[evlatests] Oddities at L-band (pointing, Tcal, Tsys, Psum, etc.)
Rick Perley
rperley at nrao.edu
Tue Aug 13 17:18:57 EDT 2013
Interesting. I've generated the (power) bandpasses for all
antennas. ea11 shows a decline in its bandpass by about 12 dB between
1020 and 1200 MHz -- far less than indicated below. But I used a tuning
to set the bandpass edges at 1009 to 2033 MHz, which seems to help to
get rid of the big low-frequency roll-off. With this tuning, some
antennas show almost no low-frequency roll-off at all -- less than -5 dB.
Barry Clark wrote:
> 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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