[evlatests] switched power issues

[Paul Demorest]

hi all,

I've recently been exploring the effect of the VLA switched power 
signals on WIDAR data.  This has turned up some notable features, some 
of which may be relevant to everyone's favorite topic, "Pdif 
compression".  If any of this sounds interesting to you, read on 
below... 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.

-Paul

------------

For background:  The default state that is set up when a VLA receiver is 
selected for use is for its noise diode to be turned on, switching at 10 
Hz.  This setting for all other (not-in-use) receivers is left at 
whatever it happened to be at previously.  The net result is that the 
typical state of the VLA is for _all_ noise cals (all receivers) to be 
switching at 10 Hz at all times, regardless of which receiver is 
actually being used.

By using WIDAR's "pulsar binning mode" it is possible to fold the 
correlations at 10 Hz and see the effect of the noise cal signals on the 
data directly, for example by observing a calibrator and solving for 
antenna gain as a function of the cal pulse phase.  Alternatively, "FRB 
mode" can be used to dump correlations at 5 ms and do the folding 
offline; both setups seem to provide consistent results.

What I've seen is that there is a notable difference in how these 
results look depending on whether the cals for the not-in-use receivers 
are left switching or are turned off.  The first attached plot shows 
gain amplitude averaged over many 10 Hz cycles for ea22 at S-band.  Top 
panel is with all cals on (the usual situation) and bottom is with all 
cals off except for S-band.  The second plot shows the corresponding 
autocorrelation data, you can see how this signal lines up with the cal 
high/low state.  Immediately following a cal low->high transition there 
is a gain dip, then some ringing.  The same thing with opposite sign 
happens at the high->low transition.  The magnitude of this effect seems 
to vary with antenna, I've shown one of the stronger examples of it 
here.  Turning off the unused cals makes this ringing signal vanish, and 
leaves only a small, ~constant gain difference between the states (this 
in turn goes away if the S-band cals are turned off).  I think this 
latter feature may be a quantization effect due to the 4-bit 
correlation.

However, I have no explanation for how the "out-of-band" cal signals 
(which in principle should have no effect) are causing the "ringing".  
If this ringing is a true amplifier gain variation, since it has 
opposite sign at each transition it will cause an apparent Pdif 
compression.. basically, the average gain being different in the cal 
high and low states will couple a small fraction of Tsys into the Pdif 
measurement (Pdif ~= G*Tcal + DeltaG*Tsys).  The sign and approximate 
magnitude of this effect seem suspiciously comparable to the Pdif 
compression results Rick has shown various times.. for example a 0.1% 
gain difference would result in ~10% change in the measured PDif value 
when Tsys is increased by a factor of 5.

I think it would be interesting to try some of the strong source 
observations that show Pdif compression with extraneous cal signals on 
vs off, and compare the results.  I don't know if this will explain the 
full effect since there could still be other non-linearity present.  But 
based on everything I've seen so far, I do expect this change to have at 
least some effect.  Also I think it would be a good idea to make "cals 
off" the default for receivers that are not in use.. I can't think of 
any reason why they need to be left on (except possibly P-band for 
commensal VLITE observing).  Finally, it would be nice to know what is 
going on in the electronics to cause this effect, I don't really have 
any idea what might be doing this.

Comments, alternate explanations, or questions about any of this are 
welcome!
-------------- next part --------------
A non-text attachment was scrubbed...
Name: gain_amp_onoff_ea22.png
Type: image/png
Size: 111667 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20160520/1d095d76/attachment-0002.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: autos_onoff_ea22.png
Type: image/png
Size: 81123 bytes
Desc: not available
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20160520/1d095d76/attachment-0003.png>