[evlatests] Frequency mislabelling

[Michael Rupen]

George/Steve -- could one of you look at this data set in CASA and confirm
the frequency offset?  I'd like to trace this back as far as possible.


This is

      L_baseband_000.55102.80582284722

The script tunes the lower baseband edge to 988.0 MHz.

The subbands should then cover
   988.0 to 988.0+128.0= 1116.0 MHz
  1116.0 to 1244 MHz
etc.

We have 256 channels per subband.  I claim this means that each channel
is 128 MHz/256 channels= 0.5 MHz wide, so the channels should be centered on
XXX.25 and XXX.75 MHz -- i.e., the AIPS labels seem correct.  Radars
emmitting at XXX.00 MHz should be halfway between two channels.

Subband central frequencies however should be (e.g.) 988+128/2= 1052.0 MHz
etc., so *if* the central frequencies in the AIPS header are supposed
to correspond to the center of the subband (rather than e.g. a channel,
as I suspect), those frequencies would be in error.  I suspect this is
a mis-interpretation of the AIPS header however.

Ken, would you check me on the above?

            Michael


>    Michael took a full 1 GHz spectrum at L-band, using 8 subands of 128
> MHz each, a day or so ago.  This allows me to check if the frequencies
> are right, as we have many 'beacons' in that passband.
>
>    It's very clear that the frequencies, as reported by AIPS, are off
> by one half of one channel:  The radars at 1030, 1090, 1310, and 1330
> MHz show up as 1030.25, 1090.25, ...  MHz
>
>    Looking at the header, I note that the central frequencies of the
> subbands are listed as:
>
>    1052.25, 1180.25, 1308.25 ... MHz.
>
>    Presumably, all of these are too high by 250 kHz.
>
>
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