[evlatests] switched power issues

[Barry Clark]

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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