[evlatests] Frequency mislabelling

[Steven T. Myers]

For the L_baseband dataset, in casa I see 8 sub-bands, with the lower edge
of the first sub-band at 988 MHz which seems like what is desired.
Spacing is 128 MHz as expected.  This puts the first channel of first sub-band
centered at 988.25 MHz.

On Tue, 29 Sep 2009, Michael Rupen wrote:

> 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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|:| Steven T. Myers                      |:|  Tenured Astronomer       |:|
|:| National Radio Astronomy Observatory |:|  Ph:  (575) 835-7294      |:|
|:| P.O. Box O, Socorro, NM 87801        |:|  FAX: (575) 835-7027      |:|
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