[evlatests] A most spectacular failure of 'set-and-remember'?

Tue Jan 14 12:36:09 EST 2014

    Impossible perhaps, but at least plausible ...

    Can a mechanism be found, or at least proposed, by which the times 
at which the correct attenuator levels are issued be spread around by 
more than one hour?  That's the time span over which the various 
antennas got to their more-or-less correct gain levels ...

    The run started at 12:12.  The first six antennas to jump gain were 
all clustered near 12:48.  The rest of them followed about an hour later. 

    Weird, eh? 


Barry Clark wrote:
> The way SETANDREMEMBER works is that a SETLEVEL command is sent,
> timed a few seconds after the beginning of the the first scan using
> the setup.  The few seconds is to allow the subreflector to stop
> moving.  The phenomena are explained if this SETLEVEL command
> is instead issued or interpreted for that time plus some 2500
> seconds.  As impossible as this is, it seems to me less impossible
> than the other suggestions.
>
> On 01/14/2014 09:26 AM, Rick Perley wrote:
>   
>>      Sirius/XM is *not* the origin of this problem.  To repair the
>> damage, I have to apply the switched power.  Review of these data show
>> the power levels to be very stable in most subbands.  Notably, it is
>> well behaved even in subband 3, which is where the Sirius/XM signal
>> resides.  These observations were at a declination of about +20 -- far
>> from the danger zones.
>>
>>      Regarding the A and C change differences -- for most antennas, the
>> changes for the two polarizations are within a few seconds of each
>> other.  Definitely different, but close.
>>
>> Keith Morris wrote:
>>     
>>> On ea18, I see A and C changing at the same time, at least to within
>>> the resolution of the monitor data archive.  However, the attenuators
>>> change from a suspicious 29 - 31 dB to a more reasonable 16dB.
>>> Somehow they knew or later determined the correct value.
>>>
>>> We don't archive LO frequencies, so it's impossible for me to know how
>>> the synthesizers are set.  It could also be the L301.  Recall that at
>>> S-band the L301 signal can contribute to the total power seen at the
>>> T304.  If the L301 signal were tuned in-band to the output detector,
>>> this may force a higher attenuator value.
>>>
>>> Alternatively, I have seen S-band observations where the Sirius/XM
>>> signal appears in the AC tuning, and not in the BD tuning.  This can
>>> cause a difference in power of about 12-15dB, which is about the step
>>> size seen in your observation.
>>>
>>>
>>>
>>>
>>> On 1/14/2014 8:35 AM, Rick Perley wrote:
>>>       
>>>>      More information:
>>>>
>>>>      Remarkably, the IF 'A' jump times are *not* the same as the IF 'C'
>>>> jump times.  Won't this invalidate the L302 theory?  In any event, if
>>>> the L302 was wrong, there should be no discernible fringes during the
>>>> time it was wrong.  But the fringes are strong and stable.
>>>>      In no cases were the jump times aligned with source changes.  They
>>>> all occurred in the middle of scans.
>>>>      There is no change in the requantizer gains, so the correlator is
>>>> exonerated.
>>>>
>>>> Keith Morris wrote:
>>>>         
>>>>> ea18 and ea19 both show the same behavior in the T304-a: the input
>>>>> power and input attenuator were constant over the jump (at the nominal
>>>>> values) and the output attenuator and power changed, to their nominal
>>>>> values. This (and the AC-dependence) implies that the L302 was wrong,
>>>>> and became correct, since the output detector is post-conversion.
>>>>>
>>>>> I can look into it further tomorrow.
>>>>>
>>>>>
>>>>>
>>>>> On 1/13/2014 5:17 PM, Rick Perley wrote:
>>>>>           
>>>>>>       I'm calibrating 20 S-band databases, taken from November 2013
>>>>>> through January 2014.    Five have been completed without any troubles
>>>>>> (other than the usual satellite RFI).  But one is 'special':
>>>>>>
>>>>>>       For this database, the gains of all antennas jumped up, by a
>>>>>> factor
>>>>>> of ~ 2 to 10, at times roughly in the middle of the 90-minute run.
>>>>>> But ...
>>>>>>
>>>>>>       1) Only the A and C IFs jumped.
>>>>>>       2) Each antenna jumped gain at a different time.
>>>>>>       3) The new gain was in fact the correct one (judging from the
>>>>>> calibrated gain values and PSum values).  Hence, *the initial power
>>>>>> setup was too low by factors of a few*.  Somehow, this error was
>>>>>> corrected in the middle of the run.
>>>>>>
>>>>>>       This SB was under program 13B-316.  The data were taken on 06 Dec
>>>>>> 2013.
>>>>>>
>>>>>>       The time patterns of the jumps are curious:
>>>>>>
>>>>>>       1) Antennas 18, 19, 20, 22, 23 and 28 all jumped between 12:47
>>>>>> and
>>>>>> 12:51 (IAT).  All jumps were 'clean' -- no more than one intermediate
>>>>>> value is seen.
>>>>>>       2) All other antennas jumped between 13:20 and 13:45.  Some of
>>>>>> these
>>>>>> jumps were not clean -- the system oscillated between the two
>>>>>> states, or
>>>>>> (in one case) hopped around three states, before stabilizing.
>>>>>>
>>>>>>       The initial scan for this run was many minutes long -- more than
>>>>>> enough time for the system to find the 'right' level, and keep it.
>>>>>> But
>>>>>> it didn't, and it didn't ...
>>>>>>
>>>>>>       Any ideas?
>>>>>>
>>>>>>
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>>>>>>
>>>>>>             
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