[evlatests] switched power issues

[Frazer Owen]

On 5/26/16 11:39 AM, Barry Clark wrote:
> That was a bad suggestion.  Rick sees an actual decrease in pdiff,
> not just a change of slope.  But, another question.  Do we need to
> worry about the receiver noise from the P band receiver leaking into
> S and raising the Tsys?
We did tests when we first put up some Lowband receivers to see if we 
could detect any changes in the stress tests and saw nothing.
>
> On 05/26/2016 09:28 AM, Barry Clark wrote:
>> Isn't this Rick's "non-linearity"?  And why it doesn't show up
>> at P band?
>>
>> On 05/26/2016 09:10 AM, Paul Demorest wrote:
>>> On 2016-05-25 12:28, Paul Demorest wrote:
>>>> On 2016-05-25 11:36, Paul Harden wrote:
>>>>> On 5/20/2016 3:49 PM, Paul Demorest wrote:
>>>>>> the short story is that I think the practice of leaving all receiver
>>>>>> cals firing at all times is maybe not so good; this may be causing a
>>>>>> small gain modulation at the cal switching frequency; we should
>>>>>> change this default behavior, and test what effect this has on the
>>>>>> Pdif compression issue.
>>>>>
>>>>> The results are interesting and fairly convincing.  It seems the next
>>>>> thing to do would be to cycle CAL on and off through the different
>>>>> receivers to see if a particular band receiver is the culprit, or a
>>>>> global effect.
>>>>>
>>>>> More to the point (thinking out loud), I have always wondered about
>>>>> the low band receiver.  The Tcal injection on the microwave receivers
>>>>> is in the order of 2-3K and confined within the RF components (i.e.,
>>>>> the cryo dewar serves as a nice shield); the low band receiver 
>>>>> Tcal is
>>>>> 20-30K, or 10dB higher.  This power is injected into the noise 
>>>>> coupler
>>>>> *before* the LNAs, thus little isolation between the LBR receiver
>>>>> inputs and the MJPs (74 MHz) or the P-band dipoles.  This 20-30K Tcal
>>>>> power may well be imposed back to the dipoles, which would be a 
>>>>> fairly
>>>>> efficient radiator located just underneath the subreflector and a
>>>>> radiator above the feed horns (in fact, could be a "double whammy")
>>>>>
>>>>> I have never figured out how to measure if the low band CAL switching
>>>>> power is being radiated by the dipoles into the microwave feeds.  You
>>>>> can see Tcal switching (about 1-2 dB jumps) on the 74 MHz low band
>>>>> outputs and about 0.5dB jumps on P-band.  Therefore, I would 
>>>>> recommend
>>>>> repeating your test with the CAL switching to the low band receiver
>>>>> turned off first.  Note that the LBR CAL switching is turned off via
>>>>> the new F318 module, not the F317s like the microwave receivers.  The
>>>>> operator knows how to turn off the low band CAL.  I know from my
>>>>> visits to antennas for low band work, the CAL switching to the LBR is
>>>>> almost always on.  I'm certainly curious myself if the low band
>>>>> dipoles are radiating Tcal into other receivers and if you can detect
>>>>> this.  If so, you have discovered a nasty lingering problem, and best
>>>>> of all ... a very easy fix!
>>>>
>>>> Thanks for bringing up the low-band cals, I neglected to describe this
>>>> in the last message.. when I did this last week, I did try turning
>>>> them off and I do see the effect you mention.  The S-band Pdif values
>>>> drop by about ~5% (or even more in some cases) when the LBR cals are
>>>> off.  In constrast with the other effect I described, this does _not_
>>>> appear to have any effect on gains.. ie, it's not detectable in the
>>>> cross-correlations, only in Pdif and autocorrelations.
>>>>
>>>> Doing this test is a little tricky because of the way the executor
>>>> controls the cal devices.  The cal setting selected for the main
>>>> receiver in use is always also sent to the F318, so it's not easy to
>>>> control the state of the LBR cals separately.  Sending a low-level
>>>> command to the F318 works temporarily (for the current scan) but it
>>>> gets reverted on the next scan when the config commands are re-sent.
>>>> Does anyone know a way around this?
>>>
>>> Here is a plot from this test observation showing Pdif values vs time
>>> for ea22 (same antenna I showed before).  These numbers are straight 
>>> out
>>> of the SysPower table.  The dashed red lines show times when I sent
>>> commands to turn cals off or on, and the dotted line show scan
>>> boundaries.  The S-band cals were left on the entire time.  At the 
>>> start
>>> and end of the plot all cals are on (usual mode).  This illustrates a
>>> few things:
>>>
>>> 1. When the cals are turned off the measured S-band Pdif drops by ~5%.
>>>
>>> 2. The LBR cals turn themselves back on at the next scan boundary, but
>>> all others stay off.  In this state the Pdif is a few percent higher
>>> than when all cals are on.  This seems consistent with the gain
>>> modulation and resulting apparent Pdif compression that I described
>>> before.
>>>
>>> 3. When all cals are turned back on (third red line) things go back to
>>> the previous state.
>>>
>>> In this test, there was a single subband in each IF, with identical
>>> tuning.  So we expect IFs A and B (also C/D) to look the same.  This
>>> does seem to be true aside from a small difference in the overall power
>>> level.
>>>
>>> -Paul
>>>
>>>
>>>
>>>
>>>
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>>
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