[evlatests] switched power issues

[Paul Demorest]

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?

-Paul