[evlatests] L-band passbands explained!

[Emmanuel Momjian]

On Mon, Aug 26, 2013 at 1:33 PM, Rick Perley <rperley at nrao.edu> wrote:

>     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.
>
>
>

a very minor correction: this tuning arrangement was recommended in Memo
#154.
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