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

[Bob Hayward]

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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