[evlatests] Using S-band is Tough!

[R. Craig Walker]

Just a reminder from the VLBA for which S band was one of the original
receivers.  We have been living with XM/Sirius since they were launched. 
They are often the whipping boy whenever there are problems with the S
band receivers.  But I have never been convinced that we have actually had
problems with those signals as long as we don't try to observe on top of
their frequencies - as long as we tune the BBCs away from them.  However,
at 8 of the 10 sites we have had to put in band limiting filters to only
allow observations between 2.2 and 2.4 GHz (without the filters, the
receiver is sensitive at some level to 1920 to 2840 MHz).  Those filters
cut out-of-band RFI that did cause problems, probably intermod products,
in all observing frequencies we can reach.  With those filters, the S band
signals are reasonably clean.  Note that XM/Sirius is right in the middle
of those filters so they are not attenuated at all.   There are much worse
signals out there than XM/Sirius so don't assume your troubles will be
over if you block them.  One possible hopeful point for the EVLA is that
the two sites that have not needed the 2.2-2.4 GHz filters are Pie Town
and Fort Davis.  Perhaps the ELVA site is also reasonably free of the
offending signals.

Cheers,

Craig





> Rick Perley wrote:
>>     Jim:
>>
>>     We'll need filters in two IFs (like, A and C), for at least a couple
>> of antennas.  I don't think we need as many as four (we're not checking
>> closure here).
>>
>>     Anybody else with an opinion?
>>
>>
>    If I understand correctly XM/Sirius affects all declinations, so it
> makes sense to block it out. However, the downlinks only affect 0 dec so
> we probably don't want to filter them out with hard wired filters. Is
> that what is planned ?
>
> ---Frazer
>>
>>
>> Jim Jackson wrote:
>>
>>> Rick,
>>>
>>> Rob has quotes coming in this week for filters to block out XM/Sirius
>>> radio. As soon as we get them all in, we'll choose a vendor and place
>>> an order for some filters to test.  I imagine there is a typical 8-12
>>> week delivery time which is typical of custom RF filters.  Just to
>>> make sure were getting the right amount, how many antennas do we need
>>> to equip with these filters as a valid test?
>>>
>>> Cheers,
>>> Jim
>>>
>>>
>>>
>>>
>>>
>>>
>>> At 02:03 PM 7/1/2011, Rick Perley wrote:
>>>
>>>>     I've finally gotten around to trying to properly calibrate the
>>>> S-band data from the Hercules A 'demo' run, from C-configuration --
>>>> taken last December.
>>>>
>>>>     It ain't easy ...
>>>>
>>>>     The major problem is RFI, the majority of which is from
>>>> geostationary satellites.  Because Herc A is near zero declination, it
>>>> essentially travels right along the equatorial belt, passing by these
>>>> satellites in turn.  Six of the 16 subbands are essentially unusable
>>>> due
>>>> to this -- subbands 2 through 5, and 13 through 16.   The first three
>>>> are taken out by XM radio, plus the emitter at 2190 MHz (whose origin
>>>> I
>>>> forget -- but it most clearly is a geostationary satellite), the
>>>> latter
>>>> three are taken out by TV downlinks.
>>>>
>>>>     The emission is so strong in subbands 2 through 5 that it causes
>>>> the
>>>> accumulators to overflow, which then completely trashes the entire
>>>> subband.  It would be useful, when we can find the time, to utilize
>>>> the
>>>> flag which is aware of accumulator overflow, so these ruined spectra
>>>> never make it to the user.
>>>>
>>>>     An interesting question is whether we should be changing the
>>>> requantizer gains to accommodate the extra power in these subbands.  I
>>>> think the answer is 'no', but we should probably discuss the issue at
>>>> some point.
>>>>
>>>>     On the brighter side, the calibrator observations on 3C286 and
>>>> 3C48
>>>> were not seriously affected by any of these emissions -- the antenna's
>>>> forward gain is sufficient to suppress these transmission.  So
>>>> observing
>>>> in the north will generally work.  The specific frequencies allocated
>>>> to
>>>> the emission in subbands 2 through 4 may be unuseable, but the
>>>> adjacent
>>>> channels will be fine.  What we don't know is how close to the
>>>> satellite
>>>> we can point, and still get useable data.
>>>>
>>>>
>>>>
>>>>
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>>>>
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    R. Craig Walker            Array Operations Center
    cwalker at nrao.edu           National Radio Astronomy Observatory
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