[evlatests] switched power issues

[Paul Harden]

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!

Paul