[evlatests] L-band passbands explained!

[Rick Perley]

    I circulated a report on Sunday, noting that the remarkable 
improvement in L-band passband shapes is entirely due to the replacement 
of the old (narrowband) receivers with new ones, and asking for an 
explanation of why the band-edge rolloff, so prominently visible in S, 
C, and X band observation (with the 8-bit system) is absent from the 
L-band data. 

    Keith and Emmanuel have provided me the explanation:

    The essence is in the use of the 64-MHz-wide spectral windows in the 
default setup for L-band. 

    There are two obvious ways to obtain a full 1024 MHz-wide coverage 
at L-band:

    1)  Utilize only one IF pair (say, A and C), and tune this to the 
988 to 2012 MHz passband.   There would be eight 128 MHz-wide spectral 
windows, with 2 MHz spectral resolution for full polarization. 

    2) Utilize both IF pairs, tuning the first to (say) 988 to 1500 MHz, 
and the second to (say) 1500 to 2012 MHz.  In this case we get 64 
MHz-wide spectral windows with 1 MHz spectral resolution and full 
polarization. 

    Simply from spectral resolution arguments alone, the second option 
is preferred.  But there is another benefit (which I had forgotten):  
The correlator is able to utilize its spectral window formation 
capabilities to place the eight 64 MHz-wide spectral windows anywhere 
within the 1024 MHz-wide input IF.  So, in the new default tuning 
arrangements:

    IFs A and C are tuned to 802 to 1826 MHz (center of 1308 MHz).  The 
correlator's first stage filter forms eight 128-MHz-wide subbands, from 
which the eight 64 MHz-wide spectral windows are selected.  The low-end 
rolloff is avoided as the eight narrow windows are taken from 2nd 
through 6th of the original subbands.  
    IFs B and D are tuned to 1170 through 2194 MHz.  The eight 
64-MHz-wide spectral windows are selected from the third through seventh 
of the original 128-MHz-wide subbands, thus avoiding the low-frequency 
rolloff, which is in the lowest two subbands. 

    Emmanuel reminds me that this tuning arrangement was recommended in 
his Memo #159.