[evlatests] Oddities at L-band (pointing, Tcal, Tsys, Psum, etc.)
Barry Clark
bclark at nrao.edu
Tue Aug 13 17:12:48 EDT 2013
Here's the soida output for receiver L38 installed in antenna
11. This is for the R side, that is, IF A.
Fsky, 1st LO, 2ndLO, Time, Gain, Trcvr, Tcal
1000 , 1000 , 0 ,45915.4 , +13.0 , 1879.1 , 49.71
1025 , 1025 , 0 ,45989.2 , +16.8 , 747.1 , 14.50
1050 , 1050 , 0 ,46062.9 , +18.0 , 653.6 , 18.89
1075 , 1075 , 0 ,46136.8 , +19.4 , 412.3 , 14.61
1100 , 1100 , 0 ,46152.7 , +39.2 , 71.7 , 6.69
1125 , 1125 , 0 ,46166.2 , +36.7 , 49.7 , 3.52
1150 , 1150 , 0 ,46180.6 , +52.7 , 30.3 , 2.04
1175 , 1175 , 0 ,46195.6 , +56.7 , 22.0 , 1.48
I can't say I blame it for getting a little confused, with the
gain dropping by 40 dB.
On 08/13/2013 02:59 PM, Barry Clark wrote:
> 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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