Subject: Re: some GBPP progress
Date: Thu, 25 Jul 2002 14:08:57 -0700 (PDT)
From: don backer 415 campbell 510-642-5128 <dbacker@astron.berkeley.edu>
To: dbacker@astron.berkeley.edu, fghigo@nrao.edu
CC: zaven@milkyway.gsfc.nasa.gov

On the Mysteries of RFIF.TBL:

Yes, you understand correctly - change 1790 to 1810
leaving all else untouched to go from 1390 center
to 1410 center.

There are 4 frequencies on the rf/if lines (which
are duplicated for the two polarizations, but my
recollection is that only the first is used.

The first is what we call the "signed sum of LOs" in
VLBI days prior to last analog IF/video conversion
done in GBPP. The next 3, in absolute value, are
the lower edge, "selected center" and upper edge
of IF passband. When frequency is inverted
coming into GBPP these are negative.

With GBPP we have hardwired the IF center frequency
in software to be 400 MHz for GBT observing to
remove one degreee of freedom from an already
complex microwave system.

The six analog LOs are placed symmetrically
around the IF center such that there are
48 channels below and 48 channels above the
IF center. As stated in previous email, we
mostly use the "p"acked channel spacing. FYI,
the "s"pread configuration maximally spaces
the LOs/blocks of 16 channels over the available
IF passband, and thus the reason for providing
the low and high sides of what is available.
And further, if available band is less than
what the 96channels need, there is an
"o"verlap mode.

C'est tout...my last days in France,

Don