[evlatests] switched power issues

[Rick Perley]

     Clearly a subject for the next 'Engineering/Tests' meeting, 
scheduled for June 9.

     Rick

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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