two!) additional correlation in the signal.  Both 
amplitude and phase are affected, but the effect is much stronger in 
amplitude.  Both 'positive' and 'negative' additions are seen.  Beyond 
about 1 MHz from the baseband end, the effect is no longer detectable in 
any of the individual spectra. 

    This is a serious 'transition' issue.  The effect does not originate 
in the EVLA's antenna or DTS electronics, nor in the VLA's baseband 
filters.  It must be due to aliasing in the digital-to-analog conversion 
system we use to enable EVLA signals to be used for science observing 
(and tests!).   Unlike most 'transition' issues, this one will not 
lessen in time -- it grows with the number of EVLA to EVLA pairs we have 
-- and will not be eliminated until the new correlator arrives. 
    I believe the effect is time-variable (based on a small amount of 
data), so a simple 'closure' correction will not be easy, and perhaps 
not viable at all.  It is perhaps important to measure this variability, 
to see if it exists, and what its characteristics are. 
    This aliasing effect is small for wide-band continuum observations, 
and for relatively wideband spectral line observations, one can always 
choose to remove the affected channels on EVLA-EVLA baselines. 
    The effect is quite serious for narrow BW spectral observations -- 
anything taken at BW less than ~3 MHz will be notably degraded.