[evlatests] Temperature-sensitive gains at Ku, K, Ka, and Q bands

Tue Nov 1 11:13:28 EDT 2011

    So far as we can tell, the application of PDif nicely removes these 
temperature-induced gain changes.  So we're not too concerned about 
this.  However -- the tests so far cannot easily discern if the noise 
diode output is varying significantly (defined here as more than ~1%).  
The 'flux density' run of last year provided some evidence that noise 
diode power was varying by about this much ... we'll need to think of a 
good test to give a more definitive judgment. 


Bob Hayward wrote:
> A few words about gain change versus temperature on the EVLA. There 
> seems to be some surprise that we might be seeing gain variation in 
> the signal path in the Vertex Cabin. I'm not surprised at all.
>
> The rule of thumb for a typical amplifier is that it will have a 
> temperature coefficient of -0.012 db / deg C / stage. Most of our amps 
> are 3-stage devices so a 1 deg C change in temperature will cause a 
> -0.036 db change in gain. Thus a 10 deg change (to use a nice round 
> number) from 15 to 25 C will cause a drop in gain of -0.36 dB (i.e., 8%).
>
> Looking at the data from last winter of the temperature sensors built 
> into each of the room temperature RF Boxes on our receivers, we saw up 
> to a 10C temperature between day to night. We also saw a 20 C change 
> from the hot of summer to the cold of winter. We suspect that not all 
> of this change is due to the ambient temperature inside the Vertex 
> Cabin. Much of it may be due to the fact that the receivers are 
> hanging off a big metal feed (or feed tower) that sticks up through 
> the roof, thus forming a nice heat pipe.
>
> The amplifiers in the LO/IF rack will see much less of a temperature 
> change - perhaps a degree or two at most - thanks to conditioned air 
> inside the enclosure. The UX Converter, on the other hand, is much 
> less regulated, and it is hanging off of the wall of the hut.
>
> So a 10 deg change in the receiver's warm RF path would give us a 0.36 
> dB gain change from its post-amps. On the UX Converter, there are 1 or 
> 2 amplifiers depending whether the signal is going through the direct 
> of converted path. If it only saw half of the 10 deg temperature 
> change that the receiver experiences, there would be another 0.18 to 
> 0.36 dB change in gain. Thanks to Murphy's Law, these will add, so 
> overall we would see a total change in gain of 0.54 to 0.72 dB, or 13 
> to 18%.
>
> All 8 of the receivers will have roughly a similar gain change but the 
> high frequency receivers will be slightly worse because of the added 
> contribution from the UX Converter.
>
> It should be noted that this performance is no different than what we 
> had on the old VLA system and it is exactly why we use switched power 
> to compensate for gain changes. Our real problem is that the 
> temperature change that we are seeing in the receivers is bigger than 
> we had expected (thanks to the heat pipe effect) so the output power 
> of our noise diodes may be changing more that we would like.
>
> More tests are planned for this winter...
>
> -Bob
>
>
> Jim Jackson wrote:
>> I just looked - the temperature inside one UX converter cycled over 
>> only about 5C over a 24 hr period.  I'll have to talk to the guys 
>> tomorrow about whether that could explain anything like this - I'm 
>> skeptical that it could.
>>
>> Jim
>>
>>
>> At 02:04 PM 10/31/2011, Rick Perley wrote:
>>>     The elevation-gain test taken early Saturday morning strongly
>>> supports Michiel's evidence for temperature-sensitive gains -- and at
>>> all the high frequency bands (Ku, K, Ka, and Q).
>>>
>>>     A strong change in PDif was noted in the tests taken the preceding
>>> week -- of a source setting from 82 degrees elevation to elevation 8.
>>> All high frequency PDif values *increased*, by typically 10%, during
>>> this period.  It was difficult to be sure that the visibilities changed
>>> by a similar amount, since the test was of a single source setting
>>> during the period.
>>>
>>>     The data taken Saturday morning were of a source which was 
>>> rising at
>>> midnight, and transited at dawn.  Temperature effects were minimized,
>>> while the elevation vs time was reversed (i.e., the source was rising,
>>> rather than setting).  The PDif values for this run rise during the run
>>> -- but by a much smaller amount than the previous week's test.
>>>
>>>     We can rule out an elevation dependency of the PDif.  And we can
>>> strongly favor a temperature-dependent gain, since the vertex room
>>> temperatures should considerably lag the external air temperature.  
>>> Both
>>> of my runs were taken when the air temperature was declining (the first
>>> one strongly so, the second one only slightly), while Michiel's test --
>>> which showed the opposite variation in PDif -- was taken with strong
>>> heating.
>>>
>>>     Application of the PDif values gives an elevation-dependence of the
>>> gains which appears to be the same for both of my runs -- this
>>> strengthens an explanation that the PDif changes are due to real gain
>>> variations, and not due to a changing value of the noise diode power.
>>>
>>>     Rick
>>>
>>> -------- Original Message --------
>>> Subject:        [evlatests] Q-band holography d.d. 2011-10-29
>>> Date:   Mon, 31 Oct 2011 12:37:02 -0600
>>> From:   Michiel Brentjens <mbrentje at nrao.edu>
>>> To:     EVLA Tests <evlatests at nrao.edu>
>>>
>>>
>>>
>>> ** [2011-10-31 Mon 12:11] Q-band holography d.d. 2011-10-29
>>>
>>> Temperature sensitive gains?
>>> ----------------------------
>>>
>>> A 3C273 Q-band holography observation from past Saturday shows a
>>> systematic decrease of PDIF over the course of six hours in virtually
>>> all antennas. This *decrease* in accompanied by a similar slow,
>>> systematic *decrease* in visibility amplitude. Typically, PDIF
>>> decreases by 20%. The visibility amplitudes also show a 20% decrease,
>>> although they are much noisier than the PDIFs.
>>>
>>> The source had a fairly constant elevation between 36 and 57
>>> degrees. The observation began at 08:00 MDT and ended at 14:00
>>> MDT. Are we seeing a temperature dependent Q-band gain here? Notable
>>> exception is IF 3C, which shows nicely constant PDIFs, yet also
>>> decreases its amplitudes along with all others.
>>>
>>>
>>> Other phenomena
>>> ---------------
>>>
>>> For some reason, 11R lost all signal around 16:50 UTC. After 17:30
>>> UTC, the PDIFs of 20L become increasingly more noisy, until they
>>> suddenly snap together at a higher value around 19:05 UTC. 12A PDIFs
>>> are consistent with noise. 4L PDIFs behave nicely until 17:00, at
>>> which point they increase by 50% and begin behaving erratically.
>>>
>>> - Michiel
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