[mmaimcal] Re: [alma-sw-ssr] question about frequency/band switching speeds and rates

[Bryan Butler]

if i understand this correctly, the only reason to do it this way is 
that it is hard to synchronize the changes (to LO and correlator) that 
quickly (i don't follow the fringe frequency argument).  otherwise why 
would you go to the bother and not do the simpler (conceptually, and 
probably in software) discrete phase + delay center changes?  but i'm 
probably missing some nuance...

and, yes, this was some time ago - ALMA is starting to get a bit long in 
the tooth, and many of the old stablemasters have gone to greener 
pastures ;)...

	-bryan


On 2/4/05 10:12, R. Lucas wrote:
> Darrel:
> 
> This is what we have in the OTF interferometry use case:
> 
> ""2. The scheme currenly envisioned for phase tracking is the following 
> (D. Emerson):
> 
> - We apply, on one of the LO's, the phase and fringe frequency needed to 
> track the center of the antenna beam (taken at the center of the 
> integration period),
> 
> - an additional phase, constant during the integration, as needed to 
> obtain a continuous phase at the beginning of the integration.
> 
> - this phase value is kept and later subtracted from that of 
> visibilities obtained during that integration. This scheme should not 
> unneededly restrain the  final data rate.""
> 
> I think this resulted of a discussion with Larry and yourself at the 
> time. We always phase-track a fixed position in the sky, corresponding 
> to the center of the primary beam at mid-integration, but make sure 
> there is no phase jump at the end of each integration.
> 
> Cheers
> 
> Robert
> 
> 
> Darrel Emerson wrote:
> 
>> I believe Robert Lucas is working on the details, but I don't think
>> we want to track the phase centers as such as they move across
>> the sky with the primary beam in OTF interferometry.
>> If you do that, you end up with artificially high fringe rates that
>> are too high to sample - Larry pointed this out.  However, if
>> you keep the phase center more or less fixed, but track the fringe
>> frequency rather than its absolute phase, that's a more slowly
>> changing quantity across the sky, & it becomes manageable.
>> Of course you do need to track the delay center with the primary
>> beam, but not necessarily the phase center.
>>
>>   There was some discussion on this issue a couple of years back.
> 
> 
> I think that was rather a couple of couples of years back.
> 
> 
>>
>>              Cheers,
>>                    Darrel.
>>
>> Bryan Butler wrote:
>>
>>>
>>>
>>> On 2/3/05 22:45, Bryan Butler wrote:
>>>
>>>>
>>>> . OTF mosaic cell phase-center saltation: rate=100 usec; speed=10
>>>>   usec.
>>>>   [for the rate, i've assumed slewing at 1 deg/sec, and 950 GHz,
>>>>   with 2 samples per FWHM; speed is a guess - it will depend on
>>>>   how fast the correlator can reset delays]
>>>
>>>
>>>
>>>
>>> OK - got this one totally wrong.  slew rate for OTF interferometry is 
>>> 3'/sec, not 1d/sec, which corresponds to 15 msec required 
>>> reconfiguration speed if the phase centers are changed discretely as 
>>> the antennas slew across the source...  but it's not clear this can 
>>> be accomplished if the correlator phase center can only be updated on 
>>> 48 msec boundaries?
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>>
>>
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> 
>