[evlatests] Oddities at L-band (pointing, Tcal, Tsys, Psum, etc.)

[Barry Clark]

There is no such thing as an A/C Psum.  There is an A Psum and
a C Psum.  I guess you are adding them together, which is OK
if they are both reasonable.  But they might not be.  In any event,
it looks like a cable/detector problem.  In which case, the
same antennas should show up the same way at all bands.

Tcals may not care about a severe receiver rolloff, but the soida
rack certainly does.  It gets confused and sputters nonsense.

On 08/13/2013 02:24 PM, Rick Perley wrote:
>      Answers to Barry's questions:
>
> Barry Clark wrote:
>> On 08/13/2013 10:05 AM, Rick Perley wrote:
>>
>> [snip]
>>
>>
>>>       3) I used a central subband (#8 -- 1480 MHz, nice and stable) to
>>> look at the PSum values.  They should all be reasonably close to each
>>> other if 'set and remember' is working right.   I'm prepared to accept
>>> that 'reasonably close' means with 50% of some median value.  But this
>>> is not the case.  The median value for the subband appears to be about
>>> 10 counts.  In LCP, all antennas are within 50% of this.  But in RCP,
>>> there is a very different story:  ea02R, ea07R and ea13R are all at 1
>>> count, or less!  That's a factor of 10 too low!!!
>>>
>>
>> What do the Psums in the other subbands do?  If there is a lot
>> of interference in some subband, you could have big numbers there
>> that make the average Psum sensible.  If not, it could be that
>> the wire between the T304 detector and the sampler has a lot of
>> attenuation.  Do both polarizations do this?
>>
>> [snip]
>>
>      They all behave in the manner described above.  A quantitative
> example for ea02 (almost exactly the same values are seen in ea07.  ea13
> is discrepant only in the A/C side -- B/D looks o.k.).
>
>           Subband#       AC PSum           BD PSum
>               1                      .41
> .25          (band edge attenuation)
>               2                      .65                        12
>               3                      1.1                         8.2
>               4                      .95                         8.2
>               5                      .82                         8.5
>               6                      .61                         7.6
>               7                      .58                         5.9
>               8                      .73                          7.1
>               9                      2.2                          20
> (RFI Zone of Death!)
>              10                      3                            23
>              11                      2.5                         15.7
>              12                      1.5                         10.3
>              13                      1.3                          9.5
>              14                      1.0                         8.0
>              15                      .83                         5.9
>              16                      .47                         3.7
> ----------------------------------------------------------------------
>
>      So, there is no evidence from these values, nor from the spectra,
> that there is any signal in the RCP side which would have caused the
> levels within the subbands to be as low as they are.
>
>      This problem seems only to be in the RCP side.  All LCP values are
> within 'norms' (+/- 50% of the median)
>
>
>>
>>
>>>       5) In perusing the Tcal tables, some curiosities are found:
>>>
>>>       a) ea07 has a Tcal about 8K -- 5 times higher than any other
>>> antenna!  Is this right?
>>>
>>
>> Right or not, that's what the Soida documents say.
>>
>      Righto.  I do note that the visibilities from this antenna (both
> polarizations) are about as expected, and the Tsys values, although
> higher than normal , are in line with the observed amplitudes.  (In
> other words, the listed Soida documents are in keeping with the data.)
> This raises the question of why is this receiver so different  than the
> others ...?
>>
>>>       b) ea11, ea14 and ea17 all have incredibly high Tcal values listed
>>> for the first two subbands (values exceeding 55K for ea11!!!), but
>>> normal (1.5K) values for all other subbands.  Can this be real?  I
>>> didn't think the noise diodes could have output powers 30 times higher
>>> than the mean in the bottom 200 MHz of the band...
>>>
>>
>> This is due to the (ahem) feature that the Tcal stowed in the SDM
>> is the interpolation to the subband reference frequency (lower
>> edge) combined with the fact that the bandpasses of these receivers
>> appear to go really whacky below about 1150 MHz.
>>
>      The Tcal values shouldn't know or care what the bandpass looks
> like.  While it is true that these antennas do have a significant
> low-frequency rolloff in their spectra, there are others which decline
> similarly, but without the spectacular (and unphsysical) rise in Tcal in
> the lowest subbands.
>      I'll be keen to see how the improved interpolation scheme works.
>
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